River, fish community assembly, functional diversity, environmental filtering, interspecific interaction

Abstract

The middle reach of the Yangtze River is a typical floodplain ecosystem consisting of a main river and a high density of lakes (one of the highest in China). It harbors a rich and unique freshwater fish fauna. In the past few decades, the lakes in the middle reaches of the Yangtze River have been disturbed by multiple environmental anthropic pressures, and the composition and structure of fish communities have undergone significant changes. Clarifying the fish community assembly mechanism has important significance for the maintenance and restoration of lake biodiversity in the Yangtze River Basin. In this study, 26 shallow lakes in the middle reaches of the Yangtze River were taken as the research object. This study shows how the multiple environmental pressures faced by the lakes in the middle reaches of the Yangtze River induced changes in the functional structure and diversity of fish communities through the action of environmental filtering and interspecific interactions. A quantitative analysis of species co-occurrence patterns allowed a better assessment of the relative importance of environmental filtering, competition, and predator-prey interactions to community assembly. This study contributes to a better knowledge of fish community assembly rules, and also provides a scientific basis for the management and scientific protection of lake fish communities in the Yangtze River Basin.

Extended abstract

The middle reach of the Yangtze River is one of the most concentrated areas of lakes in China, forming a typical river-lake complex ecosystem and forming a unique and rich freshwater fish fauna. However, in the past few decades, the lakes in the middle reaches of the Yangtze River have been disturbed by factors such as water pollution, land use change, habitat fragmentation, and overstocking, and the composition and structure of fish communities have undergone significant changes. In recent years, China has fully implemented the Yangtze River ecological environment protection and restoration strategy, and the lake recuperation system has been gradually improved.

Since then, lakes in the middle reaches of the Yangtze River have entered a new historical period of ecological restoration and succession, which provides a valuable historical opportunity and an ideal ecosystem for studying the mechanism of fish community assembly in the process of ecological restoration and succession of lakes.

Clarifying the fish community assembly mechanism in the process of lake ecological restoration succession has important theoretical significance for the maintenance of lake biodiversity in the Yangtze River Basin, and can also provide technical support for the evaluation of the effect of watershed ecological restoration measures. In this study, 26 shallow lakes in the middle reaches of the Yangtze River were taken as the research object, and the lake environmental factors and fish community data were obtained through field surveys and other methods to carry out related research. The main research revolves around the following contents and ideas: elaborate the spatial pattern of environmental factors based on the impact of multiple environmental pressures faced by the lake ecosystem on fish communities; to reveal the impact mechanism of key environmental factors on the species composition and functional structure of fish communities; proposed and verified the hypothesis that environmental pressure mediates interspecies interactions to drive changes in fish community diversity; build a prediction model to identify key predictors and explore the application of prediction models and ecological threshold detection in lake ecosystem management; to explore the main driving forces in the mechanism of fish community construction and to quantify the relative importance of environmental filtering and inter-species interactions in community assembly. The specific research results are as follows:

1. Based on the 2019 survey of fish and environmental data such as fish communities, eutrophication, land use, lake morphology, and proliferation patterns in lakes in the middle reaches of the Yangtze River, explore the spatial pattern and key factors of multiple environmental pressures. The spatial patterns of four environmental factors, namely eutrophication, land use, lake morphology, and proliferation patterns, were described respectively in the lakes in the middle reaches of the Yangtze River. The results show that there are environmental gradient differences among the four types of factors among lakes, so it is reasonable and representative to select the four types of factors as the environmental disturbances that mainly drive environmental filtration in this study. In the analysis of the inner relationship among environmental factors, the hypothesis of this study was verified. That is, eutrophication is a secondary disturbance among the four environmental pressures, and the other three environmental factors may all have an impact on eutrophication, making eutrophication a comprehensive effect of multiple disturbances. In the identification of key environmental factors affecting fish community structure, only eutrophication and proliferation patterns were directly related to fish community metrics. While proliferation patterns were only significantly correlated with fish biomass, eutrophication was also correlated with functional diversity of fish communities. Therefore, eutrophication has been identified as a key factor in the study of the relationship between lake environmental pressure and fish community structure in the middle reaches of the Yangtze River, which is conducive to revealing its specific impact on fish communities.

2. In this study, a total of 19 families and 53 species of fish were investigated in lakes in the middle reaches of the Yangtze River. On the basis of the identification of eutrophication as a key environmental factor. The impact of eutrophication as a key environmental factor on species abundance, taxonomic diversity, functional diversity, species structure, functional trait composition, and special functional groups (life history strategies) of fish communities was explored from the multidimensional level of fish community structure. The taxonomic diversity, especially the difference in species richness, cannot be directly explained by the eutrophication gradient. However, species abundance and biomass were positively correlated with eutrophication, and functional diversity was negatively correlated. After analyzing the composition of species structure and functional traits, this study found that eutrophication essentially affects species composition through trait filtering and thus affects fish community structure. It is summarized from the perspective of environmental filtering on functional traits. Eutrophication affects the quality of habitat and changes the availability of resources, which increases the dominance of pelagic zooplanktivorous species that are more adaptable to the turbid environment of eutrophication, and reduces the dominance of benthic species. From the perspective of life history strategy, we applied the triangular continuous life history strategy framework to conduct research, and the results showed that eutrophication negatively impacted equilibrium species by reducing environmental stability and reducing resource limitation. In displacing the species dominance of the equilibrium, periodic strategists have a greater advantage than opportunistic strategists, possibly because periodic strategists are in a better position in resource competition and predator-prey relationships.

3. Although no direct relationship between species richness and environmental factors was found in this study, the impact of environmental factors on fish community diversity can be explained by interspecies interactions. First of all, this study examines the relationship between biomass and species evenness, species dominance, functional evenness, functional dispersion, and functional redundancy. It was determined that the increase in community biomass was mainly caused by the increase in minority species, which increased interspecies competition. Second, a unimodal relationship between fish biomass and species and functional richness was identified through a test of the "biomass-driven competition hypothesis". It is determined that when the biomass (the degree of interspecific competition) reaches a certain level, the richness will decrease.

Finally, using structural equation modeling, we determined that the process by which biomass drives competition leading to loss of community diversity is mediated by nutrient availability, with eutrophication being the main cause of biomass rise. The mechanism by which multiple environmental factors mediate species interactions to drive changes in fish community diversity was revealed in stages. The specific role of interspecies interactions on diversity in the assembly of fish communities in lakes in the middle reaches of the Yangtze River was explored, while reflecting that environmental filtering is still the main factor mediating this process.

4. Using predictive models and identification of key predictors, from the perspective of ecosystem and fish community management, to expand the research on the mechanism of environmental filtering on fish community structure. The importance of environmental factors was assessed in terms of abundance, richness, and diversity of taxonomic structure and functional composition using random forest models. At the same time, the gradient forest model was used to evaluate the importance of environmental factors from the perspective of multivariate structure of species composition and functional traits. Twelve key predictors of four environmental stress categories, eutrophication, land use, lake morphology and proliferation patterns, were identified from the perspectives of fish community abundance, taxonomic diversity, functional diversity, species composition, and functional traits by predictive models .

The prediction performance of the prediction model was evaluated based on the construction of the prediction model, and because the data set is small for the prediction model, the resulting prediction model has predictive ability, but the prediction goodness and accuracy are not ideal. This study used the Threshold Indicator Taxa Analysis (TITAN) model based on the ecological threshold test of key factors. It further supplements and expands the mechanism of environmental filtering on fish communities, and at the same time reflects the importance of ecological threshold testing in lake ecosystems and fish community management. 5. By quantifying inter-species co-occurrence patterns, environmental filtering, inter-species competition, and predator-prey interactions, key issues in the mechanism of fish community assembly in lakes in the middle reaches of the Yangtze River are explained. Firstly, it was verified that neutral theory and niche theory are not absolutely opposite in explaining fish community assembly, but compatible and complementary.

Although fish communities in lakes in the middle reaches of the Yangtze River tend to stochastic dispersal, this pattern is due to the interaction of environmental filtering and inter-species interactions. And confirmed the relatively more important role of environmental filtering in the process of fish community assembly in lakes in the middle reaches of the Yangtze River. While explaining the relatively low importance of inter-species interactions, it reveals that the fish community assembly process in this region is greatly influenced by environmental factors. In comparing with other relevant research, explore ways to improve and expand future research.

To sum up, this study starts from the multiple environmental pressures faced by lakes in the middle reaches of the Yangtze River, and identifies key environmental factors that have important impacts on fish communities. And reveal the impact of its mediated environmental filtering and inter-species interactions on the species composition, functional structure and diversity of fish communities. At the same time, the feasibility of applying the prediction model based on key environmental factors and the ecological threshold detection model to lake ecosystem and fish community management was explored and evaluated. Finally, based on the quantification of species co-occurrence patterns, environmental filtering, interspecies competition, and predatorprey interactions, the relative importance of environmental filtering and interspecies interactions in fish community assembly was revealed from a mechanistic perspective.

This study enriches the theoretical system and method framework of global biological community assembly, and also provides a scientific basis for the management and scientific protection of lake fish communities in the Yangtze River Basin.

Chapter1 General introduction 1.1 Research background

Fish is one of the most diverse groups of vertebrates, plays key ecological functions in aquatic ecosystems, and is also an important protein source for humans (Duffy et al., 2016). Globally, the overexploitation of fish resources and the rapid degradation of aquatic ecosystems are seriously threatening the sustainable food production and biodiversity maintenance in fishery waters [START_REF] Asche | Three pillars of sustainability in fisheries[END_REF][START_REF] Fluet-Chouinard | Global hidden harvest of freshwater fish revealed by household surveys[END_REF][START_REF] Gordon | Habitat degradation negatively affects auditory settlement behavior of coral reef fishes[END_REF]. Changes in the structure of fish communities directly affect the structure and function of water ecosystems, thereby having an important impact on fishery productivity, biodiversity, nutrient cycling, water purification, and food web dynamics [START_REF] Cohen | Climate warming reduces fish production and benthic habitat in Lake Tanganyika, one of the most biodiverse freshwater ecosystems[END_REF]. Therefore, systematically elucidating the mechanism of fish community construction is an important prerequisite for sustainable management of lake ecosystems and restoration of biodiversity.

The Yangtze River Basin is one of the regions with the most concentrated lakes in China [START_REF] Wang | Fisheries and wetland protection in the Yangtze River Basin[END_REF]. Thousands of lakes are distributed in the Yangtze River Basin, with a total area of 1.5210 4 km 2 , which is close to 1/5 of the total lake area in China [START_REF] Wang | Predictive limnology research on small and medium-sized lakes in the middle and lower reaches of the Yangtze River[END_REF]. The Yangtze River and its lake ecosystem are rich in water resources and biodiversity, and are one of the most important wetland areas in the world.

The whole world pays great attention to the biodiversity of the Yangtze River and its lake ecosystem [START_REF] Fu | Spatial pattern and resource analysis of fish diversity in the Yangtze River Basin[END_REF]. There are more than 400 species of fish in the Yangtze River system, including about 350 species of pure freshwater fish and 156 species of endemic fish [START_REF] Cao | Current status and protection countermeasures of fish resources in the Yangtze River[END_REF]. The natural catch production in the Yangtze River Basin accounts for 63%-64% of the total catch of freshwater fish in China, and it is the most important production area of freshwater fish in China [START_REF] Yu | Preliminary evaluation of the impact of large-scale water conservancy projects on fish resources in the Yangtze River (part 1)[END_REF]. In the middle reaches of the Yangtze River plain area, the river surface is wide, the water flow is gentle, and the lakes along the coast are dotted with fertile water quality and rich food organisms. In history, most of the lakes were once connected with the main stream or tributaries of the Yangtze River, forming a typical river-lake complex ecosystem, forming a unique and rich environment. It is one of the most important freshwater fishery bases in China and the world [START_REF] Chen | Discussion on the Pearl River's fish fauna and its zoogeographic division[END_REF][START_REF] Xie | Biodiversity crisis in the Yangtze River: The culprit was dams, followed by overfishing[END_REF]. This area is also one of the areas in China where fish ecology research was first carried out and long-term historical data are accumulated most continuously and abundantly.

Most of the lakes in the middle reaches of the Yangtze River were originally connected to the river, that is, they were closely connected with the Yangtze River and naturally connected with each other, forming a unique and complete river-lake complex ecosystem with the main stream and tributaries of the Yangtze River [START_REF] Chang | Significance of fishery in river-connected lakes and resource management strategies. Resources and Environment in The Yangtze Basin[END_REF]. But today, most of the lakes have lost their connection with the Yangtze River due to the construction of water conservancy and other reasons since the 1950s [START_REF] Wang | The ecological impact of river and lake barriers on fish resources in the Zhangdu Lake area. Resources and Environment in The Yangtze Basin[END_REF][START_REF] Hu | Fish species diversity and its temporal and spatial changes in West Dongting Lake. Resources and Environment in The Yangtze Basin[END_REF]. Factors such as the reclamation of lakes and the acceleration of natural sedimentation induced by the loss of lake connectivity further lead to the shrinkage or even disappearance of the lake water surface [START_REF] Wang | The ecological impact of river and lake barriers on fish resources in the Zhangdu Lake area. Resources and Environment in The Yangtze Basin[END_REF][START_REF] Zeng | Fishery resources in the Yangtze River system[END_REF]). As a result, it has a profound impact on the structure and dynamics of the population or community of fish and other aquatic organisms [START_REF] Xie | Threats to biodiversity in Chinese inland waters[END_REF][START_REF] Li | Biodiversity and protection of important fish species in the Yangtze River[END_REF]. Fish species in many lakes have declined by as much as 50%, while wild fish production has declined by as much as 80% [START_REF] Wang | The ecological impact of river and lake barriers on fish resources in the Zhangdu Lake area. Resources and Environment in The Yangtze Basin[END_REF][START_REF] Jin | Comprehensive research on Jianghan Lake Group[END_REF][START_REF] Hu | Structure and dynamics of fish species diversity in lakes in the middle reaches of the Yangtze River[END_REF]. For the lakes in the Yangtze River Basin, at different times, the increasing dam barrier and reclamation gradually fragmented the river-lake complex ecosystem that was originally interconnected with the Yangtze River. Among the different factors affecting the survival of wild fish, river and lake barriers are the most damaging, but the impact of overfishing cannot be ignored [START_REF] Hu | Structure and dynamics of fish species diversity in lakes in the middle reaches of the Yangtze River[END_REF]. At the same time, in disorderly purse seine farming activities, the super-density, large-area, and high-input farming model has caused the loss of fish habitats in lakes and exacerbated the eutrophication of lakes, which has also affected lake ecosystems, especially fish habitats. caused serious damage. In general, in the past few decades, the lakes in the middle reaches of the Yangtze River have been disturbed by factors such as river and lake isolation, agricultural expansion, overstocking, habitat fragmentation, and water pollution. Significant changes have taken place in the composition and structure of fish communities, mainly manifested in the sharp decline in the number of species, the sharp decline in resources, and the miniaturization and younger age of fish. Past studies have generally focused on the time-series changes in the taxonomic diversity of fish communities [START_REF] Jia | Lake fisheries in China: Challenges and opportunities[END_REF][START_REF] Jiang | Impacts of pen culture on alpha and beta diversity of fish communities in a large floodplain lake along the Yangtze River[END_REF], and the mechanism of fish community construction and key driving factors in the process of lake ecological succession. There is a serious lack of awareness in this area, resulting in a lack of theoretical basis for the utilization and protection of fish resources in lakes in the middle reaches of the Yangtze River, and various deviations in practice, which have caused major losses to China's aquatic resources and biodiversity.

In recent years, the country has comprehensively implemented the Yangtze River ecological environment protection and restoration strategy, such as the in-depth promotion of measures such as the withdrawal of the "three nets" of key lakes in the Yangtze River Basin, "returning embankments to lakes", and "completely banning fishing in aquatic organisms nature reserves". The lake recuperation system has been gradually improved. Since then, lakes in the middle reaches of the Yangtze River have entered a new historical period of ecological restoration and succession, which provides a valuable historical opportunity and an ideal ecosystem for studying the mechanism of fish community construction in the process of ecological restoration and succession of lakes. Clarifying the fish community construction mechanism in the process of lake ecological restoration succession has important theoretical significance for the sustainable food production and biodiversity maintenance of lakes in the Yangtze River Basin, and can also provide technical support for the evaluation of the effect of watershed ecological restoration measures.

Biodiversity

Overview of biodiversity

Biodiversity is defined in the United Nations Convention on Biological Diversity as: the variability among living organisms from all sources, including terrestrial, marine and other aquatic ecosystems and the ecological complexes they constitute. This includes diversity within species, between species and in ecosystems [START_REF] Heywood | Global biodiversity assessment[END_REF]. Biodiversity determines the functions and services of ecosystems, and is the basis for the survival of human beings and the development of human society [START_REF] Ma | On the concept of biological diversity[END_REF]. Biodiversity provides humans with sufficient food and resources through various life forms, and also plays a pivotal role in regulating climate, protecting soil, and conserving water sources through biological functions [START_REF] Hooper | Effects of biodiversity on ecosystem functioning: a consensus of current knowledge[END_REF]. However, with the rapid growth of the world's population, environmental problems caused by human activities have caused drastic changes in the global ecological environment, resulting in unprecedented impacts and losses on biodiversity [START_REF] Tilman | Forecasting agriculturally driven global environmental change[END_REF]. This will cause the ecosystem to lose its proper functions and services, and have direct or indirect impacts on human development [START_REF] Díaz | Biodiversity loss threatens human well-being[END_REF][START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF].

Biome diversity is divided into four levels: genetic diversity, species diversity, ecosystem diversity and landscape diversity, and different levels are divided according to the different contents they reflect [START_REF] Ma | The measurement method of biological community diversity: the measurement method of α diversity (Part 1)[END_REF]. Genetic diversity, also known as gene diversity, generally refers to the sum of genetic variation among individuals within a species, which is the result of long-term natural selection and mutation of individuals of the same species due to their different living environments [START_REF] Ma | The measurement method of biological community diversity: the measurement method of α diversity (Part 1)[END_REF]. Species diversity is one of the most direct parameters to characterize biodiversity. Species are classified by morphological and genetic means, and include origin, change, composition, distribution characteristics and structure maintenance based on species level [START_REF] Zhou | Species and Species Diversity[END_REF]. Ecosystem diversity refers to the diversity of habitats, biological communities and ecosystems within the biosphere, as well as the diversity of habitat differences and ecological process changes within the ecosystem [START_REF] Ma | The measurement method of biological community diversity: the measurement method of α diversity (Part 1)[END_REF].

Landscape diversity refers to the diversification or variability in spatial structure, functional mechanism and temporal dynamics composed of different types of landscape elements or ecosystems. A landscape is a highly spatially heterogeneous area composed of interacting landscape elements or ecosystems in a certain order [START_REF] Fu | Types of landscape diversity and their ecological significance[END_REF].

Community diversity

Community ecology is an important part of ecological research, and the diversity of biological communities is also the main component of the level of ecosystem diversity in biodiversity [START_REF] Mcneely | Conserving the world's biological diversity[END_REF]. Biome diversity refers to the differences in community composition, functional characteristics, and distribution dynamics [START_REF] Clarke | A further biodiversity index applicable to species lists: variation in taxonomic distinctness[END_REF]. Among the various levels of biodiversity, biome diversity is the earliest level to be studied, both in terms of definition concepts and quantitative methods are relatively mature [START_REF] Ma | The measurement method of biological community diversity: the measurement method of α diversity (Part 1)[END_REF]. The research content of community diversity mainly focuses on species diversity related to structural composition, and covers the diversity of functional structural characteristics and composition changes as well as the diversity of species phylogeny (Lyashevska and Farnsworth, 2012). The study of biological community diversity has its theoretical and practical significance. In theory, it can help ecologists understand more deeply the structural changes and functional characteristics of biological communities in ecosystems; in practice, it can help ecosystem managers provide references and standards in system management and biological protection [START_REF] Mccann | The diversity-stability debate[END_REF]. A biome with higher diversity tends to have richer genetic and genetic diversity. From the perspective of biological integrity, the significance of this community is greater, so that corresponding protection measures are taken according to the diversity of the community in conservation practice. There are many successful cases of this application in practice [START_REF] Magurran | Ecological diversity and its measurement[END_REF]. However, it is one-sided to use biodiversity to evaluate the stability of biological communities, and the communities that adapt to the environmental conditions of the local ecosystem are the truly stable communities.

For example, a biome in the climax successional stage may be less diverse than a community in the process of succession, but it can maintain a stable state under local environmental conditions [START_REF] Ma | The measurement method of biological community diversity: the measurement method of α diversity (Part 1)[END_REF].

Fish community diversity is an important part of biodiversity, and fish diversity can provide rich fishery resources and food sources for human society [START_REF] Ziv | Trading-off fish biodiversity, food security, and hydropower in the Mekong River Basin[END_REF].

At the same time, it also provides diversified genetic resources, and continuously breeds new varieties through various breeding techniques to contribute to the development of human society [START_REF] Wang | Historical review and prospect of transgenic fish development in my country[END_REF]. In addition to providing direct value, fish diversity can also maintain the balance and stability of the ecosystem and ensure ecological service functions [START_REF] Jeppesen | The structuring role of submerged macrophytes in lakes[END_REF][START_REF] Holmlund | Ecosystem services generated by fish populations[END_REF]. In the early stage of fish community diversity research, due to the intuitiveness of species taxonomy, it was mainly focused on the level of species diversity [START_REF] Fisher | The relation between the number of species and the number of individuals in a random sample of an animal population[END_REF]. With the development of fish community diversity research levels, on the basis of species diversity, multi-dimensional diversity concepts such as functional diversity [START_REF] Cilleros | Taxonomic and functional diversity patterns reveal different processes shaping European and Amazonian stream fish assemblages[END_REF] and phylogenetic diversity are gradually widely used in research. And the concept of fish community diversity has been extended [START_REF] Pool | Species contribute differently to the taxonomic, functional, and phylogenetic alpha and beta diversity of freshwater fish communities[END_REF]. At the same time, the genetic variation between and within fish species (genetic diversity) and the different relationships between fish communities and their habitats (ecosystem diversity) are also included in the category of fish community diversity.

Research dimensions of diversity

The significance of community diversity is based on three distinctly studied dimensions of community structure and composition, namely species composition, functional traits, and phylogeny. Biome diversity represents different ecological meanings in different dimensions, providing complementary ecological information for ecologists on the meaning of community diversity [START_REF] Gallardo | How to choose a biodiversity indicator-Redundancy and complementarity of biodiversity metrics in a freshwater ecosystem[END_REF]. As a basic component dimension of biodiversity, species diversity has been widely used as a representative of community diversity in the study of biodiversity-related issues in ecology since it was proposed [START_REF] Fisher | The relation between the number of species and the number of individuals in a random sample of an animal population[END_REF][START_REF] Ma | On the concept of biological diversity[END_REF]. The breadth of species diversity in biological communities is generally quantified by biodiversity indices (species richness, Shannon-Wiener index, Simpson index, Pielou index, Margalef index, etc.) based on species number and species abundance data [START_REF] Solow | Measuring biological diversity[END_REF][START_REF] Magurran | Measuring Biological Diversity[END_REF]. However, ecologists have found in recent studies that the traditional species diversity index is not good at quantifying the position of different species in the ecological function and evolutionary relationship when studying the differences in human disturbance, environmental factors, interactions between organisms and species evolution [START_REF] Connell | Diversity in tropical rain forests and coral reefs: high diversity of trees and corals is maintained only in a nonequilibrium state[END_REF]Mouillot et al., 2006). In order to improve the framework of biodiversity, while describing the species composition of biological communities, related indicators based on functional traits and phylogenetic characteristics were incorporated into the diversity evaluation system. This facilitates a comprehensive description of the role of species in ecosystems and biomes [START_REF] Webb | Phylogenies and community ecology[END_REF]Olga and Keith, 2012). In summary, the study of biome diversity should be based on a threedimensional diversity space, which includes species diversity, functional diversity and phylogenetic diversity. The study of multidimensional diversity is conducive to comprehensively explaining the changing rules and determining mechanisms of community structure [START_REF] Mason | Functional richness, functional evenness and functional divergence: the primary components of functional diversity[END_REF][START_REF] Mcgill | Rebuilding community ecology from functional traits[END_REF].

The proposal of functional diversity greatly compensates for the inability of species diversity to reflect the specific functions of species in ecosystem processes.

Even in two biological communities with the same species diversity, due to the differences in functional traits of different species, the communities occupy completely different functional trait spaces in the ecosystem (Shuai et al., 2017). Functional diversity based on species functional traits effectively establishes the relationship between species and ecosystem function [START_REF] Petchey | Functional diversity: back to basics and looking forward[END_REF]. From the perspective of functional traits, functional diversity is defined as the value or range of functional traits of species that affect ecosystem functions, and describes the spatial variation of functional traits in a community [START_REF] Dı Áz | Vive la différence: plant functional diversity matters to ecosystem processes[END_REF]. Functional diversity can establish a direct relationship with the ecosystem environment and reveal the response mechanism of biological communities to environmental disturbances.

However, its quantification requires more trait parameters than species diversity [START_REF] Pease | Ecoregional, catchment, and reach-scale environmental factors shape functional-trait structure of stream fish assemblages[END_REF].

Species diversity and functional diversity of biological communities describe community diversity from two aspects of species composition and functional traits.

However, when measuring the biodiversity of a community, the relationship or phylogenetic distance between species is also very important [START_REF] Harper | Biodiversity: measurement and estimation[END_REF]. Phylogenetic diversity indicates the relatedness among species in a biome. In general, species with close relatives have similar functional traits, so phylogenetic diversity is often used as a comprehensive diversity index. The use of phylogenetic diversity integrates concepts from evolutionary biology with community ecology, providing new insights into the adaptive relationship between ecological environments and species evolution [START_REF] Ricklefs | Community diversity: relative roles of local and regional processes[END_REF][START_REF] Safi | Understanding global patterns of mammalian functional and phylogenetic diversity[END_REF]. However, traditional phylogenetic diversity calculates phylogenetic distances based on evolutionary relationships among species such as phylogenetic trees, and the process of data acquisition and processing is cumbersome and inconvenient to use [START_REF] May | Taxonomy as destiny[END_REF][START_REF] Williams | Measuring biodiversity: taxonomic relatedness for conservation priorities[END_REF]. [START_REF] Clarke | A taxonomic distinctness index and its statistical properties[END_REF] were the first to quantify interspecies phylogenetic distances of biomes using a series of indices of taxonomic difference between species. These indices effectively reflect the evolutionary relationship between species and are used by researchers as a measure of phylogenetic diversity, especially in communities that lack complete phylogenetic information [START_REF] Jog | Can land use indicate wetland floristic quality and taxonomic distinctness?[END_REF].

Research levels of diversity

The research levels of biological community diversity can be divided into αdiversity, β-diversity, and γ-diversity according to different scales and measurement methods [START_REF] Whittaker | Vegetation of the Siskiyou mountains, Oregon and California[END_REF]. α-diversity refers to the diversity of biomes based on a specific area, and γ-diversity describes the sum of the diversity of multiple biomes [START_REF] Whittaker | Vegetation of the Siskiyou mountains, Oregon and California[END_REF]. In fact, α-diversity and γ-diversity have the same characteristics, that is, they describe the diversity of biological communities in a single area (different scales), so they are collectively called cataloged diversity [START_REF] Jurasinski | Inventory, differentiation, and proportional diversity: a consistent terminology for quantifying species diversity[END_REF]. βdiversity characterizes the difference in diversity among communities, and measures the temporal and spatial differences in species composition, functional traits, and phylogeny among different sampling units in a specific geographical area. β-diversity acts as a bridge linking α-diversity and γ-diversity [START_REF] Anderson | Multivariate dispersion as a measure of beta diversity[END_REF]. Research on diversity initially focused on the α-diversity level. In recent years, due to the more comprehensive characteristics of community diversity explained by β-diversity, more research and development have gradually been obtained. β-diversity can be divided into two categories in terms of measurement methods. One is differentiation diversity, starting from the specific composition of species and comparing the similarities of different communities. The second is proportional diversity, which compares the differences in diversity indices at different scales, and is usually associated with αdiversity and γ-diversity [START_REF] Jurasinski | Inventory, differentiation, and proportional diversity: a consistent terminology for quantifying species diversity[END_REF][START_REF] Chen | Research progress on Beta diversity[END_REF]. [START_REF] Baselga | Partitioning the turnover and nestedness components of beta diversity[END_REF] further decomposed β-diversity into components arising from species turnover and species nesting. At the same time, β-diversity extends from the category of species diversity to the category of functional β-diversity and phylogenetic β-diversity [START_REF] Graham | Phylogenetic beta diversity: linking ecological and evolutionary processes across space in time[END_REF]Villéger et al., 2013). β-diversity can be used as an effective tool to explore the reasons for the homogeneity of biological communities caused by human disturbance, thereby revealing the underlying drivers of community construction. The division of species turnover and species nesting components is beneficial for ecologists to further explore the mechanism of community construction [START_REF] Hylander | Differences in habitat quality explain nestedness in a land snail meta-community[END_REF][START_REF] López-Delgado | Local environmental factors influence beta-diversity patterns of tropical fish assemblages more than spatial factors[END_REF].

Functional diversity

Overview of functional diversity

In the context of global change, the mechanisms by which biome functions affect ecosystem properties and how ecosystem functions respond to changes in environmental conditions are prerequisites for understanding ecosystem processes. In order to better explain these problems, research related to the field of biome function has emerged as the times require. Ecologists have found that the functional traits of species can be used as a link between ecological environment, biodiversity and ecosystem function. On this basis, functional diversity based on functional traits has been greatly developed and has become a hot spot in the field of ecological research.

As an important part of biodiversity, functional diversity is based on the functions of species in ecosystems and biomes, and by establishing the link between species and ecosystem functions, it can explain and predict ecosystem functions [START_REF] Petchey | Functional diversity: back to basics and looking forward[END_REF]. Although functional diversity plays an important role in ecology-related research, and a large number of studies on functional diversity have been published, its definition has not been unified. The main reason for the ambiguity of the concept of functional diversity is that its connotation is more complex than that of species diversity, many related indices and methods have been proposed, and there is no unified measurement standard [START_REF] Dı Áz | Vive la différence: plant functional diversity matters to ecosystem processes[END_REF]. Some relatively broad definitions cannot accurately cover the meaning of functional diversity. Such as "diversification of functions in communities (Tesfaye et al., 2003)", "number, type and distribution of functions of organisms in ecosystems [START_REF] Dı Áz | Vive la différence: plant functional diversity matters to ecosystem processes[END_REF]" and so on. [START_REF] Tilman | Forecasting agriculturally driven global environmental change[END_REF] defines functional diversity as "the functional components of biological diversity that affect the function and operation of ecosystems". This definition regards functional diversity as the connection between ecosystem function and biodiversity, but the content covered by ecosystem is too broad and complex. This definition does not allow a quantitative description of functional diversity [START_REF] Petchey | Functional diversity: back to basics and looking forward[END_REF]. The definition of functional diversity currently widely accepted by ecologists is "refers to the value, distribution, range and relative abundance of all species functional traits in a specific ecosystem community" [START_REF] Dı Áz | Vive la différence: plant functional diversity matters to ecosystem processes[END_REF][START_REF] Tilman | Forecasting agriculturally driven global environmental change[END_REF]Petchey and Gaston, 2002b). This definition makes it clear that the measurement of functional diversity is essentially the measurement of the diversity of functional traits, that is, functional diversity refers to the overall difference or diversity of functional traits among all species in a community. Functional traits are biological characteristics that affect ecosystem function [START_REF] Tilman | Forecasting agriculturally driven global environmental change[END_REF][START_REF] Petchey | Functional diversity: back to basics and looking forward[END_REF]. This definition realizes the quantification of the multi-faceted functional characteristics of species and at the same time links the functional traits of species with ecosystem functions and services [START_REF] Lavorel | Predicting changes in community composition and ecosystem functioning from plant traits: revisiting the Holy Grail[END_REF]Violle et al., 2007).

There are two main approaches for the study of functional diversity based on functional traits. One is to divide species into different functional groups based on single or multiple functional traits [START_REF] Pla | Quantifying functional biodiversity[END_REF], and the other is to calculate the functional diversity index based on species functional traits [START_REF] Mouchet | Functional diversity measures: an overview of their redundancy and their ability to discriminate community assembly rules[END_REF].

Functional groups refer to species clusters with similar functional traits related to ecosystem service functions. Functional groups can be divided by a key trait, or can be classified comprehensively based on multiple traits that can reflect ecosystem functions [START_REF] Pla | Quantifying functional biodiversity[END_REF]. The method of calculating functional trait index is to construct functional space according to the functional traits of all species in the community, integrate the location and abundance of species in the space, and reflect the community functional diversity from the perspective of functional richness, functional evenness, functional dispersion and other measures [START_REF] Mouchet | Functional diversity measures: an overview of their redundancy and their ability to discriminate community assembly rules[END_REF]. In specific studies, the comprehensive use of functional groups and functional diversity index methods can often effectively study the functional structure and characteristics of communities [START_REF] Mouillot | Functional over-redundancy and high functional vulnerability in global fish faunas on tropical reefs[END_REF].

In ecological research, the application of functional diversity in terms of ecosystem function, dynamics, services, and stability is becoming more and more popular, so it is particularly important to standardize the use of functional diversity methods for quantitative analysis of ecological problems. Petchey and Gatson (2006) proposed that quantitative analysis of functional diversity should ideally follow the following four steps: [START_REF] Feng | Eutrophication induces functional homogenization and traits filtering in Chinese lacustrine fish communities[END_REF] Reasonable selection of functional traits should include biologically appropriate functional information into the analysis and exclude irrelevant information. Specifically, we should start from selecting the appropriate functional type [START_REF] Naeem | Ecosystem consequences of biodiversity loss: the evolution of a paradigm[END_REF], and select the functional characteristics of the research focus in this classification, and exclude useless information [START_REF] Naeem | Disentangling biodiversity effects on ecosystem functioning: deriving solutions to a seemingly insurmountable problem[END_REF]. At the same time, select a suitable and sufficient number of functional traits to ensure the stability of species functional classification [START_REF] Rosenfeld | Functional redundancy in ecology and conservation[END_REF]. Finally, the collected functional traits should be selected according to the quality of functional information and the way of acquisition [START_REF] Hodgson | Allocating CSR plant functional types: a soft approach to a hard problem[END_REF]. ( 2) The trait values are weighted according to the relative functional importance of the selected functional traits. Although there is still no uniform standard on how to objectively weight functional traits according to the specific ecological question under study. But there is no doubt that different weights can produce completely different functional classifications and have important implications for the relationship between taxonomy and functional diversity [START_REF] Walker | Plant attribute diversity, resilience, and ecosystem function: the nature and significance of dominant and minor species[END_REF]Petchey and Gaston 2002;[START_REF] Roscher | The role of biodiversity for element cycling and trophic interactions: an experimental approach in a grassland community[END_REF]). (3) Reasonably choose statistical methods or mathematical models to analyze the diversity of functional traits, such as the selection of continuous and discontinuous measures. When the data of community functional traits is only classified data, you can choose to use the method of dividing functional groups for analysis, but continuous quantitative data can often better reflect the actual situation of biological community functional characteristics than discrete categorical data [START_REF] Dı Áz | Vive la différence: plant functional diversity matters to ecosystem processes[END_REF][START_REF] Mason | An index of functional diversity[END_REF][START_REF] Ricotta | A note on functional diversity measures[END_REF].

(4) Assess whether the adopted methods can explain and predict processes and changes at the level of ecosystem function. Effective analytical methods for functional diversity should be able to link biological traits and ecosystem processes through predictions and theoretical models [START_REF] Tilman | Plant diversity and ecosystem productivity: theoretical considerations[END_REF][START_REF] Dı Áz | Vive la différence: plant functional diversity matters to ecosystem processes[END_REF].

Functional traits

Functional traits of species refer to biological characteristics that can affect ecosystem functions [START_REF] Tilman | Forecasting agriculturally driven global environmental change[END_REF], and are the basis of functional diversity research. These traits are quantifiable at the individual level. For freshwater fishes, functional traits mainly refer to the characteristics of species in terms of nutritional behavior, morphology, movement, habitat use, spatial distribution and life history (Violle et al. 2007;Pease et al. 2012). Functional traits can affect both the function of organisms and the function of the ecosystem, thus establishing the link between community species and ecosystem function [START_REF] Tilman | Forecasting agriculturally driven global environmental change[END_REF][START_REF] Verberk | Delivering on a & Evolution[END_REF]. At present, species traits have been fully used in freshwater fish community management and research, which is conducive to ecologists to explain the formation mechanism of fish community. For example, the impact of habitat changes on the functional composition of fish communities [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF][START_REF] Goldstein | Comparisons of fish species traits from small streams to large rivers[END_REF], Blank et al. 2007), alien species invasion and functional homogeneity [START_REF] Olden | Life-history strategies predict fish invasions and extirpations in the Colorado River Basin[END_REF][START_REF] Teresa | Influence of forest cover and mesohabitat types on functional and taxonomic diversity of fish communities in Neotropical lowland streams[END_REF]Villéger et al., 2014;Casatti et al., 2015), species extinction and functional vulnerability [START_REF] Angermeier | Ecological attributes of extinction-prone species: loss of freshwater fishes of Virginia[END_REF][START_REF] Johnston | The relationship of spawning mode to conservation of North American minnows (Cyprinidae)[END_REF], impacts of climate change on fish communities [START_REF] Poff | Fish diversity in streams and rivers[END_REF][START_REF] Daufresne | Climate change impacts on structure and diversity of fish communities in rivers[END_REF].

Taxonomically based species diversity has dominated our perception of the global pattern of biodiversity for centuries (Gatson, 2000). In recent years, the analysis of functional traits in single or multiple species has been recognized as a reliable biomonitoring tool that can replace traditional taxon-based approaches [START_REF] Bonada | Developments in aquatic insect biomonitoring: a comparative analysis of recent approaches[END_REF][START_REF] Culp | Incorporating traits in aquatic biomonitoring to enhance causal diagnosis and prediction[END_REF]. Trait-based approaches have three major advantages over traditional taxon-based diversity measures (taxa-based abundance, taxonomic diversity, and richness): [START_REF] Feng | Eutrophication induces functional homogenization and traits filtering in Chinese lacustrine fish communities[END_REF] applicable to a wide range of geographic regions; ( 2) is conducive to the understanding of the mechanism of habitat condition change, including the prior prediction of species traits along the environmental disturbance gradient; (3) has the ability to distinguish different types and degrees of human disturbance (Statzner et al., 2001;[START_REF] Liess | The footprint of pesticide stress in communities-species traits reveal community effects of toxicants[END_REF][START_REF] Menezes | Beyond taxonomy: a review of macroinvertebrate trait-based community descriptors as tools for freshwater biomonitoring[END_REF]. Several studies have shown that changes in biomes depend on the ecological characteristics of species (i.e., body size, defense capabilities, predation strategies, competition, etc.) in response to human disturbance. Thus, considering measures of species diversity based on functional traits (such as behavioral, morphological, physiological, and ecological traits of individuals) (functional diversity) are more sensitive in capturing these variations than traditional measures of taxonomic diversity (Violle et al., 2007;[START_REF] Braghin | Effects of dams decrease zooplankton functional βdiversity in river-associated lakes[END_REF]. Changes in traditional indicators of species diversity can take decades to emerge (Edgar et al., 2009;Babcock et al., 2010). In contrast, functional or nutritional traits of the community may allow detection of earlier or more subtle community-level changes. In particular, those changes related to ecological processes and functions, such as increases in the abundance of larger, higher trophic species detected in the early establishment of marine fish sanctuaries (Coleman et al., 2015).

From the perspective of ecological theory, the reason why the method based on functional traits is helpful to reveal the impact of environmental changes on biological communities is that the nature of environmental filtering is to act on functional traits related to ecosystem functions, while the selection of species is only the result of environmental filtering. The ecological theoretical basis for the analysis of species functional traits is the "habitat template" concept proposed by [START_REF] Southwood | Habitat, the templet for ecological strategies?[END_REF][START_REF] Southwood | Tactics, strategies and templets[END_REF], which was later refined by [START_REF] Townsend | Species traits in relation to a habitat templet for river systems[END_REF]. According to this theory, temporal and spatial characteristics of a habitat (e.g., physical and chemical factors), including human disturbance, act as a series of filters that eliminate certain traits related to life processes (e.g., body size and shape, reproductive patterns, life cycle) or to ecosystem function (e.g., predation behavior, dispersal capacity). Thus, forming adaptive biomes composed of similarly shaped species. In the study of fish community ecology, based on this theory, many ecologists used fish functional traits to reveal the effects of environmental filterning such as habitat environmental factors, landscape differences and human disturbance on fish community construction [START_REF] Schlosser | Environmental variation, life history attributes, and community structure in stream fishes: implications for environmental management and assessment[END_REF][START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF][START_REF] Goldstein | Comparisons of fish species traits from small streams to large rivers[END_REF][START_REF] Growns | A numerical classification of reproductive guilds of the freshwater fishes of south-eastern Australia and their application to river management[END_REF]Blank et al., 2007).

Functional diversity index

The essence of functional diversity is the description of the diversity of functional traits [START_REF] Tilman | Forecasting agriculturally driven global environmental change[END_REF][START_REF] Petchey | Functional diversity: back to basics and looking forward[END_REF]. Functional traits cover all aspects of an organism's function. For fish, functional traits can represent population dynamics (e.g., size, growth rate, longevity), reproductive strategies (e.g., absolute egg size, egg size, parental care behavior), predation behavior (e.g., hunting style, feeding habits), trophic characteristics (e.g., trophic level), morphological characteristics (e.g., mouth position, eye position, body shape), dispersal capacity (e.g., swimming style), and habitat selection (e.g., flowing water preference, vertical distribution) [START_REF] Mouchet | Functional diversity measures: an overview of their redundancy and their ability to discriminate community assembly rules[END_REF]. Therefore, it is difficult to use a single index or measure to describe the distribution characteristics of biome species and abundance in such a complex functional space [START_REF] Mouillot | Assessment of coastal lagoon quality with taxonomic diversity indices of fish, zoobenthos and macrophyte communities[END_REF]. [START_REF] Mason | Functional richness, functional evenness and functional divergence: the primary components of functional diversity[END_REF] proposed that the functional diversity index should describe the degree of dispersion of species in the space of functional traits, and thus developed the concept of functional dispersion. It is generally accepted by ecologists that functional diversity can be divided into three categories, namely functional richness, functional uniformity and functional dispersion. This is the conclusion drawn by [START_REF] Mouchet | Functional diversity measures: an overview of their redundancy and their ability to discriminate community assembly rules[END_REF] after evaluating a series of functional diversity indices. Among them, functional richness measures how much functional trait space is occupied by all species in a biome. Functional evenness describes whether the distribution of species in the occupied trait space is uniform. Functional dispersion calculates the distance of each species in the trait space, thereby quantifying the degree of difference in the position of all species in the community in the functional space. [START_REF] Pla | Quantifying functional biodiversity[END_REF] summarized 15 functional diversity indices from the perspective of species functional traits and availability of species abundance, as well as according to research needs. It is divided into three types of functional diversity indexes: using single traits of species and species abundance, using multiple traits of species without considering species abundance, and using multiple traits while considering abundance. This can help researchers to select functional diversity indices according to the specific situation and purpose. In addition, the index of function specialization (Villéger et al., 2008), function redundancy and function vulnerability [START_REF] Ricotta | Measuring the functional redundancy of biological communities: a quantitative guide[END_REF], function rarity (Violle et al., 2017) and other indexes are proposed, providing a more diversified connotation for functional diversity.

With the continuous deepening of research by functional ecologists, the indices describing different aspects of the functional diversity of biological communities are becoming more and more abundant. There is no standard to follow which functional diversity index to choose for specific research, and researchers need to choose according to the purpose and characteristics of the research. Here, the author selects and summarizes 10 functional diversity indices that are often used in fish functional ecology based on his own understanding for a brief introduction. These indexes cover the use of single traits or multiple traits, at the species level or community level, considering the abundance of species or only the presence and absence of species, etc., while there are more classic functional diversity indices and newly proposed functional diversity indices:

1. Community weighted mean (CWM) is an index based on single traits, species abundance, and community level. It is defined as the abundance-weighted average of the functional traits of the biome. This index facilitates the dynamic evaluation of the level of a single functional trait in a biome [START_REF] Lavorel | Assessing functional diversity in the field-methodology matters![END_REF].

2. Rao's quadratic entropy (RaoQ) is an index based on multiple traits, species abundance, and community level. The index integrates information on species richness and functional trait differences between species pairs. It is calculated from the distance matrix calculated by species functional traits and the species relative abundance vector, so the index has both functional richness and functional dispersion [START_REF] Rao | Diversity and dissimilarity coefficients: a unified approach[END_REF].

3. The functional richness (FRic) is an index based on multiple traits and community levels. Researchers can consider whether to add species abundance information or only use missing data of species (generally without considering abundance) according to the research situation, but the number of species is required to be greater than the number of functional traits. The functional richness index measures the size of the functional space occupied by all species in the community. A low index means that potential resources are not fully utilized [START_REF] Cornwell | A trait-based test for habitat filtering: convex hull volume[END_REF]Villéger et al., 2008). 4. Functional evenness (FEve), which is an index based on multiple traits, species abundance, and community level. This index measures the uniformity of the species abundance distribution in the functional space, and is obtained by measuring the uniformity of the branch length of the minimum spanning tree in the trait space. A lower index implies gaps in the distribution of species in functional space, and a higher index implies an even distribution (Villéger et al., 2008). 5. Functional divergence (FDiv) is an index based on multiple traits, species abundance, and community level. This index explains the divergence of species abundance in the functional space. First, the centroid is determined by the divergent species in the functional space (abundance is not considered at this time), and then the abundanceweighted average distance between each species and the centroid is calculated. A higher index means stronger complementarity and weaker competition among species in the functional space (Villéger et al., 2008).

6. Functional dispersion (FDis) is an index based on multiple traits, species abundance, and community level. This index explains the dispersal of species in the functional space. First, the centroid of species dispersal is calculated in the functional trait space of all species with species abundance as the weight, and then the average distance between each species and the centroid is calculated. A lower index means that the distribution of species in functional space is more concentrated, and a higher index means that the distribution is more scattered [START_REF] Laliberté | A distance-based framework for measuring functional diversity from multiple traits[END_REF]. 7. Functional specialization (FSpe) is an index based on multiple traits and species abundance. First, the center of gravity of the functional traits of all species in the functional space was calculated (abundance was not considered at this time), and the distance between each species and the center of gravity was the degree of functional specialization at the species level. The value obtained after the abundance weighted average is the functional specialization index at the community level. A lower index indicates a lower number or abundance of functionally specialized species in the community [START_REF] Bellwood | Functional versatility supports coral reef biodiversity[END_REF][START_REF] Mouchet | Functional diversity measures: an overview of their redundancy and their ability to discriminate community assembly rules[END_REF].

8. Functional redundancy (FRed), which is an index based on multiple traits, species abundance, and community level. The concept of functional redundancy is based on functional uniqueness (FUni), which is calculated by RaoQ/Gini Simpson, and the functional redundancy index is calculated by 1-FUni. A high redundancy index means that species in the community are similar in function and have high dominance [START_REF] Ricotta | Measuring the functional redundancy of biological communities: a quantitative guide[END_REF]. 9. Species-level functional vulnerability is an index based on multiple traits, species abundance, and species level. The index combines species functional uniqueness (distance to all other species in the community functional space; [START_REF] Mouillot | A functional approach reveals community responses to disturbances[END_REF] and taxonomic scarcity (relative abundance) to measure the vulnerability of a particular species in a given community [START_REF] Ricotta | Measuring the functional redundancy of biological communities: a quantitative guide[END_REF].

10. The species-level functional rarity index is an index based on multiple traits, species abundance, and species level. In fact, the species-level index is an extension of the functional vulnerability from the specified community at the local scale to the regional scale. On the one hand, the functional uniqueness of species in the community [START_REF] Mouillot | A functional approach reveals community responses to disturbances[END_REF] is extended to the functional uniqueness of the regional species library (the distance from similar species in the functional space of the regional species library to measure whether there are other redundant species in the species; [START_REF] Buisson | Toward a loss of functional diversity in stream fish assemblages under climate change[END_REF].

On the other hand, it extends the local scarcity of taxonomy to the distribution restriction (occurrence ratio or distribution range) of the regional scale [START_REF] Dapporto | Species richness, rarity and endemicity on Italian offshore islands: complementary signals from island-focused and species-focused analyses[END_REF]Dennis, 2008) (Violle et al., 2017).

Functional group

According to niche theory, each species occupies a specific ecological niche.

However, in the biome of the ecosystem, there are always one or more groups of species that have similar functions, use environmental resources in similar ways, and thus occupy similar ecological niches. This cluster is defined as a functional group [START_REF] Stanley | Macroevolution: pattern and process[END_REF][START_REF] Pla | Quantifying functional biodiversity[END_REF]. The division of functional groups is not based on the subjective judgment of species characteristics, but on the objective classification of the key functional traits that the research focuses on. It is more objective to apply cluster analysis methods to functional traits to divide functional groups [START_REF] Jiang | Ecological morphology and functional diversity of fishes in Erhai Lake[END_REF]. Each cluster obtained through the division of functional groups should have clear biological and ecological explanations, and the consistency of functional traits between groups should be high [START_REF] Xia | Fish functional groups in Huayang Lake Group and their response to seine culture[END_REF]. Reasonable use of functional group research methods is conducive to the study of ecological issues such as species responses to environmental disturbances, species redundancy, resistance to alien organisms, trophic level interactions, species coexistence rules, and community construction mechanisms [START_REF] Naeem | Ecosystem consequences of biodiversity loss: the evolution of a paradigm[END_REF][START_REF] Dumay | Functional groups of lagoon fish species in Languedoc Roussillon, southern France[END_REF][START_REF] Voigt | Using functional groups to investigate community response to environmental changes: two grassland case studies[END_REF][START_REF] Luck | Changes in bird functional diversity across multiple land uses: interpretations of functional redundancy depend on functional group identity[END_REF].

In fish communities, the application of life history strategy theory and the division of different life history strategies are examples of the application of functional group methods based on functional traits. Life history strategies are central concepts in ecology and evolutionary biology [START_REF] Marquet | On theory in ecology[END_REF]. Life history is the set of coadaptive functional traits that a species develops to maximize adaptation to a particular environment [START_REF] Stearns | The evolution of life histories[END_REF]. Life history theory provides a framework for understanding environmental changes based on species traits, which can help us understand how the environment affects the distribution and changes of communities [START_REF] Charnov | Evolutionary assembly rules for fish life histories[END_REF]. The simplest framework that can be thought of is to assume that there are two opposing strategies for describing the continuum on which species lie.

The most common continuum is the "r-K" selection model (Pianka, 1970). From rselection for species with short lifespan, fast growth, early maturity, small size, high fecundity and no parental care to K-selection for species with opposite traits. Another "slow-fast" continuum is more commonly used in mammals. On this continuum, organisms with high adult mortality evolve to be fast-growing, fecund, and short-lived, while organisms with low adult mortality evolve to be slow-growing, fecund, and longlived [START_REF] Harvey | Patterns of mortality and age at first reproduction in natural populations of mammals[END_REF].

Neither the 'r-K' framework nor the 'slow-fast' framework fully satisfactorily described the combination of demographic traits observed in fish. According to these frameworks, large fish are expected to be less fecund, a pattern that does not account for the presence of many large fish with high fecundity (e.g., the four major carps).

Thus, [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF] proposed that variability in fish demographics can be nicely summarized by a triangular continuum. The endpoints of this triangle describe three contrasting life-history strategies: opportunistic, periodic, and equilibrium. Under undesirable environmental conditions, those long-lived species that delay sexual maturation to have large size and high fecundity are defined as "periodic" species.

Species with low fecundity maximizes the survival rate of young fish through parental care and larger eggs correspond to "equilibrium" strategies. Fast-spreading species with small size, short lifespan, early maturity and low fertility (but high relative reproductive effort) are defined as "opportunistic" strategies. When the environment affecting early life survival is seasonal and has interannual variation, the fitness of cycle one strategy species is maximized. Due to their long lifespan, they can tolerate many years and reproduce in large numbers in favorable years. The equalitarian strategy improves juvenile fish survival through high levels of parental care, a life-history strategy that relies on stable and predictable habitats with low environmental change. Species of opportunistic strategies typically adapt to highly variable, unpredictable, disturbed and transient habitat conditions [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF]. In this process, environmental disturbance acts as a filter, and fish communities act as an environmental screen, making species with different life-history strategies (functional traits) respond completely differently to environmental changes.

Fish communities and environmental disturbances in lakes

Types of environmental disturbances

Disturbance is defined as an event that may affect the local ecosystem, biome or population structure, physical or chemical environment that alters the availability of material resources [START_REF] Glasby | Sampling to differentiate between pulse and press perturbations[END_REF][START_REF] Shade | Lake microbial communities are resilient after a whole-ecosystem disturbance[END_REF]. Play a key role in the construction of ecosystem structure [START_REF] Sousa | The role of disturbance in natural communities[END_REF][START_REF] White | The ecology of natural disturbance and patch dynamics[END_REF].

Disturbances can be divided into three categories according to their sources, disturbances caused by human factors, biological factors and environmental factors [START_REF] Mouillot | A functional approach reveals community responses to disturbances[END_REF]. Anthropogenic disturbance is generally due to direct human exploitation and exploitation of biological and environmental resources [START_REF] Mouillot | A functional approach reveals community responses to disturbances[END_REF].

Human activities have resulted in enormous impacts on Earth's geology, climate, and ecosystems [START_REF] Steffen | The Anthropocene: From global change to planetary stewardship[END_REF][START_REF] Waters | The Anthropocene is functionally and stratigraphically distinct from the Holocene[END_REF]. In fact, most ecosystems in the world, including freshwater ecosystems, are threatened by multiple anthropogenic disturbances [START_REF] Vörösmarty | Global threats to human water security and river biodiversity[END_REF][START_REF] Mccluney | Riverine macrosystems ecology: sensitivity, resistance, and resilience of whole river basins with human alterations[END_REF]Albert et al., 2020;[START_REF] Birk | Impacts of multiple stressors on freshwater biota across spatial scales and ecosystems[END_REF]. Lakes are no exception, as they are threatened by eutrophication, land-use change, intensive farming and overfishing, altered lake morphology and habitat loss, climate change, loss of connectivity, invasive species [START_REF] Dudgeon | Freshwater biodiversity: importance, threats, status and conservation challenges[END_REF][START_REF] Smol | Under the radar: long-term perspectives on ecological changes in lakes[END_REF][START_REF] García-Girón | Biotic interactions hold the key to understanding metacommunity organisation[END_REF]. Disturbance can have significant negative impacts on natural and managed lake ecosystems and fish communities [START_REF] Shade | Lake microbial communities are resilient after a whole-ecosystem disturbance[END_REF]. Therefore, studying the influence of various disturbances on lake ecology and fish communities and the internal relationship between various disturbances is an important prerequisite for exploring the role of environmental filtering in the process of lake fish community construction.

Rapid anthropogenic nutrient enrichment (nitrogen or phosphorus) and the resulting eutrophication have caused widespread environmental degradation in natural ecosystems around the world [START_REF] Smith | Eutrophication science: where do we go from here?[END_REF]. There is no doubt that eutrophication has dramatically altered fish communities around the world [START_REF] Jeppesen | The impact of nutrient state and lake depth on topdown control in the pelagic zone of lakes: a study of 466 lakes from the temperate zone to the arctic[END_REF] and is one of the major anthropogenic disturbances facing lake fish.

Increased nutrient levels in lakes lead to environmental changes that could potentially affect the composition, function, and dynamics of fish communities [START_REF] Jeppesen | The impact of nutrient state and lake depth on topdown control in the pelagic zone of lakes: a study of 466 lakes from the temperate zone to the arctic[END_REF]. Eutrophication leads to the transformation of clear lakes dominated by macrophytes to turbid lakes dominated by phytoplankton, with changes in both habitat and nutrient resources [START_REF] Scheffer | Alternative equilibria in shallow lakes[END_REF]. Excessive algal growth and bio-clogging lead to hypoxia at the bottom of the water column, leading to a decline in the habitat quality of benthic communities [START_REF] Jacobson | Disentangling the effects of a century of eutrophication and climate warming on freshwater lake fish assemblages[END_REF]. Eutrophication thus leads to the loss or degradation of habitats where fish feed, spawn or inhabit, such as gravel spawning grounds and vegetation habitats at the bottom of shallow lakes due to hypoxia [START_REF] Argillier | Development of a fish-based index to assess the eutrophication status of European lakes[END_REF]. Eutrophication favors the proliferation of toxic cyanobacteria, with detrimental consequences for biodiversity [START_REF] Smith | Eutrophication science: where do we go from here?[END_REF][START_REF] Jia | Unravelling fish community assembly in shallow lakes: Insights from functional and phylogenetic diversity[END_REF].

Changes in land use at the watershed scale directly affect water quality and coastal habitat quality, and affect important ecological processes such as hydrology, primary productivity, metabolism, and organic matter turnover [START_REF] Young | Effects of land use on stream metabolism and organic matter turnover[END_REF][START_REF] Clapcott | Quantifying relationships between land-use gradients and structural and functional indicators of stream ecological integrity[END_REF]. These changes can directly or indirectly affect fish communities, potentially causing detrimental effects [START_REF] Allan | Landscapes and riverscapes: the influence of land use on stream ecosystems[END_REF]. Changes in land use are often driven by human activities (Prieto-Benítez and Méndez), and transitions from forests or pastures to croplands or urban settlements strongly affect water quality and biodiversity [START_REF] Allan | Landscapes and riverscapes: the influence of land use on stream ecosystems[END_REF]. Land use development is known to reduce water quality and lead to eutrophication [START_REF] Chadwick | Urbanization affects stream ecosystem function by altering hydrology, chemistry, and biotic richness[END_REF]. Landscape changes such as agricultural production, urban development or deforestation can increase the specific conductivity of water bodies [START_REF] Cañedo-Argüelles | Salinisation of rivers: an urgent ecological issue[END_REF]. Land-water interactions greatly affect the health of shallow water systems, especially those located in watersheds dominated by agriculture. In such systems, runoff from intensively cultivated areas can lead to high nutrient and sediment loads in aquatic systems, which can alter their trophic status [START_REF] Scheffer | Alternative equilibria in shallow lakes[END_REF]van Nes, 2007). In addition to the impact on water quality, agriculture and deforestation cause sediment to enter the water column, often resulting in sedimentation, and this change in sediment leads to the reduction of fish species with specific habitat needs, especially benthic predators and spawning fish [START_REF] Roth | Landscape influences on stream biotic integrity assessed at multiple spatial scales[END_REF]. At the same time, high levels of urbanization affect sensitive fish, leading to fish communities dominated by tolerant fish (Diedel et al., 2010). Lake fishery has contributed to China's economic development and aquatic product supply. However, with the rapid development of lake fishery, China's lake fish protection and lake fishery are facing a series of problems. Most lake fisheries in China have unreasonable resource development and utilization, such as the lack of conventional resource dynamic analysis, unrestrained overfishing has destroyed lake aquatic biological resources, lake fish resources have been depleted, and fish community structures have been miniaturized [START_REF] Qian | Current status of fishery resources and environmental monitoring in Poyang Lake[END_REF][START_REF] Liao | Current status and changes of fishery resources in Dongting Lake[END_REF].

Unreasonable fish stocking and fishing management schemes, lack of reasonable assessment and planning of the fishery carrying capacity of the water ecosystem, lead to changes in the fish community and population structure, and at the same time led to changes in the structure of other aquatic organisms in the lake [START_REF] Zhang | Effects of fish stocking on lake ecosystems in china[END_REF]. In the high-density lake fishery with intensive and semi-intensive methods as the main methods, a large amount of feed and fertilizer input lead to serious eutrophication of the lake water and reduced transparency [START_REF] Cui | Fishery resources and environmental protection of lakes in the Yangtze River Basin[END_REF]. At the same time, the seine nets of lakes and reservoirs lead to ecological isolation, destroy the integrity of the water ecosystem, decrease biodiversity, destroy the habitat of many fish, and lead to a large decline in species [START_REF] Cui | Fishery resources and environmental protection of lakes in the Yangtze River Basin[END_REF].

The morphological characteristics of lakes, and the alteration of these characteristics by human disturbance, can affect the structure and composition of fish communities in lakes [START_REF] Schindler | Habitat coupling in lake ecosystems[END_REF]. Lake morphology plays an important role in shaping lake habitat complexity and the structure of fish communities. Lake depth can significantly alter the composition of lake fish communities [START_REF] Garcia | Correlations between type-indicator fish species and lake productivity in German lowland lakes[END_REF][START_REF] Søndergaard | Water Framework Directive: ecological classification of Danish lakes[END_REF]. Larger lake areas may imply more complex habitat types, so positive correlations between lake area and fish species richness have often been reported in studies [START_REF] Dodson | The relationship in lake communities between primary productivity and species richness[END_REF][START_REF] Mehner | Rapid recovery from eutrophication of a stratified lake by disruption of internal nutrient load[END_REF]. The complexity of lake shorelines is often associated with the diversity of fish habitats [START_REF] Winfield | Fish in the littoral zone: ecology, threats and management[END_REF].

In lakes around the world, lake shorelines are increasingly being developed and altered by humans [START_REF] Jennings | Cumulative effects of incremental shoreline habitat modification on fish assemblages in north temperate lakes[END_REF][START_REF] Radomski | Consequences of human lakeshore development on emergent and floating-leaf vegetation abundance[END_REF][START_REF] Beeton | Large freshwater lakes: present state, trends, and future[END_REF].

Conversion of nearshore habitats can adversely affect fish communities because they form part of the physical template in which lake ecosystems reside [START_REF] Jackson | What controls who is where in freshwater fish communities the roles of biotic, abiotic, and spatial factors[END_REF]Olden and Jackson 2001;Olden et al., 2001). At the same time, the shape of the lake is also related to the nutritional status of the lake. The water depth and area of the lake also mean the carrying and buffering capacity of the lake for external pollutants.

Compared with circular lakes, lakes with high perimeter-area ratio (higher shoreline development index) can receive relatively higher terrestrial material input [START_REF] Wetzel | Limnology: lake and river ecosystems[END_REF].

Eutrophication, land use change, intensive stocking and overfishing, changes in lake morphology and habitat loss are the main environmental disturbances faced by lakes in the middle reaches of the Yangtze River. And the other three environmental disturbances, as relatively primary human disturbances, will directly or indirectly cause lake eutrophication. Environmental disturbances such as climate change, biological invasions, and loss of lake connectivity can also affect lake fish communities. Fish community structure and diversity have received more attention in the context of climate change in recent years. Many studies have shown that lake fish are directly affected by temperature changes, mainly as an increase in fish abundance and species richness as temperature rises [START_REF] Abell | Freshwater ecoregions of the world: a new map of biogeographic units for freshwater biodiversity conservation[END_REF][START_REF] Jeppesen | Impacts of climate warming on lake fish community structure and potential effects on ecosystem function[END_REF]. The invasion of alien species caused by fishing activities also has a very serious impact on the lake ecosystem [START_REF] Wang | Countermeasures for aquatic invasive alien species in China[END_REF]. Alien species often have the advantages of rapid growth, strong stress resistance, wide range of food, and high reproduction rate. The invasion of alien species has changed the structure of lake fish communities, making characteristic indigenous fish gradually become endangered fish [START_REF] Xie | Threats to biodiversity in Chinese inland waters[END_REF].

Landscape connectivity facilitates the movement of organisms and abiotic flows between sites, so biodiversity change and ecosystem dynamics are affected by landscape connectivity [START_REF] Mitchell | Linking landscape connectivity and ecosystem service provision: current knowledge and research gaps[END_REF][START_REF] Heino | The importance of metacommunity ecology for environmental assessment research in the freshwater realm[END_REF]. Therefore, maintaining natural hydrological connectivity is important for the conservation of fish and other organisms [START_REF] Saunders | Freshwater protected areas: strategies for conservation[END_REF][START_REF] Dudgeon | Freshwater biodiversity: importance, threats, status and conservation challenges[END_REF]. From the 1950s to the 1970s, most lakes in the floodplain of the Yangtze River were cut off from the Yangtze River through dams or sluices, and few lakes are currently connected [START_REF] Liu | Estimation of minimum area requirement of riverconnected lakes for fish diversity conservation in the Yangtze River floodplain[END_REF].

Environmental filtering

Key processes that influence community construction include interbiotic interactions such as interspecific competition and predation [START_REF] M'closkey | Niche separation and assembly in four species of Sonoran Desert rodents[END_REF][START_REF] Connor | The assembly of species communities: chance or competition?[END_REF], environmental filtering (Heino, 2013a;[START_REF] Kraft | Community assembly, coexistence and the environmental filtering metaphor[END_REF] and historical effects such as diffusion limitations due to physical barriers (Dias et al., 2014). Spatial scale is an important factor affecting fish community structure, so the effects of physical barriers may not dominate at local scales. Inter-species interactions in biological communities mainly depend on the differences in functional traits of species caused by phylogenetic differences. Environmental filtering emphasizes the relationship between organisms and the environment, highlighting the selection power of the environment (abiotic process), but at the same time it is based on the functional characteristics of different species in the species pool and the differences in environmental adaptation. Although the relative importance of different driving forces in influencing biome construction is difficult to draw conclusions [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF]. But in the context of the current global change, abiotic environmental changes undoubtedly play a central role in shaping the structure and distribution of biological communities. By focusing more on the selective effects of environmental change on organisms, ecologists can gain a clearer understanding of the many ways in which environmental change shapes community diversity patterns. Therefore, the role of environmental filtering on biological communities is undoubtedly the key to explaining the mechanism of fish community construction [START_REF] Kraft | Community assembly, coexistence and the environmental filtering metaphor[END_REF].

The concept of environmental filtering arose from studies of plant community assembly and dynamics in the late 1970s and early 1980s (Nobel and Slatyer 1977;[START_REF] Van Der Valk | Succession in wetlands -a Gleasonian approach[END_REF][START_REF] Bazzaz | Habitat selection in plants[END_REF][START_REF] Woodward | Functional approaches to predicting the ecological effects of global change[END_REF]. These studies liken environmental condition descriptions to "sieves" or "filters" that only allow species with specific characteristics or traits to establish populations and survive, while excluding all other species. This concept has been frequently used since it was proposed, and has played an important role in many studies of community assembling, succession, invasion biology, and biogeography [START_REF] Weiher | Community assembly rules, morphological dispersion, and the coexistence of plant species[END_REF][START_REF] Webb | Exploring the phylogenetic structure of ecological communities: an example for rain forest trees[END_REF][START_REF] Cornwell | A trait-based test for habitat filtering: convex hull volume[END_REF][START_REF] Whitfeld | Change in community phylogenetic structure during tropical forest succession: evidence from New Guinea[END_REF]. This concept has been widely used in subsequent research, and a series of terms with similar but not identical meanings have emerged, such as "environmental filtering ", "abiotic filtering ", "habitat filtering " and so on. [START_REF] Maire | Habitat filtering and niche differentiation jointly explain species relative abundance within grassland communities along fertility and disturbance gradients[END_REF] differentiate and explain these terms, while arguing that 'habitat filtering ' seems more appropriate as an umbrella term covering both biotic and abiotic interactions. But [START_REF] Kraft | Community assembly, coexistence and the environmental filtering metaphor[END_REF] believe that "environmental filtering" can minimize ambiguity. This concept more purely emphasizes the focus on abiotic factors, and is closer to the original concept of this ecological process, while the interaction between organisms should be strictly separated and studied in other ecological processes.

Central to the concept of environmental filtering is a focus on the relationship between organisms and their environment, recognizing that not all organisms can successfully establish populations and survive under all abiotic conditions. From this perspective, the environment is seen as a selective force that weeds out species that cannot tolerate the environmental conditions of a particular site. If species that are able to survive in a particular environment share common functional and phenotypic characteristics, resulting in a common tolerance to abiotic environments, then there may be some functional and phenotypic similarities among species in the community that reflect this tolerance [START_REF] Kraft | Community assembly, coexistence and the environmental filtering metaphor[END_REF]. This logic has led to some of the most widely validated general predictions in environmental filtering studies, specifically, how species in the same environment perform on key ecological dimensions compared to a null expectation based on random sampling from species pools. Convergence of function and phenotype [START_REF] Weiher | Community assembly rules, morphological dispersion, and the coexistence of plant species[END_REF][START_REF] Cornwell | A trait-based test for habitat filtering: convex hull volume[END_REF][START_REF] Kraft | Functional traits and niche-based tree community assembly in an Amazonian forest[END_REF][START_REF] Swenson | Opposing assembly mechanisms in a Neotropical dry forest: implications for phylogenetic and functional community ecology[END_REF]. In specific studies this may manifest as the abundance of similar species in biomes [START_REF] Whittaker | Vegetation of the Siskiyou mountains, Oregon and California[END_REF], phylogenetic relatedness [START_REF] Webb | Phylogenies and community ecology[END_REF], concordance of functional traits increases with environmental gradients [START_REF] Cornwell | Community assembly and shifts in plant trait distributions across an environmental gradient in coastal California[END_REF][START_REF] Swenson | The biogeography and filtering of woody plant functional diversity in North and South America[END_REF]. These patterns are often interpreted as evidence of environmental filtering [START_REF] Kraft | Community assembly, coexistence and the environmental filtering metaphor[END_REF].

Responses of fish communities

Global spatial patterns of fish distribution and abundance are the result of thousands of years of evolution. On a global scale, the main moderators of these patterns are geographic barriers, which may limit species dispersal (Dias et al., 2014).

In addition, with the focus of the research scope, other environmental filters at different levels from regional to local scales also have an impact on fish community patterns. At the regional landscape scale, climate change [START_REF] Tonn | Climate change and fish communities: a conceptual framework[END_REF], flood and drought (Junk et al., 1989), landscape connectivity (Rolls et al., 2013), watershed vegetation cover (Casatti et al., 2006) etc. On a finer local or local scale, local lake characteristics and human disturbance factors are more important to the structure of fish communities. For example, eutrophication [START_REF] Lopes | Drivers of fish trophic guild composition in lakes of the Upper Paraná River floodplain[END_REF], land use change [START_REF] Effert-Fanta | Effects of riparian forest buffers and agricultural land use on macroinvertebrate and fish community structure[END_REF], lake morphology and limnological characteristics [START_REF] Matthews | Morphology, habitat use, and life history[END_REF], overfishing and irrational lake fisheries, etc. [START_REF] Liu | Fish diversity reduction and assemblage structure homogenization in lakes: A case study on unselective fishing in China[END_REF]. The above four environmental disturbances played a pivotal role in the environmental filtering in the lakes in the middle reaches of the Yangtze River. For the study of the response of fish communities to environmental filtering, ecologists have conducted a large number of studies on various aspects of fish communities for a variety of disturbance types. For example, traditional species diversity index, species composition, functional diversity index, functional trait structure, functional group, etc.

From the perspective of four main disturbances of eutrophication, land use change, lake morphology and high-density lake fisheries, this review gives examples of the research on the response of fish communities to environmental filtering at all levels.

Thus, the importance of environmental filtering in the process of fish community construction is summarized. For eutrophication: [START_REF] Menezes | Homogenization of fish assemblages in different lake depth strata at local and regional scales[END_REF] found that the degree of eutrophication in 53 lakes in Denmark would lead to a decrease in community species diversity, an increase in structural homogeneity, and an increase in the dominance of individual species. [START_REF] Ritterbusch | European fish-based assessment reveals high diversity of systems for determining ecological status of lakes[END_REF] found that the trophic functional structure of fish communities in European lakes changed significantly with eutrophication, such as the decrease of piscivorous perch (Perca fluviatilis) and the increase of omnivorous roach (Rutilus rutilus). For land use change: Britton et al. (2019) used the urbanization ratio along the shore of Lake Tanganyika to represent the intensity of human disturbance, combined with stable isotope methods to study the feeding habits of cichlids. [START_REF] Giacomazzo | Linking fisheries to land use: How anthropogenic inputs from the watershed shape fish habitat quality[END_REF] revealed in Lake St. Pierre, Canada that sediment deposition caused by land use change on the lakeshore led to a series of cascading effects of "water transparency reduction -submerged plant reduction -habitat loss of perch -biomass reduction of perch". [START_REF] Miyazono | Assemblage patterns of fish functional groups relative to habitat connectivity and conditions in floodplain lakes[END_REF] found that the density of fish with both equilibristic and opportunistic life histories increased with increasing turbidity in lakes in the Yazoo River floodplain, USA. For lake morphology, positive relationships between lake area and fish species richness have often been reported [START_REF] Dodson | The relationship in lake communities between primary productivity and species richness[END_REF][START_REF] Mehner | Rapid recovery from eutrophication of a stratified lake by disruption of internal nutrient load[END_REF], consistent with expectations for island habitats in general. Lake depth increases habitat heterogeneity within the lake, and depth acts in conjunction with other environmental disturbances on fish community structure. For example, [START_REF] Menezes | Homogenization of fish assemblages in different lake depth strata at local and regional scales[END_REF] found that eutrophication in deep lakes did not cause homogenization of fish community structure in different areas of the lake, but showed a trend of homogeneity in shallow lakes. For lake fisheries and fishing: Liu et al. ( 2022) compared the differences in fish communities between the fishing season and the non-fishing season of lakes in eastern China, and concluded that fishing would reduce species diversity and fish abundance. [START_REF] Lamothe | Evaluating functional diversity conservation for freshwater fishes resulting from terrestrial protected areas[END_REF] compared the functional diversity of fish communities in freshwater protected areas and lakes outside protected areas in Lake Ontario, Canada. It is concluded that the protected fish communities have higher functional richness, functional dispersion and functional divergence, and more functional rare species.

Assembling mechanism of fish communities

Overview of community assembling mechanism

In biomes, species are not scattered randomly, but gather together based on certain rules. The content of community assembly research is how different species in the community combine and coexist, and the mechanism of biodiversity formation and maintenance [START_REF] Niu | Neutral theory and niche theory of community construction[END_REF]. The study of community assembling mechanism has always been a core topic in the field of community ecology. Revealing the underlying mechanism of community species composition changes has broad practical significance while enriching and developing the theory of community ecology. In the context of current global changes, studying the mechanism of community assembling is helpful to explain and predict the formation mechanism and maintenance principle of biological communities under environmental and human disturbances, so as to contribute to the protection of rare species, management of biological communities, and restoration of ecosystems (Cavender -Bares et al., 2009;[START_REF] Mori | β-diversity, community assembly, and ecosystem functioning[END_REF]. Research theories on the mechanism of community assembling emerge in endlessly and are still under development, but there are currently two main theories to explain the mechanism of community assembling. One holds that deterministic factors such as niche differentiation among co-occurrence species dominate community assembling, namely the niche theory [START_REF] Chase | Towards a really unified theory for metacommunities[END_REF]. Another theory holds that dispersal and stochastic action are the key to the coexistence of species and the maintenance of biodiversity in the community, that is, the neutral theory [START_REF] Hubbell | The unified neutral theory of biodiversity and biogeography[END_REF]. With the development of community assembling mechanism research, ecologists believe that the integration of niche theory and neutral theory in the study of community assembling can provide a more comprehensive and reasonable explanation [START_REF] Tilman | Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly[END_REF][START_REF] Gravel | Reconciling niche and neutrality: the continuum hypothesis[END_REF][START_REF] Leibold | Coexistence of the niche and neutral perspectives in community ecology[END_REF].

Niche is a basic concept in ecological theory, which mainly refers to the position and demand of species in time, space, resources and the relationship with other species in natural ecosystems, as well as its impact on habitat [START_REF] Chase | Ecological niches: linking classical and contemporary approaches[END_REF]. The main claim of niche theory is that deterministic local processes play a decisive role in the mechanism of species coexistence and diversity maintenance in communities, and these local processes are related to species niches [START_REF] Chase | Towards a really unified theory for metacommunities[END_REF]. Based on the niche theory, community assembling is considered to be the process by which species in a regional species pool enter local communities through multi-layer environmental filtering and interactions between organisms, and the two provide opposing forces [START_REF] Keddy | Assembly and response rules: two goals for predictive community ecology[END_REF][START_REF] Zobel | The relative of species pools in determining plant species richness: an alternative explanation of species coexistence?[END_REF]. Specifically, environmental filtering selects species with similar functional traits and similar adaptability to local environmental conditions, that is, similar ecological niches, to form the same community, so that species exhibit assembling and dispersal or trait convergence [START_REF] Prinzing | The niche of higher plants: evidence for phylogenetic conservatism[END_REF]Weiher and Keddy, 2001). Organismal interactions lead to competitive exclusion between species with similar ecological niches, resulting in species segregation or trait divergence [START_REF] Weiher | Assembly rules, null models, and trait dispersion: new questions from old patterns[END_REF][START_REF] Webb | Phylogenies and community ecology[END_REF]. Niche theory clarifies that both environmental filtering and interbiotic interactions (such as interspecies competition and predator-prey relationships) play an important role in niche differentiation during community assembling [START_REF] Silvertown | Plant coexistence and the niche[END_REF].

The neutral theory holds that stochastic and diffusion processes play a decisive role in community assembling. [START_REF] Hubbell | The unified neutral theory of biodiversity and biogeography[END_REF] integrated ideas of dispersal and migration from the theory of island biogeography [START_REF] Macarthur | Limiting similarity convergence and divergence of coexisting species[END_REF] with ideas from molecular genetics where the randomness of allelic mutations leads to new speciation and neutral drift [START_REF] Kimura | Evolutionary rate at the molecular level[END_REF]. In terms of spatial scale, the neutral theory holds that regional processes play a decisive role in the assembling and maintenance of communities, such as the speciation and extinction of the global species pool and the migration and dispersal of species at the regional scale [START_REF] Zobel | The relative of species pools in determining plant species richness: an alternative explanation of species coexistence?[END_REF]. Neutral theory introduces randomness into the study of community assembling mechanisms, emphasizing the importance of secondary dispersal and speciation for the maintenance of diversity above the regional scale. At the same time, dispersal limitation is an important corollary of neutral theory, that is, due to the spatial effect of dispersal, community similarity will decrease with increasing geographical distance [START_REF] Alonso | The merits of neutral theory[END_REF][START_REF] Hubbell | The unified neutral theory of biodiversity and biogeography[END_REF][START_REF] Mcgill | A test of the unified neutral theory of biodiversity[END_REF]. Nonetheless, one of the basic assumptions of neutral theory is ecological equivalence, which ignores differences in species in terms of form and function. However, differences in these functional traits actually affect the species' ability to compete, reproduce, and migrate, thus failing to support the hypothesis of ecological equivalence [START_REF] Gilbert | Neutrality, niches, and dispersal in a temperate forest understory[END_REF][START_REF] Hubbell | Neutral theory and the evolution of ecological equivalence[END_REF][START_REF] Mcgill | Rebuilding community ecology from functional traits[END_REF].

The focus of debate between niche theory and neutral theory is mainly on which one plays a leading role in the process of community assembling, deterministic process or stochastic process. With the progress of research on the mechanism of community assembling, ecologists tend to think that the niche theory and neutral theory of community assembling are not absolutely opposite. The two describe the characteristics of communities from different aspects, and the compatibility and complementarity between these two theories can better reveal the mechanism of community assembling [START_REF] Tilman | Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly[END_REF][START_REF] Chase | Towards a really unified theory for metacommunities[END_REF][START_REF] Leibold | Coexistence of the niche and neutral perspectives in community ecology[END_REF]. [START_REF] Tilman | Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly[END_REF] proposed the stochastic niche theory, which introduced the role of neutral processes into the process of niche-dominated community assembling. [START_REF] Gravel | Reconciling niche and neutrality: the continuum hypothesis[END_REF] proposed the nicheneutral continuum hypothesis, integrating the niche and neutral theories. This hypothesis affirms that both competitive and stochastic processes can act on community assembly, and that their relative magnitudes determine the dynamics of the continuum. [START_REF] Adler | A niche for neutrality[END_REF] believed that the stochastic process in the neutral theory and the deterministic process in the niche theory are essentially special forms in the species coexistence model. [START_REF] Hérault | Reconciling niche and neutrality through the Emergent Group approach[END_REF] attempted to use the concept of resource groups to reconcile the contradictions arising from the integration of neutral theory and niche theory. All in all, the integration of niche theory and neutral theory is the general trend and final destination of research on community assembling mechanism [START_REF] Chai | Research progress on mechanism of plant community construction[END_REF][START_REF] Wennekes | The neutral-niche debate: a philosophical perspective[END_REF].

Environmental filtering and biological interactions

Based on the niche theory and neutral theory, community aggregation can be classified into three types according to different mechanisms of action. ( 1) Under the neutral effect, the species in the community are composed randomly, and there is no regular relationship between the coexistence patterns of species. ( 2) Under the interaction between organisms, the competitive exclusion of species in the community leads to niche separation and similarity constraints between coexisting species, manifested as segregation dispersal or trait divergence. (3) Under the effect of environmental filtering, only species adapted to a specific local environment can coexist, and species in the community show aggregation and distribution or trait convergence.

In niche theory, the relative importance of environmental filtering and interactions between organisms to community assembly has been widely discussed. The community assembly rule based on the niche theory considers that community assembly is a filtering process in which species in a regional species pool enter a local area through multi-layer environmental filtering and interactions between organisms [START_REF] Diamond | Ecology and evolution of communities[END_REF]. Environmental filtering and interactions between organisms together constitute the two opposing driving forces of community assembly [START_REF] Geheber | The effect of spatial scale on relative influences of assembly processes in temperate stream fish assemblages[END_REF].

If the distribution model of co-occurrence species in the community shows convergence of traits relative to the null model, then environmental filtering is the main driving force of community assembly; otherwise, if it shows divergence of traits, interactions between organisms play a dominant role [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF]. In addition, the relative importance of these two opposing driving forces in the assembly process of different biomes is different, and there is a scale correlation. For example, environmental filtering is the main driving force determining plant community assembly at regional scales, while interactions between organisms play a role at smaller scales [START_REF] Götzenberger | Ecological assembly rules in plant communities-approaches, patterns and prospects[END_REF]. However, in some animal community studies, the opposite conclusion was drawn, and the study found that the interaction between organisms played a decisive role in the process of animal community assembly [START_REF] Trisos | Unraveling the interplay of community assembly processes acting on multiple niche axes across spatial scales[END_REF]Fitzgerald et al., 2017a).

Regarding the research on the mechanism of fish community assembly, environmental filtering and interactions between organisms are considered to directly affect the functional traits of fish, thereby driving the formation of fish communities and maintenance of diversity. Therefore, the species-based functional trait method is considered by ecologists to be an important means to study the assembly mechanism of fish communities (Stuart-Smith et al., 2013;[START_REF] Troia | Functional strategies drive community assembly of stream fishes along environmental gradients and across spatial scales[END_REF][START_REF] Drew | Community assembly of coral reef fishes along the Melanesian biodiversity gradient[END_REF][START_REF] Mou-Tham | Assembly rules of fish communities in Tuamotu archipelago atoll lagoons: The case of Fangatau, a lagoon dominated by giant clam habitats[END_REF]. The functional traits of fish mainly include: habitat selection, feeding nutrition, life history strategy and locomotion behavior (Stuart-Smith et al., 2013;[START_REF] Buisson | Toward a loss of functional diversity in stream fish assemblages under climate change[END_REF][START_REF] Escobar-Toledo | Temporal shifts in functional traits of the fish community in Terminos Lagoon (Mexico) in three periods (1980, 1998 and 2011)[END_REF][START_REF] Fitzgerald | Using trophic structure to reveal patterns of trait-based community assembly across niche dimensions[END_REF][START_REF] Colin | Functional diversity measures revealed impacts of non-native species and habitat degradation on species-poor freshwater fish assemblages[END_REF]Peoples and Midway, 2018;Wilkinson et al., 2018). In general, environmental factors such as water quality, substrate, habitat structure, land use, hydrological characteristics, and climatic factors, as well as disturbances from human activities, have important effects on fish functional traits [START_REF] Teresa | Trait-based metrics as bioindicators: Responses of stream fish assemblages to a gradient of environmental degradation[END_REF][START_REF] Córdova-Tapia | The role of environmental filtering in the functional structure of fish communities in tropical wetlands[END_REF]. At the same time, fish functional traits are also affected by biological factors such as interspecific competition, predator-prey interactions, and food resources [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF][START_REF] Geheber | The effect of spatial scale on relative influences of assembly processes in temperate stream fish assemblages[END_REF][START_REF] Fitzgerald | Using trophic structure to reveal patterns of trait-based community assembly across niche dimensions[END_REF]. [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF] studied the assembly mechanism of fish communities in American rivers using species-based functional traits and found that 75%-85% of river fish communities are determined by non-stochastic patterns.

Among them, environmental filtering plays a decisive role, followed by predator-prey interactions (predator-prey interactions), while interspecific competition plays the least role. Fish community assembly mechanisms are also affected by temporal and spatial scales. Fitzgerald et al. (2017a) used fish functional traits and functional diversity to study the assembly of fish communities in the main tributaries of the Amazon Basin, and found that two key drivers of fish community assembly will change with seasonal changes. [START_REF] Geheber | The effect of spatial scale on relative influences of assembly processes in temperate stream fish assemblages[END_REF] found that environmental filtering plays a decisive role in the assembly process of river fish communities, and its role is enhanced with the expansion of the spatial scale. In China, research on the assembly mechanism of freshwater fish communities started relatively late. The vast majority of fish community studies still focus on species surveys, taxonomic diversity, and resource dynamic analysis, and there are few reports explaining the lake fish community assembly mechanism from the perspective of ecological functions.

Purpose and significance of this study

The middle reach of the Yangtze River is one of the most concentrated areas of lakes in China, and most of the lakes have been connected with the main stream or tributaries of the Yangtze River in history, forming a typical river-lake complex ecosystem and forming a unique and rich freshwater fish fauna. It is one of the most important freshwater fishery bases in China and even in the world. This area is also one of the areas in China where fish ecology research was first carried out and long-term historical data are accumulated most continuously and abundantly. However, in the past few decades, lakes in the middle reaches of the Yangtze River have been disturbed by factors such as water pollution, land use change, habitat fragmentation, and overstocking, and the composition and structure of fish communities have undergone significant changes. The main manifestations are the sharp decline in the number of species, the sharp decline in resources, and the miniaturization and younger age of fish.

Past studies have generally focused on the time-series changes in the taxonomic diversity of fish communities, but there is a serious lack of understanding of fish community assembly mechanisms and key driving factors in the process of lake ecological succession. As a result, there is a lack of theoretical basis for the utilization and protection of fish resources in lakes in the middle reaches of the Yangtze River, and various deviations have occurred in practice, which has caused major losses to China's aquatic resources and biodiversity.

In recent years, China has fully implemented the Yangtze River ecological environment protection and restoration strategy, and the lake recuperation system has been gradually improved. Since then, the lakes in the middle reaches of the Yangtze River have entered a new historical period of ecological restoration and succession.

Therefore, it provides a valuable historical opportunity and an ideal ecosystem for studying the mechanism of fish community construction in the succession process of lake ecological restoration. Clarifying the mechanism of fish community construction in the process of lake ecological restoration succession has important theoretical significance for the sustainable food production and biodiversity maintenance of lakes in the Yangtze River Basin, and can also provide technical support for the evaluation of the effect of watershed ecological restoration measures.

In this study, 26 shallow lakes in the middle reaches of the Yangtze River were taken as the research object, and the lake environmental disturbance data, fish community composition and functional trait data were obtained through field surveys and other methods. Key environmental factors were screened based on the interrelationships between environmental and human disturbance factors (eutrophication, land use, lake morphology, proliferation patterns) and their impact on fish communities. The characteristics of lake fish communities in the middle reaches of the Yangtze River were described from the dimensions of structural composition, species abundance, species diversity, and functional diversity. At the same time, we will further study the response mechanism of fish communities to environmental factors, and predict fish communities through environmental factors. After revealing the relationship between fish communities and environmental factors, the role of environmental filtering and interbiotic interactions (interspecies competition and predator-prey interactions) during fish community assembly in lakes in the middle reaches of the Yangtze River were quantified, respectively. At the same time, it is analyzed which role plays a driving role in the assembly of fish communities, so as to reveal the assembly mechanism of fish communities under multiple environmental pressures in lakes in the middle reaches of the Yangtze River. The flow chart of present study is shown in Figure 1-1. In this study, the lakes in the middle reaches of the Yangtze River were studied according to the idea of "environmental disturbance factors-fish community structure-community assembly mechanism", and the following scientific questions were planned to be answered. [START_REF] Feng | Eutrophication induces functional homogenization and traits filtering in Chinese lacustrine fish communities[END_REF] What is the relationship between different environmental factors and which factors have a key impact on fish communities? ( 2) What are the structural characteristics and response mechanisms of fish communities under the disturbance gradient of different environmental factors? (3) Can multiple environmental factors predict changes in fish community structure, thereby serving lake ecosystem management? (4) Among environmental filtering, interspecific competition, and predator-prey interactions, which process plays a major driving role in lake fish community assembly?

Chapter2 Spatial patterns and key factors of multiple environmental pressures 2.1 Introduction

The Yangtze River Basin is one of the regions with the most concentrated lakes in China [START_REF] Wang | Fisheries and wetland protection in the Yangtze River Basin[END_REF]. Thousands of lakes are distributed in the Yangtze River Basin, with a total area of 1.5210 4 km 2 , which is close to 1/5 of the total lake area in China [START_REF] Wang | Predictive limnology research on small and medium-sized lakes in the middle and lower reaches of the Yangtze River[END_REF]. Most of the lakes in the middle reaches of the Yangtze River were originally connected to the river, that is, they were closely connected with the Yangtze River and naturally connected with each other, forming a unique and complete riverlake complex ecosystem with the main stream and tributaries of the Yangtze River [START_REF] Chang | Significance of fishery in river-connected lakes and resource management strategies. Resources and Environment in The Yangtze Basin[END_REF]. But today, most of the lakes have lost their connection with the Yangtze River due to the construction of water conservancy and other reasons since the 1950s [START_REF] Wang | The ecological impact of river and lake barriers on fish resources in the Zhangdu Lake area. Resources and Environment in The Yangtze Basin[END_REF][START_REF] Hu | Fish species diversity and its temporal and spatial changes in West Dongting Lake. Resources and Environment in The Yangtze Basin[END_REF]. Factors such as the reclamation of lakes and the acceleration of natural sedimentation induced by the loss of lake connectivity further lead to the shrinkage or even disappearance of the lake water surface [START_REF] Wang | The ecological impact of river and lake barriers on fish resources in the Zhangdu Lake area. Resources and Environment in The Yangtze Basin[END_REF][START_REF] Zeng | Fishery resources in the Yangtze River system[END_REF]). As a result, it has a profound impact on the structure and dynamics of the population or community of fish and other aquatic organisms [START_REF] Xie | Threats to biodiversity in Chinese inland waters[END_REF][START_REF] Li | Biodiversity and protection of important fish species in the Yangtze River[END_REF]. Fish species in many lakes have declined by as much as 50%, while wild fish production has declined by as much as 80% [START_REF] Wang | The ecological impact of river and lake barriers on fish resources in the Zhangdu Lake area. Resources and Environment in The Yangtze Basin[END_REF][START_REF] Jin | Comprehensive research on Jianghan Lake Group[END_REF][START_REF] Hu | Structure and dynamics of fish species diversity in lakes in the middle reaches of the Yangtze River[END_REF]. For the lakes in the Yangtze River Basin, at different times, the increasing dam barrier and reclamation gradually fragmented the river-lake complex ecosystem that was originally interconnected with the Yangtze River. Among the different factors affecting the survival of wild fish, river and lake barriers are the most damaging, but the impact of overfishing cannot be ignored [START_REF] Hu | Structure and dynamics of fish species diversity in lakes in the middle reaches of the Yangtze River[END_REF]. At the same time, in disorderly purse seine farming activities, the super-density, large-area, and high-input farming model has caused the loss of fish habitats in lakes and exacerbated the eutrophication of lakes, which has also affected lake ecosystems, especially fish habitats. caused serious damage. In general, in the past few decades, the lakes in the middle reaches of the Yangtze River have been disturbed by factors such as river and lake isolation, agricultural expansion, overstocking, habitat fragmentation, and water pollution. Significant changes have taken place in the composition and structure of fish communities, mainly manifested in the sharp decline in the number of species, the sharp decline in resources, and the miniaturization and younger age of fish.

Environmental pressures affecting fish communities vary across spatial scales [START_REF] Jackson | What controls who is where in freshwater fish communities the roles of biotic, abiotic, and spatial factors[END_REF]. At the global scale, the main moderators of these patterns are geographic barriers (Dias et al., 2014). With the focus of the research scope, at the regional landscape scale, climate change [START_REF] Tonn | Climate change and fish communities: a conceptual framework[END_REF], flood and drought (Junk et al., 1989), landscape connectivity (Rolls et al., 2013), watershed vegetation cover (Casatti et al., 2006), etc. On a finer local or local scale, local lake characteristics and human disturbance factors are more important for the structure of fish communities, such as eutrophication [START_REF] Lopes | Drivers of fish trophic guild composition in lakes of the Upper Paraná River floodplain[END_REF], land use change [START_REF] Effert-Fanta | Effects of riparian forest buffers and agricultural land use on macroinvertebrate and fish community structure[END_REF], lake morphology and limnological characteristics [START_REF] Matthews | Morphology, habitat use, and life history[END_REF], overfishing and unreasonable lake fisheries, etc. [START_REF] Liu | Fish diversity reduction and assemblage structure homogenization in lakes: A case study on unselective fishing in China[END_REF]. For lakes in the middle reaches of the Yangtze River, although the loss of landscape connectivity has historically had a significant impact on fish communities, there are few lakes with high connectivity under the premise that river-lake barriers are prevalent in lakes at present. Therefore, eutrophication, land use, lake morphology, and fishery proliferation patterns play a major role in environmental filtering of fish communities in lakes in the middle reaches of the Yangtze River. There is no doubt that eutrophication has dramatically altered fish communities around the world [START_REF] Jeppesen | The impact of nutrient state and lake depth on topdown control in the pelagic zone of lakes: a study of 466 lakes from the temperate zone to the arctic[END_REF] and is one of the major anthropogenic disturbances facing lake fish. Increased nutrient levels in lakes lead to environmental changes that could potentially affect the composition, function, and dynamics of fish communities [START_REF] Jeppesen | The impact of nutrient state and lake depth on topdown control in the pelagic zone of lakes: a study of 466 lakes from the temperate zone to the arctic[END_REF]. Changes in land use at the watershed scale directly affect water quality and coastal habitat quality, and affect important ecological processes such as hydrology, primary productivity, metabolism, and organic matter turnover [START_REF] Young | Effects of land use on stream metabolism and organic matter turnover[END_REF][START_REF] Clapcott | Quantifying relationships between land-use gradients and structural and functional indicators of stream ecological integrity[END_REF]. These changes can directly or indirectly affect fish communities, potentially causing detrimental effects [START_REF] Allan | Landscapes and riverscapes: the influence of land use on stream ecosystems[END_REF]. Land use development is known to reduce water quality and lead to eutrophication [START_REF] Chadwick | Urbanization affects stream ecosystem function by altering hydrology, chemistry, and biotic richness[END_REF]. Landscape changes such as agricultural production, urban development or deforestation can increase the specific conductivity of water bodies [START_REF] Cañedo-Argüelles | Salinisation of rivers: an urgent ecological issue[END_REF]. Land-water interactions greatly affect the health of shallow water systems, especially those located in watersheds dominated by agriculture. In such systems, runoff from intensively cultivated areas can lead to high nutrient and sediment loads in aquatic systems, which can alter their trophic status [START_REF] Scheffer | Alternative equilibria in shallow lakes[END_REF]van Nes, 2007).

Morphological characteristics characteristic of lakes, and alterations of these characteristics by human disturbance, can affect the structure and composition of fish communities in lakes [START_REF] Schindler | Habitat coupling in lake ecosystems[END_REF]. Lake morphology plays an important role in shaping lake habitat complexity and the structure of fish communities.

At the same time, the shape of the lake is also related to the nutritional status of the lake.

The water depth and area of the lake also mean the carrying and buffering capacity of the lake for external pollutants. Compared with circular lakes, lakes with high perimeter-area ratio (higher shoreline development index) can receive relatively higher terrestrial material input [START_REF] Wetzel | Limnology: lake and river ecosystems[END_REF]. Unreasonable resource development and utilization exist in most lake fisheries in China. Unrestrained overfishing has destroyed lake aquatic biological resources, depleted lake fish resources, and miniaturized fish community structure [START_REF] Qian | Current status of fishery resources and environmental monitoring in Poyang Lake[END_REF][START_REF] Liao | Current status and changes of fishery resources in Dongting Lake[END_REF]. In the high-density lake fishery with intensive and semi-intensive methods as the main methods, large inputs of feed and fertilizers lead to serious eutrophication of lake water and reduced transparency [START_REF] Cui | Fishery resources and environmental protection of lakes in the Yangtze River Basin[END_REF].

The above four environmental pressures directly or indirectly affect fish communities from different aspects, and are the main environmental disturbances faced by lakes in the middle reaches of the Yangtze River. Moreover, based on the relationship between different environmental disturbances, it is reasonable to assume that the impacts of the other three environmental stresses on fish communities may include indirect components caused by changing the eutrophication status of lakes. That is, eutrophication is a secondary disturbance among the four environmental pressures, and it may be a combination of the combined effects of multiple disturbances, which can better reflect the structural impact and filtering effect of environmental pressure on fish communities.

To sum up, the purpose of the research in this chapter is: [START_REF] Feng | Eutrophication induces functional homogenization and traits filtering in Chinese lacustrine fish communities[END_REF] To describe the spatial pattern of the four environmental pressures in the middle reaches of the Yangtze River, and to grasp the gradient characteristics of multiple environmental pressures. [START_REF] Feng | Application of artificial spawning substrates to support lacustrine fish recruitment and fisheries enhancement in a Chinese lake[END_REF] Analyze the correlation between various environmental factors, and based on this relationship, identify the key environmental factors that affect fish communities, so that follow-up research can reveal the filtering process of environmental pressure on fish communities in a more targeted manner.

Materials and methods

Study area and sample setting

In this study, 26 shallow lakes in the lake group in the middle reaches of the Yangtze River were selected as the research objects (Figure 2-1, Table 2-1). The lakes range in size from 2.30 km 2 to 274.60 km 2 . Most of these lakes are shallow, with a maximum water depth between 0.8 m and 5.7 m. These lakes have different gradients of trophic levels (total phosphorus concentration 0.01-0.77 mg L -1 ). The land use in this region is mainly agricultural, and different lake basins have experienced different degrees of agricultural expansion [START_REF] Dong | Carbon burial by shallow lakes on the Y angtze floodplain and its relevance to regional carbon sequestration[END_REF] and lake fishery development [START_REF] Wang | Paradigm changes in freshwater aquaculture practices in China: Moving towards achieving environmental integrity and sustainability[END_REF]. The fish community survey was conducted from March 2019 to May 2019, and three sampling points were set up in each lake. The water quality survey time is from October to November 2019, and the sampling points are at the same location as the fish community sampling points.

Sample and data collection

The data in this study are divided into two parts: environmental data and fish community data. The environmental data include eutrophication indicators, land use ratio, lake morphology data and lake fishery proliferation model data. Fish community data include species composition, abundance and biomass, and fish functional trait values.

Environmental data

Eutrophication index includes 12 indicators related to water quality and environment, all of which were obtained through field investigation. For each sampling site, 12 water quality variables related to water physicochemistry and eutrophication were measured: Secchi depth (SD, m), dissolved oxygen (DO, mg L -1 ), conductivity (Cond, μS cm -1 ), pH, turbidity (Turb, NTU), total phosphorus (TP, mg L -1 ), total nitrogen (TN, mg L -1 ), chlorophyll a (Chla, μg L -1 ), chemical oxygen demand (CODMn, mg L -1 ), ammonia nitrogen (NH4 + -N, mg L -1 ), nitrite nitrogen (NO2 --N, mg L -1 ) and nitrate nitrogen (NO3 --N, mg L -1 ). SD was measured using Secchi disc. DO, pH, Cond and Turb were measured on site using multi-parameter water quality meter (YSI ProDSS, Xylem Analytics). The pooled water samples (0.5 m below the surface, middle, and 0.5 m above the bottom) were taken with a modified 1-L Van Dorn water sampler at all sampling sites. TP and TN concentrations were measured with ascorbic acid method and cadmium reduction method, respectively, following persulfate digestion. Land use/cover types are grouped into five main groups: forest, grassland, agriculture, pond, and urban. However, in data acquisition, land use/cover types are only expressed as the total area of each cover type, without considering their spatial configuration, nor the possibility of differences in the role of land use/cover types in watershed areas.

Lake morphology variables include lake area (km 2 ), shoreline length (m), watershed area (km 2 ), maximum water depth (WD, m) and shoreline development index (SDI). The maximum water depth is the maximum value actually measured in the field sampling process using a hand-held depth sounder. Lake area and lake shoreline length were obtained from satellite maps during the study period (2019).

Watershed areas were obtained with the GIS program ArcGIS (version 10.7). The lake shoreline development index is calculated from the lake area and the length of the lake shoreline. The minimum value is 1, which means the lake is a regular circle. The larger the value, the more complex the lake shoreline [START_REF] Pan | Fractal characteristics of lake shorelines and their ecological significance[END_REF].

The lake fishery proliferation model data include the annual yield per unit water area (yield, kg hm-2), the annual stocking amount per unit water area (stocking, kg hm- 2) and the input level of feed and fertilizer (input). The lake fishery multiplication model data are all obtained by questionnaire survey mode, and for lakes with management entities, they are obtained by investigating the production data of the management entities. For lakes where there are only scattered fishing by fishermen, the yield is obtained by investigating the annual yield of each fisherman's single boat and the number of fishing boats, and the stocking amount is obtained by investigating the recorded data of the lake enhancement and release department. Feed and fertilizer input levels are also obtained through surveys, which are divided into five levels (0-4) from low to high. 0 means no feed and fertilizer input at all, 1 means only natural feed such as wheat is used, 2 means only one of commercial compound feed or fertilizer is used, 3 means fertilizer is used with natural feed such as wheat, and 4 means a large amount of commercial compound feed and fertilizer is used at the same time. The survey was conducted in 2019, but the actual year for the relevant data is determined by when the lake's fishery ceased. For the lakes where the fishery has been banned, the relevant data of the last year before the ban was investigated, and for the lakes where the lake fishery still existed, the relevant data of the latest year with complete records were investigated.

Fish data

A multi-mesh composite gill net of 30 m in length and 2.0 m in height (modified from European standard methods, CEN, 2015) was used in fish sampling. The composite gill net was composed of 12 panels (2.5 x 2 m) with respective mesh sizes of 5, 6.25, 8, 10, 12.5, 15.5, 19.5, 24, 30, 35, 43 and 55 mm. Mesh sizes are randomly ordered within a net, but all gillnets have the same order. A benthic fyke net (15 m in length, 0.4 m in width and 0.3 m in height) with 5 mm mesh size and 20 entrances on each side was also used. The relative abundance of fish during each sampling period at each study site was determined using 4 pelagic multi-mesh gill nets, 4 benthic multi-mesh gill nets and 4 benthic fyke nets. The nets were exposed for 12h (18:00 to 6:00 the following day).

Catches were registered as number of individuals. The total wet weights of all species sampled were obtained from fresh specimens using an electronic balance (precision 0.01 g). All collected fish specimens (N = 73056) were identified to species level according to Department of Ichthyology, Institute of Hydrobiology, CAS (1976). The scientific binomials of the 53 species collected (Table 1) were updated according to

FishBase (Froese and Pauly, 2020). In this study, the functional traits of 22 species were selected for the study of functional diversity. The functional traits belong to the three major functional categories of fish, namely life history strategy, feeding and habitat use (

Data analysis

The data analysis of this study is divided into three parts. ( 1 with the results of principal component analysis to describe the multivariate feature gradient pattern of each type of environmental factor in the lake, and define the characteristics of cluster groups according to the actual situation. Combined with the results of PCA, multivariate analysis of variance was performed between each group of lakes to test whether the differences between different cluster groups were significant.

The levels of each individual indicator are then described according to cluster groups.

Before cluster analysis and principal component analysis, all data were transformed to better remove the skewed distribution of data. Among them, the land use percentage data is transformed by square root arcsine, the pH and rank order data are not transformed, and other data are transformed by ln(n+1). Hierarchical clustering was analyzed using the "hcluster" function in the "stats" package (R Core Team, 2022), and principal component analysis was analyzed using the "PCA" function in the "FactoMineR" package [START_REF] Lê | FactoMineR: an R package for multivariate analysis[END_REF].

Correlation of environmental factors

According to the hypothesis of this study, the degree of eutrophication of lakes will be affected by three types of environmental factors: land use, lake morphology and lake proliferation mode, that is, eutrophication belongs to secondary environmental disturbances, while the other three types belong to primary environmental disturbances.

In order to verify this conjecture and analyze the correlation between environmental factors, this study uses the Pearson correlation coefficient to analyze the correlation between each index of eutrophication and each index of the other three environmental factors. And the p value was used to test the significance of the correlation. Correlation analysis and testing were performed using the "corr.test" function of the "psych" package [START_REF] Revelle | psych: Procedures for personality and psychological research[END_REF].

Fish community metrics

In this study, the structural characteristics of fish communities were measured from the perspectives of species abundance, species diversity and functional diversity, so as to explore the correlation between environmental factors and fish community indicators. Species abundance is expressed in terms of harvested biomass and quantity per unit effort. For each sample, biomass per unit effort (BPUE, g net -1 h -1 ) and quantity per unit effort (NPUE, ind. net -1 h -1 ) were calculated from the catch of each net. Species diversity is represented by species richness and the Shannon-Wiener diversity index [START_REF] Shannon | The mathematical theory of communication[END_REF]Shannon and Wiener, 1949). Functional diversity is expressed in terms of functional richness (FRic;Villéger et al., 2008) and functional dispersion (functional dispersion, FDis; [START_REF] Laliberté | A distance-based framework for measuring functional diversity from multiple traits[END_REF]. All metrics are calculated from the data at each sampling point and then averaged across each lake to represent the level of the lake. Species diversity was calculated using the function "diversity" of the R package "vegan" [START_REF] Oksanen | Community Ecology Package (R Package Version 2.6-2)[END_REF], and functional diversity was calculated using the function "dpFD" of the R package "FD" [START_REF] Laliberté | Measuring functional diversity from multiple traits, and other tools for functional ecology[END_REF].

Correlation between environmental factors and fish community metrics

In order to identify the environmental factors that have a key impact on fish community structure, this study preliminarily identified which environmental factors play a key role in the environmental filtering process of fish community through the correlation between four types of environmental factors and fish community metrics.

In this study, the Pearson correlation coefficient was used to analyze the correlation between fish community indicators and the indicators of the four types of environmental factors, and the p value was used to test the significance of the correlation. The specific method is the same as 2.2.3.3.

Results

Spatial pattern of environmental factors

In this study, cluster analysis and principal component analysis were performed on the four environmental factors of eutrophication, land use, lake morphology, and proliferation mode of all lakes, and the individual index levels were displayed. The gradient characteristics of four types of environmental factors in lakes in the middle reaches of the Yangtze River were revealed, and the spatial pattern of environmental factors was described.

Eutrophication

Through the cluster analysis of 12 eutrophication and water physical and chemical characteristics indicators of 26 lakes, all lakes were clustered into four groups (Group1-Group4) in the results. Among them, Group1 includes 3 lakes: Chonghu Lake, Wuhu Lake and Niushan Lake. Group2 includes 10 lakes including Wangmu Lake, Yanzhi Lake, Changhu Lake, Baoan Lake, Zhupo Lake, Yanzhi Lake, Huama Lake, Liangzi Lake, Huanggai Lake and Luhu Lake. Group3 includes 9 lakes including Futou Lake, Xiliang Lake, Chidong Lake, Taibai Lake, Niulang Lake, Honghu Lake, Cehu Lake, Yuni Lake and Shangjin Lake. Group4 includes four lakes: Longsai Lake, Jingyue Lake, Combining the results of cluster analysis and principal component analysis, the four groups of lakes were separated on the gradient of the first principal component, and the results of multivariate analysis of variance among groups were significant (p<0.05). This indicated that the lakes of Group1-Group4 showed an increasing trend of eutrophication. On the second principal component axis, however, there is little difference, suggesting that the second principal component cannot describe the differences among lakes. Therefore, through multivariate analysis, it can be concluded that 26 lakes have obvious eutrophication gradient.

The eutrophication gradient among the lakes can be more intuitively displayed by the difference between the four clusters of a single index. For the main eutrophication indicators (total nitrogen, total phosphorus, chlorophyll a, permanganate index, turbidity), all showed an increasing trend from Group1-Group4. SD shows a decreasing trend from Group1-Group4. And the index span between lakes is relatively large

Land use

Through the cluster analysis of 5 land use types of 26 lakes, all lakes were clustered into three groups (Group1-Group3) in the results. Among them, Group1 includes 8 lakes: Taibai Lake, Chidong Lake, Huama Lake, Liangzi Lake, Futou Lake, Xiliang Lake, Huanggai Lake, and Zhupo Lake. Group2 includes 8 lakes including Yanzhi Lake, Wangmu Lake, Yanzhi Lake, Yuni Lake, Shangjin Lake, Niulang Lake, Longsai Lake and Changhu Lake. Group3 includes 10 lakes including Wangjiada Lake, Chonghu Lake, Honghu Lake, Wuhu Lake, Nanhai Lake, Baoan Lake, Luhu Lake, Jingyue Lake, Niushan Lake and Cehu Lake. The results are shown in Figure 23456. Combining the results of cluster analysis and principal component analysis, the three groups of lakes were separated on the gradient of the first principal component, and the results of multivariate analysis of variance among groups were significant (p<0.05). This shows that the lakes of Group1-Group3 show an increasing trend of agriculturalization, while natural vegetation (forest and natural grassland) tends to decrease. On the second principal component axis, however, there was little difference, suggesting that the land use for pond culture between lake catchments of each group could not describe the differences among lakes. Therefore, through multivariate analysis, it can be concluded that 26 lakes have obvious agricultural expansion gradients.

The differences between the three clusters for a single indicator can more intuitively show the land use gradient among the lakes. From the perspective of the overall land use situation, the land use form in the lake basin of the middle reaches of the Yangtze River is dominated by agriculture, and the proportion of agricultural land shows an increasing trend from Group1-Group3. The proportion of forest showed a decreasing trend from Group1-Group3. And the index span between lakes is relatively large (compared with Group1 and Group3), the results are shown in Figure 2-8. Among other land use types, natural grassland has little difference between Group1 and Group2, but Group3 is significantly lower than the other two groups. Between-group differences were not significant for urban and pond.

Lake morphology

Through the cluster analysis of 5 lake morphological parameters of 26 lakes, all lakes were clustered into three categories (Group1-Group3) in the results. Among them, Group1 includes 11 lakes, including Yanzhi Lake, Wangjiada Lake, Wangmu Lake, Chonghu Lake, Nanhai Lake, Longsai Lake, Jingyue Lake, Cehu Lake, Huama Lake, Zhupo Lake and Niulang Lake. Group2 includes 9 lakes: Shangjin Lake, Chidong Lake, Yuni Lake, Taibai Lake, Wuhu Lake, Yezhu Lake, Niushan Lake, Luhu Lake and Baoan Lake. Group3 includes six lakes: Liangzi Lake, Honghu Lake, Futou Lake, Chang Lake, Combining the results of cluster analysis and principal component analysis, the three groups of lakes were separated on the gradient of the first principal component, and the results of multivariate analysis of variance among groups were significant (p<0.05). This shows that the lakes of Group1-Group3 show a trend of increasing lake area and lake shoreline complexity. However, there is little difference on the second principal component axis, which indicates that the water depth of lakes in each group has little difference. Therefore, through multivariate analysis, it can be concluded that the 26 lakes have obvious gradients in lake size and habitat complexity.

The difference between the three clusters of a single index can more intuitively show the lake morphological gradient existing among the lakes. Parameters related to lake volume (lake area and watershed area) and shoreline complexity (lake shoreline length and shoreline development index) showed an increasing trend from Group1-Group3. The results are shown in Figure 2-11. There was no significant difference between the groups in the maximum water depth of the lake.

Fishery proliferation model

Through the cluster analysis of 3 items of data related to the lake fishery proliferation model of 26 lakes, all lakes were clustered into three categories (Group1-Group3) in the results. Among them, Group1 includes 7 lakes: Huanggai Lake, Liangzi Lake, Xiliang Lake, Chonghu Lake, Niushan Lake, Futou Lake and Honghu Lake.

Group2 includes 10 lakes including Baoan Lake, Wangmu Lake, Yezhu Lake, Huama Lake, Yanzhi Lake, Wuhu Lake, Niulang Lake, Jingyue Lake, Luhu Lake and Shangjin Lake. Group3 includes 9 lakes including Nanhai Lake, Wangjiada Lake, Taibai Lake, Zhupo Lake, Longsai Lake, Yuni Lake, Chang Lake, Chidong Lake and Cehu Lake.

The results are shown in Figure 2-12. Combining the results of cluster analysis and principal component analysis, the three groups of lakes were separated on the gradient of the first principal component, and the results of multivariate analysis of variance among groups were significant (p<0.05). This shows that the lakes of Group1-Group3 show a trend of increasing lake fishery intensification. Therefore, through multivariate analysis, it can be concluded that 26 lakes have obvious gradients of lake fishery intensity.

The gradient of fishery intensification among lakes can be more intuitively shown by the differences among the three clusters of a single index. The lake fish yield, fish stocking amount and feed fertilizer input all showed an increasing trend from Group1-Group3. The results are shown in Figure 2-14.

Internal relationship of environmental factors

According to the hypothesis of this study, the degree of eutrophication of lakes will be affected by three types of environmental factors: land use, lake morphology and lake proliferation mode, that is, eutrophication belongs to secondary environmental disturbances, while the other three types belong to primary environmental disturbances.

In order to verify this conjecture and analyze the correlation between environmental factors, this study analyzed the correlation between each index of eutrophication and each index of other three environmental factors, so as to explore the internal relationship between different environmental pressures. The results showed that among the three types of environmental factors, land use, lake morphology and proliferation pattern, there were indicators that were significantly correlated with eutrophication indicators.

For land use index and eutrophication index, significant correlation was as follows:

there was a negative correlation between electrical conductivity and forest proportion (-0.46); there was a significant positive correlation between natural grassland proportion and chlorophyll a (0.44), dissolved oxygen (0.42) and pH (0.47); and there was a significant positive correlation between agricultural land proportion and electrical conductivity (0.52). Among them, the correlation between electrical conductivity and forest and agricultural land can reflect that agricultural expansion can increase the nutrient concentration of water body, which is thus related to the degree of For the lake morphology index and eutrophication index, the significant correlation is: there is a negative correlation (-0.46) between electrical conductivity and shoreline length. In addition to the significant correlation, total phosphorus, potassium permanganate index. There were generally weak negative correlations (>0.3) between conductivity, turbidity, and parameters related to lake volume (lake area, watershed area, shoreline complexity, water depth). This proves that the morphological characteristics of the lake, such as water depth and area, mean the carrying and buffering capacity of the lake for external pollutants. The correlation between lake morphology indicators and eutrophication indicators is shown in Figure 2-16. indicators. Except for transparency and lake fish production (-0.42), which was significantly negatively correlated with feed and fertilizer inputs (-0.50), all others were positively correlated. There was a strong correlation between total phosphorus and yield (0.85), total nitrogen and yield (0.75), total phosphorus and feed fertilizer input (0.72), permanganate index and yield (0.62), total nitrogen and stocking amount (0.61), total phosphorus and stocking amount (0.57), chlorophyll A and yield (0.52). This proves that the degree of lake fishery intensification is highly related to the degree of Integrating the relationship between three types of environmental factors, land use, lake morphology, and proliferation pattern, and eutrophication, there is a significant correlation between these three types of environmental pressures and eutrophication.

Among them, the correlation between the fishery proliferation mode index and the eutrophication index is the most extensive and strongest, followed by the land use index and the weakest lake morphology.

Influencing factors of fish community

In this study, the correlation between four types of environmental factors and fish community indicators (species richness, functional richness, density, biomass, species diversity and functional diversity) was analyzed to preliminarily explain the key environmental factors affecting the fish community. The importance of environmental factors was mainly determined by the strength and significance of their correlations with fish community indicators, and whether they were correlated with indicators of more dimensions (richness, abundance and diversity).

As for eutrophication index, the significant correlation between it and fish community index is as follows: There was a positive correlation between total phosphorus and biomass (0.58), a positive correlation between nitro nitrogen and density (0.42), a negative correlation between transparency and density (-0.39), a positive correlation between transparency and functional dispersion (0.40), a negative correlation between turbidity and functional dispersion (-0.40). In summary, there was no significant correlation between eutrophication index and fish richness index, but significant correlation between eutrophication index and species abundance (biomass and density) and diversity (functional diversity), that is, eutrophication index was positively correlated with species abundance and negatively correlated with diversity.

The correlation between fish community indexes and eutrophication indexes is shown in Figure 2-18. Among the four types of environmental factors, there was no significant correlation between land use and lake morphology and fish community indexes. The fishery proliferation pattern was only significantly correlated with fish biomass, and eutrophication was significantly correlated with fish biomass, density and fish community diversity.

Discussion

In the analysis of this study, the spatial patterns of four environmental factors, namely eutrophication, land use, lake morphology, and proliferation patterns, were described separately for the lakes in the middle reaches of the Yangtze River. The results

show that there are environmental gradient differences among the four types of factors among lakes, so it is reasonable and representative to select the four types of factors as the environmental disturbances that mainly drive environmental filtering in this study.

In the analysis of the internal relationship among environmental factors, the hypothesis of this study was verified, that is, eutrophication is a secondary disturbance among the four environmental stresses. The other three environmental factors may all have an impact on eutrophication, making eutrophication a comprehensive effect of multiple disturbances. In the identification of key environmental factors affecting fish community structure, only eutrophication and proliferation patterns were directly related to fish community metrics. While proliferation patterns were only significantly correlated with fish biomass, eutrophication was also correlated with functional diversity of fish communities.

Spatial patterns of multiple environmental pressures

The lakes in the middle reaches of the Yangtze River showed high environmental heterogeneity in the gradients of four environmental factors: eutrophication, land use, lake morphology and proliferation mode. This provides an ideal research object for exploring the influence of multiple environmental factors on fish community structure.

Regarding land use types, most of the middle reaches of the Yangtze River Basin are highly developed agricultural planting areas. Therefore, different degrees of lake reclamation led to different degrees of agricultural expansion in different lake basins [START_REF] Dong | Carbon burial by shallow lakes on the Y angtze floodplain and its relevance to regional carbon sequestration[END_REF]. In this study, the difference in the proportion of agricultural land and the proportion of natural vegetation (forest and grassland) formed the main land use type gradient among the lakes. In addition, the pond aquaculture area also has a large proportion around the lake, which is related to the high-density freshwater aquaculture in the middle reaches of the Yangtze River in China, and these ponds are likely to be reclamated from lakes [START_REF] Wang | Freshwater aquaculture in PR C hina: trends and prospects[END_REF]. However, in this study, the proportion of pond area cannot well explain the differences in land use among lakes in the middle reaches of the Yangtze River, which may be because the pond area in different lake basins does not have a complete gradient difference. Since urban land occupies a small proportion of the region, mostly between 1-3%, it cannot explain the inter-lake differences in land use.

The Yangtze River Basin is one of the regions with the most concentrated lakes in China [START_REF] Wang | Fisheries and wetland protection in the Yangtze River Basin[END_REF]. Thousands of lakes are distributed in the Yangtze River Basin, with a total area of 1.5210 4 km 2 , which is close to 1/5 of the total lake area in China [START_REF] Wang | Predictive limnology research on small and medium-sized lakes in the middle and lower reaches of the Yangtze River[END_REF]. The Yangtze River and its lake ecosystem are rich in water resources and biodiversity, and are one of the most important wetland areas in the world.

The whole world pays great attention to the biodiversity of the Yangtze River and its lake ecosystem [START_REF] Fu | Spatial pattern and resource analysis of fish diversity in the Yangtze River Basin[END_REF]. Most of the lakes in the middle reaches of the Yangtze River were originally connected to the river, that is, they were closely connected with the Yangtze River and naturally connected with each other, forming a unique and complete river-lake complex ecosystem with the main stream and tributaries of the Yangtze River [START_REF] Chang | Significance of fishery in river-connected lakes and resource management strategies. Resources and Environment in The Yangtze Basin[END_REF]. Factors such as the reclamation of lakes and the acceleration of natural sedimentation induced by the loss of lake connectivity further lead to the shrinkage or even disappearance of the lake water surface [START_REF] Wang | The ecological impact of river and lake barriers on fish resources in the Zhangdu Lake area. Resources and Environment in The Yangtze Basin[END_REF][START_REF] Zeng | Fishery resources in the Yangtze River system[END_REF]). In this study, lake area, watershed area, shoreline length, and shoreline development index, which are related to lake volume and habitat complexity, mainly explained the morphological differences among lakes. Since most of the lakes selected in this study are shallow lakes, there is little difference in water depth between lakes.

Lake fishery has contributed to China's economic development and aquatic product supply. However, with the rapid development of lake fishery, China's lake fish protection and lake fishery are facing a series of problems. Most lake fisheries in China have unreasonable resource development and utilization, such as the lack of conventional resource dynamic analysis, unrestrained overfishing has destroyed lake aquatic biological resources, lake fish resources have been depleted, and fish community structures have been miniaturized [START_REF] Qian | Current status of fishery resources and environmental monitoring in Poyang Lake[END_REF][START_REF] Liao | Current status and changes of fishery resources in Dongting Lake[END_REF].

Unreasonable fish stocking and fishing management schemes, lack of reasonable assessment and planning of the fishery carrying capacity of the water ecosystem, lead to changes in the fish community and population structure, and at the same time led to changes in the structure of other aquatic organisms in the lake [START_REF] Zhang | Effects of fish stocking on lake ecosystems in china[END_REF]. In the high-density lake fishery with intensive and semi-intensive methods as the main methods, a large amount of feed and fertilizer input lead to serious eutrophication of the lake water and reduced transparency [START_REF] Cui | Fishery resources and environmental protection of lakes in the Yangtze River Basin[END_REF]. In this study, fish production, fish stocking amount, and bait and fertilizer input were selected as three parameters to evaluate the lake fishery proliferation model, and the lakes were divided into three categories. It is basically consistent with the extensive, semi-intensive and intensive in the classification of freshwater value-added fishery types by [START_REF] Welcomme | Current approaches to the enhancement of fisheries[END_REF]. It reflects the different fishery development intensity of the lakes in the middle reaches of the Yangtze River [START_REF] Wang | Paradigm changes in freshwater aquaculture practices in China: Moving towards achieving environmental integrity and sustainability[END_REF].

Eutrophication is one of the most important human disturbances that cause the degradation of the ecological environment of lakes [START_REF] Jeppesen | The impact of nutrient state and lake depth on topdown control in the pelagic zone of lakes: a study of 466 lakes from the temperate zone to the arctic[END_REF][START_REF] Smith | Eutrophication science: where do we go from here?[END_REF], and there are many reasons for the enrichment of nutrients. Land use development is known to reduce water quality and lead to eutrophication [START_REF] Chadwick | Urbanization affects stream ecosystem function by altering hydrology, chemistry, and biotic richness[END_REF]. Landscape changes such as agricultural production, urban development or deforestation can increase the specific conductivity of water bodies [START_REF] Cañedo-Argüelles | Salinisation of rivers: an urgent ecological issue[END_REF]. At the same time, the lake morphology is also related to the nutrient state of the lake. The water depth and area of the lake also mean the carrying and buffering capacity of the lake for external pollutants. Compared with circular lakes, lakes with high perimeter-area ratio (higher shoreline development index) can receive relatively higher terrestrial material input [START_REF] Wetzel | Limnology: lake and river ecosystems[END_REF]. Unreasonable intensive high-density lake fisheries will also lead to serious eutrophication of lake water and reduce transparency [START_REF] Cui | Fishery resources and environmental protection of lakes in the Yangtze River Basin[END_REF]. Different levels of disturbance in environmental pressures such as land use, lake morphology, and proliferation patterns will inevitably lead to differences in the degree of eutrophication in lakes. This was confirmed in this study, and among the four groups obtained by cluster analysis, the two groups with the highest and lowest levels of eutrophication had a relatively large span of nutrient indicators. At the same time, in the analysis of the internal relationship of environmental factors, the hypothesis that eutrophication is a secondary disturbance has also been validied, and the other three environmental factors have different degrees of significant correlation with eutrophication.

Key environmental factors affecting fish communities

In theoretical analysis, the four environmental factors selected in this study will all have an impact on the fish community structure. Eutrophication has dramatically altered fish communities around the world [START_REF] Jeppesen | The impact of nutrient state and lake depth on topdown control in the pelagic zone of lakes: a study of 466 lakes from the temperate zone to the arctic[END_REF] and is one of the major anthropogenic disturbances facing lake fish. Increased nutrient levels in lakes lead to environmental changes that could potentially affect the composition, function, and dynamics of fish communities [START_REF] Jeppesen | The impact of nutrient state and lake depth on topdown control in the pelagic zone of lakes: a study of 466 lakes from the temperate zone to the arctic[END_REF]. Changes in land use at the watershed scale directly affect water quality and coastal habitat quality, and affect important ecological processes such as hydrology, primary productivity, metabolism, and organic matter turnover [START_REF] Young | Effects of land use on stream metabolism and organic matter turnover[END_REF][START_REF] Clapcott | Quantifying relationships between land-use gradients and structural and functional indicators of stream ecological integrity[END_REF]. These changes can directly or indirectly affect fish communities, potentially causing detrimental effects [START_REF] Allan | Landscapes and riverscapes: the influence of land use on stream ecosystems[END_REF]. Agriculture and deforestation cause sediment to enter the water column, often resulting in sedimentation, and this change in sediment leads to the reduction of fish species with specific habitat needs, especially benthic predators and spawning fish [START_REF] Roth | Landscape influences on stream biotic integrity assessed at multiple spatial scales[END_REF]. At the same time, high levels of urbanization affect sensitive fish, leading to fish communities dominated by tolerant fish (Diedel et al., 2010). The morphological characteristics of lakes, and the alteration of these characteristics by human disturbance, can affect the structure and composition of fish communities in lakes [START_REF] Schindler | Habitat coupling in lake ecosystems[END_REF]. Lake morphology plays an important role in shaping lake habitat complexity and the structure of fish communities.

Lake depth can significantly alter the composition of lake fish communities [START_REF] Garcia | Correlations between type-indicator fish species and lake productivity in German lowland lakes[END_REF][START_REF] Søndergaard | Water Framework Directive: ecological classification of Danish lakes[END_REF]. Larger lake areas may imply more complex habitat types, so positive correlations between lake area and fish species richness have often been reported in studies [START_REF] Dodson | The relationship in lake communities between primary productivity and species richness[END_REF][START_REF] Mehner | Rapid recovery from eutrophication of a stratified lake by disruption of internal nutrient load[END_REF]. The complexity of lake shorelines is often associated with the diversity of fish habitats [START_REF] Winfield | Fish in the littoral zone: ecology, threats and management[END_REF].

Most lake fisheries in China have unreasonable resource development and utilization, such as the lack of conventional resource dynamic analysis, unrestrained overfishing has destroyed lake aquatic biological resources, lake fish resources have been depleted, and fish community structures have been miniaturized [START_REF] Qian | Current status of fishery resources and environmental monitoring in Poyang Lake[END_REF][START_REF] Liao | Current status and changes of fishery resources in Dongting Lake[END_REF]. Unreasonable fish stocking and fishing management schemes, lack of reasonable assessment and planning of the fishery carrying capacity of the water ecosystem, lead to changes in the fish community and population structure, and at the same time led to changes in the structure of other aquatic organisms in the lake [START_REF] Zhang | Effects of fish stocking on lake ecosystems in china[END_REF]. At the same time, the seine nets of lakes and reservoirs lead to ecological isolation, destroy the integrity of the water ecosystem, decrease biodiversity, destroy the habitat of many fish, and lead to a large decline in species [START_REF] Cui | Fishery resources and environmental protection of lakes in the Yangtze River Basin[END_REF].

However, in the analysis results of this study, there was no significant correlation between land use and lake morphology and the selected indicators of fish community structure. Of course, this may be due to the fact that the measurement of the traditional diversity index cannot reflect the specific changes in the species composition and functional traits of fish communities (Violle et al., 2007;Edgar et al., 2009;Coleman et al., 2015). But this more reflects that the direct impact of these two types of environmental factors on fish communities may not play a decisive role in the lakes in the middle reaches of the Yangtze River. However, some weak correlation effects still indicate that these two types of environmental factors play an environmental filtering role on fish communities. Such as the positive correlation between species and functional richness and maximum water depth, the positive correlation between natural grassland and functional dispersion, etc. The effect of lake fishery proliferation pattern on fish community structure is predictable, but in this study it was only shown as an effect on fish biomass. In contrast, eutrophication was directly related to fish community abundance and diversity, reflecting its broad impact on lake fish communities. At the same time, because eutrophication is a secondary disturbance and is affected by the other three environmental factors, it represents the main indirect impact of other environmental factors on the fish community structure. For the above reasons, eutrophication was identified as a key environmental factor.

Conclusion

This chapter lays a theoretical foundation for the study of the environmental filtering effect of multiple environmental pressures on fish communities in the middle reaches of the Yangtze River, which is mainly reflected in two aspects. First, the gradient patterns of four types of environmental factors were described, thus emphasizing that the selected environmental pressures can comprehensively and effectively explain the environmental heterogeneity among lakes, and verifying its rationality as the main driving force of environmental filtering. Second, we validated eutrophication as a secondary disturbance, influenced by other environmental factors, and broadly acting on multiple dimensions of the structure of fish communities.

Therefore, eutrophication was identified as a key factor in studying the relationship between environmental pressure and fish community structure in lakes in the middle reaches of the Yangtze River. Identifying eutrophication, a key environmental factor, among many environmental pressures will help to reveal its specific impact on the species composition and functional structure of fish communities in subsequent studies.

Chapter3 Response of fish community taxonomic and functional structure to eutrophication

Introduction

Nutrient enrichment induced by human activity is likely to act as a set of environmental filters [START_REF] Smith | Eutrophication science: where do we go from here?[END_REF] which not only regulate the species composition of the community, but also affect the functional structure [START_REF] Petriki | A fish-based index for the assessment of the ecological quality of temperate lakes[END_REF]. Assessing the threat of anthropogenic disturbance (e.g., eutrophication) to the loss of fish diversity in freshwater ecosystems traditionally relied on abundance, biomass, species richness and taxonomic diversity (Stuart-Smith et al., 2013;Coleman et al. 2015). However, diversity measurement that takes into account the functional traits of species could be more sensitive to environmental changes, could help understanding the underlying mechanistic processes and identifying changes in ecosystem functioning [START_REF] Mouchet | Functional diversity measures: an overview of their redundancy and their ability to discriminate community assembly rules[END_REF].

Identifying indicator species responding to human perturbations is critical to environmental management [START_REF] Bal | Selecting indicator species for biodiversity management[END_REF], but this approach can only be applied where the indicator species occur, which limits its geographical scope. For instance, in Europe, Perca fluviatilis and Rutilus rutilus are frequently used as indicator species for eutrophication, as intolerant and tolerant species respectively [START_REF] Ritterbusch | European fish-based assessment reveals high diversity of systems for determining ecological status of lakes[END_REF].

In this regard, functional traits are supposed to provide a more general approach [START_REF] Mouchet | Functional diversity measures: an overview of their redundancy and their ability to discriminate community assembly rules[END_REF][START_REF] Mouillot | A functional approach reveals community responses to disturbances[END_REF]. Traits based approaches assumes that the local habitat acts as a filter, selecting the species having the appropriate combination of traits allowing them to persist given the local environmental conditions. By revealing traits-environment relationships that are potentially independent from taxonomic composition, such approaches could help identifying similar trait-habitat relationships in two different regions having few or no species in common [START_REF] Su | Human impacts on global freshwater fish biodiversity[END_REF]. However, studies using functional approaches to assess the effects of eutrophication on lacustrine fish communities are still very rare and strongly geographically biased toward Europe.

In a recent review of fish-based assessments of ecological status of European lakes, [START_REF] Ritterbusch | European fish-based assessment reveals high diversity of systems for determining ecological status of lakes[END_REF] pointed out that fish exhibited an integrated reaction to multiple stressors (including eutrophication) particularly with regard to their feeding ecology. The relative contribution of piscivorous fishes tend to decrease with eutrophication, while the opposite is observed for benthivorous and omnivorous fishes [START_REF] Ritterbusch | European fish-based assessment reveals high diversity of systems for determining ecological status of lakes[END_REF]. Those patterns suggest the existence of environmental filtering in which species sharing the same traits covary along the eutrophication gradient. This trait filtering process observed for Europe [START_REF] Jeppesen | Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient[END_REF][START_REF] Argillier | Development of a fish-based index to assess the eutrophication status of European lakes[END_REF][START_REF] Petriki | A fish-based index for the assessment of the ecological quality of temperate lakes[END_REF]) is not necessarily general and could depend on the particular evolutionary history of this region, the type of lakes encountered or the causes for nutrient enrichment. In this study we will address the existence of traitseutrophication relationships for lacustrine fish communities in a different geographical context (the Yangtze River floodplain, China) and its consistency with previous studies.

Life history strategy is a core concept in ecology and evolutionary biology [START_REF] Marquet | On theory in ecology[END_REF]. Life history is a set of co-adaptive traits that a species develops to maximize adaptation to a particular environment [START_REF] Stearns | The evolution of life histories[END_REF]. This specific combination of demographic traits defines the life history strategy of species (Winemiller et al., 2015), and the process by which these species experience different survivals in different environmental gradients is called environmental filtering [START_REF] Cornwell | A trait-based test for habitat filtering: convex hull volume[END_REF]. The life history strategy of a species largely determines its response to the variability, predictability (stability) and periodicity (seasonality) of environmental conditions [START_REF] Winemiller | Life history strategies, population regulation, and implications for fisheries management[END_REF][START_REF] Mims | Life history theory predicts fish assemblage response to hydrologic regimes[END_REF]. Life history theory provides a framework for understanding environmental changes based on species traits and can help us understand how the environment affects the distribution and change of communities [START_REF] Charnov | Evolutionary assembly rules for fish life histories[END_REF].

Neither the r-K (Pianka 1970) nor the slow-fast [START_REF] Harvey | Patterns of mortality and age at first reproduction in natural populations of mammals[END_REF] frameworks are entirely satisfactory to describe the combinations of demographic traits observed in fishes. According to these frameworks, large species are predicted to have a low fecundity, a pattern that does not account for the existence of many fish species that are both large and fecund (e.g., Hypophthalmichthys molitrix and Hypophthalmichthys nobilis). Therefore, [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF] proposed that the variability in fish demographic traits was well captured by a triangle with end points describing three contrasted life-history strategies: opportunistic, periodic, and equilibrium. Long-lived species that delay maturation to have large body size and high fecundity during periods of suboptimal environmental conditions are identified as "periodic" species; species having low fecundity and maximizing juvenile survival through parental care and large eggs correspond to the "equilibrium" strategy; rapid colonized species with small size, short life span, early maturation and low fecundity (yet high relative reproductive effort) are considered "opportunistic" strategists.

Periodic strategy maximizes fitness when the environmental variation influencing survival during early life stages is seasonal coupled with inter-annual variability, and represents an iteroparous "bet-hedging tactic" on a scale of interannual variation.

Equilibrium strategists improve larval survival through high levels of parental care, a life-history strategy that relies on stable and predictable habitats with low environmental variation. Opportunistic strategist fishes usually adapt to highly variable, unpredictable, disturbed and ephemeral habitat conditions [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF].

This framework has been used to study fish community responses to an array of environmental factors such as natural hydrological variability [START_REF] Tedesco | River hydrological seasonality influences life history strategies of Constance example revisited[END_REF][START_REF] Mims | Life history theory predicts fish assemblage response to hydrologic regimes[END_REF][START_REF] Hitt | Fish life history trends indicate increasing flow stochasticity in an unregulated river[END_REF], anthropogenic flow control [START_REF] Mims | Fish assemblages respond to altered flow regimes via ecological filtering of life history strategies[END_REF], land use change [START_REF] Manfrin | Effect of river restoration on life-history strategies in fish communities[END_REF][START_REF] Hitt | Fish life history trends indicate increasing flow stochasticity in an unregulated river[END_REF], fishing [START_REF] Rose | Compensatory density dependence in fish populations: importance, controversy, understanding and prognosis[END_REF], exotic species invasion [START_REF] Olden | Life-history strategies predict fish invasions and extirpations in the Colorado River Basin[END_REF], temporal and spatial variability of estuarine habitat conditions (Teichert et al., 2017), climate warming [START_REF] Pecuchet | From traits to life-history strategies: Deconstructing fish community composition across European seas[END_REF] and latitude (Vila-Gispert et al., 2002). In contrast with lotic habitats, studies on the variability of life history strategy of fish among lakes are sparse and limited to the effect of landscape connectivity [START_REF] Miyazono | Assemblage patterns of fish functional groups relative to habitat connectivity and conditions in floodplain lakes[END_REF][START_REF] Dembkowski | Comparison of fish assemblages in two disjoined segments of an oxbow lake in relation to connectivity[END_REF].

In comparison with nearby rivers, lakes are more stable and predictable, at least with regard to hydrological variability, and, therefore, are expected to be more favorable to equilibrium species which is generally observed [START_REF] Zeug | Relationships between hydrology, spatial heterogeneity, and fish recruitment dynamics in a temperate floodplain river[END_REF].

However, in some conditions, the stability and predictability of lacustrine environments could be reversed. This is in particular the case for eutrophic lakes. Eutrophic lakes

often alternate between clear water and turbid states [START_REF] Carpenter | Eutrophication of aquatic ecosystems: bistability and soil phosphorus[END_REF], which makes them unstable systems. The occurrence of hypoxia and harmful algal blooms in eutrophic lakes is usually not gradual, but abrupt (Scheffer et al. 2001) and could be induced by slight changes in the environment (e.g., calm and cloudy days). It could therefore be assumed that, along a eutrophication gradient, environmental stability and predictability decrease making the habitat less and less suitable for equilibrium species.

Moreover, because primary productivity increases with eutrophication, eutrophic lakes are unlikely to be strongly resource-limited, a situation that is not favorable to equilibrium species and species with higher reproductive effort (opportunistic and periodic) will prevail [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF]. As a whole, it is expected a negative impact of eutrophication on equilibrium species, but, to our knowledge the impact of eutrophication on life history strategies of lacustrine fish communities has not been studied yet. However, [START_REF] Lin | Eutrophication as a driver of r-selection traits in a freshwater fish[END_REF] found that eutrophication drives a lacustrine fish species to have more r-selected traits, and therefore be more opportunistic, which is in line, at the population level, with our expectations at the community level.

This study investigated the effects of eutrophication on the taxonomic and

Materials and methods

Study area and sample setting

See Chapter 2.2.1 for the setting of the study area and sample points.

Sample and data collection

For the collection and acquisition methods of fish community data, fish functional traits, eutrophication data, and lake area and maximum depth data, see Chapter 2.2.2.

Data analysis

In this study, we selected 12 eutrophication and water body physical and chemical indicators as environmental variables, and at the same time analyzed the basic morphological attributes of lakes, lake area and water depth, as environmental variables.

Because the morphology of our lakes varies significantly, this will inevitably cause differences between fish communities, so the lake morphology data is taken into account in the data analysis. The environmental variables selected in this study are as follows: Secchi depth (SD, m), dissolved oxygen (DO, mg L -1 ), conductivity (Cond, μS cm -1 ), pH, turbidity (Turb, NTU), total phosphorus (TP, mg L -1 ), total nitrogen (TN, mg L -1 ), chlorophyll a (Chla, μg L -1 ), chemical oxygen demand (CODMn, mg L -1 ), ammonia nitrogen (NH4 + -N, mg L -1 ), nitrite nitrogen (NO2 --N, mg L -1 ), nitrate nitrogen (NO3 --N, mg L -1 ), lake area (km 2 ), maximum water depth (WD, m). A correlation circle was used to visually display the correlation between the above 14 variables.

In this study, the responses of fish communities to eutrophication were analyzed from the aspects of abundance and diversity index, taxonomic composition, functional traits, and life history strategies, the special functional groups derived from functional traits. All analyzes in this study were performed under R version 4.2.2 (R Core Team, 2022).

Response of fish community index to eutrophication

We selected three metrics, species richness (SRic), total biomass (BPUE) and functional dispersion (FDis) to describe fish community structure. Functional dispersion is the mean distance of individual species in functional trait space to the centroid of all species in a community [START_REF] Laliberté | A distance-based framework for measuring functional diversity from multiple traits[END_REF]. Note that both the position of the centroid and the distances to it are weighted by the relative abundances of the species. The function dpFD of package 'FD' [START_REF] Laliberté | Measuring functional diversity from multiple traits, and other tools for functional ecology[END_REF] was used to calculate this metrics. For each sample, total biomass is given per unit effort (g net -1 h -1 ) and species richness is the total number of species observed. All metrics were computed for each fish sample site and then averaged per lake.

We analyzed the responses of each community metrics to water quality and lake morphology with a multiple regression model assuming a gaussian error distribution.

To improve normality of the response variables, SRic was square transformed, BPUE was square-root transformed. All predictor variables but DO, pH and WD were ln transformed to obtain more regular environmental gradients. Finally, to improve the comparability of regression coefficients within and among models, the response and predictor variables were standardized (zero mean and unit variance).

To identify simple but explanatory models, for each response variable, a set of predictor variables was selected by a stepwise algorithm (backward selection starting with all the 14 predictor variables) using the Akaike Information Criterion (AIC) to compare models. In this aim the function stepAIC of package 'MASS' [START_REF] Venables | Modern Applied Statistics with S. Fourth Edition[END_REF] was used. The function vif of package 'car' [START_REF] Fox | An {R} Companion to Applied Regression, Third Edition[END_REF] was used to calculate variance inflation factors (VIF) for the models. Predictor variables with a VIF value greater than 5 were removed to minimize the effect of multicollinearity in the models. In order to figure out the relative importance of predictor variables, the function calc.relimp of package 'relaimpo' [START_REF] Grömping | Relative importance for linear regression in R: the package relaimpo[END_REF] was used to calculate relative importance of predictor variables with method lmg.

Response of taxonomic composition of fish community to eutrophication

A Redundancy Analysis (RDA) was used to quantify and visualize the environmental gradients that contribute the much to community changes in species composition (Braak and Verdonschot, 1995). Only species occurring in more than 3 lakes within 26 lakes were retained for analysis (36 species). Abundance data were ln (n+1) transformed. The identity of environmental variables and their transformations are as in the multiple regression analyses. The overall significance of the communityenvironment relationship and the contribution of each environmental variable was assessed with a permutation test (the tested statistic is a "pseudo-F", quantifying the contribution of the environmental descriptors to the among-communities variability)

and the effect size of the total model was evaluated by the adjusted R 2 coefficient. RDA was performed using the R packages "vegan" [START_REF] Oksanen | Community Ecology Package (R Package Version 2.6-2)[END_REF].

Response of functional traits of fish community to eutrophication

The relationships between species traits and environmental gradients were analyzed with RLQ analyses using the R packages "ade4" [START_REF] Dray | The ade4 package: implementing the duality diagram for ecologists[END_REF].

The RLQ analysis is a multivariate method allowing quantifying and visualizing relationships between species traits and environmental descriptors, and in particular identifying species traits that co-vary along synthetic environmental gradients described by ordination axes. This analysis relates functional traits of species (Q) to environmental variables of sites (R) as mediated by species distribution across sites (L) (Dolé dec et al., 1996). To be meaningful, a traits-environment relationship requires that a) species are not distributed at random with regard to environmental features and b) species with similar traits covary in space (among sites) more than expected by chance.

Hypothesis a) was already tested with an RDA (see above) and hypothesis b) by comparing the observed strength of the traits-environment relationship with a null distribution obtained by permuting the species labels in the traits table ("model4" permutation type), then run a new RLQ and repeating this operation several times [START_REF] Dray | Combining the fourth-corner and the RLQ methods for assessing trait responses to environmental variation[END_REF].

To help interpreting the traits-environment relationships, three groups of traits have been considered and used as input for different RLQ analyses: those related to life history strategy and reproduction, trophic ecology, and habitat use. Environmental variables and species abundance were transformed as RDA (see above) and numeric traits were ln transformed.

Response of life history strategies of fish community to eutrophication

Eight life history traits were selected for their association with body size, longevity, fecundity, larval survival or generation time. The traits include maximum standard length, longevity, resilience, age at maturity, absolute fecundity, standard length at maturity, egg diameter and parental care index. Traits values per species were obtained by consulting published literature, books, scientific reports, expert knowledge, field observations and online databases (Appendix 1 and Appendix 2).

According to the triangular continuum model of [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF], we defined three life history strategies (periodic, opportunistic, equilibrium) based on species traits. They represent the endpoints of a triangular continuum resulting from trade-offs between basic population parameters of fecundity, generation time, and larval survival. Correspondingly, we plan to represent these three axes with absolute fecundity, age at maturity and parental care index, respectively. However, age at maturity of the studied species does not vary much and is not discriminant. Therefore, we only use absolute fecundity and parental care index to approximately divide the three life history strategies in two-dimensional space (Figure 3-1). First, we select the species with obvious parental care behavior (parental care index ≥ 2) as the "equilibrium" group ( 14species). The highest absolute fecundity among the equilibrium species is used as a threshold, and species greater than this value are assigned to the "periodic" group (18 species). Species with fecundity below this threshold and no parental care behavior were classified as "opportunistic" (16 species). The term "opportunistic" is used here for convenience, but strictly speaking, this last strategy is a non-equilibrium and nonperiodic strategy and includes species that are no pure opportunistic and could belong to intermediate strategies. Our approach divides the parental care-absolute fecundity plane into three contiguous life-history strategy regions, whereas no species are present in the fourth region (upper right in Figure 3-1). This suggests that there is no life history strategy with both high fecundity and high parental care, which is consistent with the concept of a triangular continuum model. The hypothesis is first tested from the perspective of species composition. The percentages of the total species for each of the three life history strategies were calculated and regressed against TP. In this study, total phosphorus concentration was used to represent the level of lake eutrophication, because it was shown in the results of the above analysis that TP can better represent the impact of eutrophication on fish functional traits. Species percentage data were square root arcsine transformed, and TP data were ln transformed. Under our working hypothesis, it is expected that the percentage of equilibrium species and TP are negatively correlated.

We then tested our hypothesis in terms of species abundance. To quantify the relationship between species abundance and eutrophication, we used a "response to eutrophication index", which is the Pearson correlation coefficient between TP (ln transformed) and species abundance (Hellinger transformed). In order to illustrate the meaning of this index, we selected two species with contrasting responses:

Chanodichthys dabryi with a positive response to eutrophication, i.e., abundance and TP are positively correlated, and Acanthorhodeus chankaensis with a negative response, i.e., abundance and TP are negatively correlated (Figure 3-2). We then tested whether there was a significant difference in the response to eutrophication indices between equilibrium and non-equilibrium species by ANOVA, the expectation being that in average indices should be lower for equilibrium species compared to non-equilibrium species.

Figure 3-2 Examples of species with contrasted responses to eutrophication

We additionally analyzed species responses to eutrophication with regard to life history features without defining strategies a priori to check for the possible existence of unanticipated patterns. A principal component analysis (PCA) was performed on the demographic traits of all species, and the coordinates of the species on all axes (PCA1-PCA8) of the PCA were extracted. Then the relationship between response to eutrophication indices and PCA1-PCA8 coordinates were investigated by a multiple linear regression model.

Results

Correlation between eutrophication indicators

In the system of lakes considered in this study, the correlations among the variables supposed to be related to eutrophication (TP, TN, Chla, Turb, SD, COD) are all of the expected sign (Figure 3-3), but could be low (e.g., r = 0.216 between COD and Turb).

All the other water quality variables but NH4 + -N display good or moderate positive correlations with the variables increasing with eutrophication.

Response of fish community index to eutrophication

SRic and BPUE are poorly explained by environmental descriptors (Table 3-1) with R 2 values lower than 0.3, while for FDis a better fit has been obtained (near 0.5).

In the model of SRic, TN, COD and NO3 --N were removed due to the multicollinearity.

For all metrics but SRic, at least one variable related to eutrophication is among the significant ones retained in the regression model (Table 3-1). When selected, eutrophication related variables were positively correlated with BPUE and negatively with FDis. Among the other water quality variables, only pH contributed significantly to one model (FDis), in the opposite direction than eutrophication. One variable related to lake morphology, WD, was significantly correlated to SRic and FDis, positively and negatively, respectively.

Response of taxonomic composition of fish community to eutrophication

According to the RDA, the species composition of fish communities was significantly related to environmental descriptors of the lakes (Figure 3-4, r 2 adj. = 0.181, F14.11 = 1.39, P < 0.05). In particular, two variables related to eutrophication, TP and Chla, contributed significantly to community variability (P < 0.05, Table 3-2). Note also the contribution of NO3 --N. Therefore, the first axis described grossly a eutrophication gradient and, according to the permutation test, it explained a significant part of community variability. No other axis was significant (P < 0.05). Lakes with the highest level of eutrophication (with positive coordinates along the first axis, Figure 3-4) were characterized by species with different ecologies. Toxabramis swinhonis and Coilia nasus are small pelagic zooplanktivores, Chanodichthys dadryi is a pelagic piscivore, Pseudobrama simoni is a detritivore mainly feeding on plant debris and periphyton. In the less eutrophicated lakes (along the negative coordinates of the first

Response of functional traits of fish community to eutrophication

In the three RLQ analyses conducted (life history strategy, feeding, habitat use), the environmental gradients described by the first axis were very similar. For life history strategy and feeding the first axis opposes small turbid lakes with high TP concentration and conductivity to large lakes with clearer water and lower TP concentration and conductivity, the situation is reversed for habitat use (Figure 3-5). According to those results, the first axis acted as an inverse eutrophication (life history strategy and feeding)

or eutrophication (habitat use) gradient in all analyses. It is worth noting that Area was correlated positively with the first axis in all the RLQ analyses. Therefore, the relationship of a trait with the first axis of a RLQ plot provides information about how eutrophication acted as an environmental filter with respect to this trait. In the RLQ analysis of the life history strategy traits of community the cumulative projected inertia obtained by the RLQ analysis is 86% when axis 1 and axis 2 are combined (Figure 3-6a), and according to the permutation test, the relationship between life history traits and lakes environmental descriptors is significant (P = 0.013).

Standard body length (S.SL) and absolute fecundity (S.AF) were positively associated with eutrophication, and the trait "demersal eggs" (S.ET2) negatively. This result suggests that eutrophication significantly favors species with large body size and high fecundity over species with demersal eggs and parental care.

In the RLQ analysis of the feeding traits of community the cumulative projected inertia obtained by the RLQ analysis is 84% when axis 1 and axis 2 are combined (Figure 3-6c) and, according to the permutation test, the relationship between traits and environment is not significant (P = 0.208). The biological traits the most notably correlated with the first axis, are inferior mouth position (F.MP1), substrate explorer (F.FH5), detritivore (F.TG3), scraper (F.FH4), omnivore (F.TG4) (positive correlations) and terminal-superior mouth position (F.MP4), predator active search (F.FH2), piscivore (F.TG7), predator sit and wait (F.FH1) (negative correlations). Although not significant, eutrophication still showed a positive relationship with piscivory.

In the RLQ analysis of the habitat use traits of community the cumulative projected inertia obtained by the RLQ analysis is 88% when axis 1 and axis 2 are combined (Figure 3-6e) and, according to the permutation test, the relationship between traits and environment is significant (P = 0.024). The traits "cylindrical body shape" (H.BS5), "top position of the eye" (H.EP1) and "Rheophilic" (H.HP2) display the strongest negative response to eutrophication. This result suggests that low eutrophication lakes favor species with traits adapted to benthic habitats and species that prefer running, well oxygenated water. Two traits, "Potamodromous" (H.HP4) and "lateral-bottom relative position of the eye" (H.EP4) are notably correlated with eutrophication, but those traits are strongly associated with some possibly stocked species (Appendix2).

Response of life history strategies of fish community to eutrophication

After testing, the relative species richness of equilibrium species decreased significantly with increasing TP (p < 0.05, Figure 3-7a). In addition, the relative species richness of periodic species showed an upward trend (Figure 3-7b), while the relative species richness of opportunistic species was relatively stable (Figure 3-7c). The results of the analysis show that the relative contribution of equilibrium species in lake fish communities decreases significantly along the eutrophication gradient and that among the non-equilibrium species, the proportion of periodic strategists increases with eutrophication. The response to eutrophication indices of the equilibrium species were in average negative and significantly lower than that of non-equilibrium species (p < 0.01, Figure 345678). The response to eutrophication indices of periodic and opportunistic species are almost equally distributed on both sides of the zero value. 3-3), both with positive correlations (Figure 3-10). The results of PCA showed (Figure 3-9) that PCA1 is positively correlated with body size, lifespan, absolute fecundity, and age at maturity and negatively with population resilience. and that PCA2 is negatively correlated with parental care index and egg size. PCA1 is positively related to life history traits associated with the periodic strategy, while PCA2 is negatively related to traits typical of equilibrium strategists.

Different roles of eutrophication indicators in fish community changes

Eutrophication is a multifactorial environmental change [START_REF] Smith | Eutrophication science: where do we go from here?[END_REF], the effects of which are not necessarily well captured by one variable alone and the impact of eutrophication on fish communities could result from the action of a limited number of variables among those than constitute the signature of eutrophication.

In our study the most relevant variables were those related to nutrient concentration (in particular total phosphorus) and water transparency. As a whole, total phosphorus appeared to be the better proxy of eutrophication in our study. The critical role of phosphorus in lake eutrophication has been reported in previous studies [START_REF] Carpenter | Phosphorus control is critical to mitigating eutrophication[END_REF][START_REF] Schindler | Reducing phosphorus to curb lake eutrophication is a success[END_REF]. Considering that, for most of the analyses conducted, at least one variable related to eutrophication is among the significant ones, and that only one metrics, species richness, is better explained by lake morphology, it emerges that eutrophication is a consistent and major factor shaping fish community structure, based either on taxonomy or functional traits.

Increasing productivity affects fish communities of lakes

A signature of eutrophication is an increased primary productivity. The relationship between ecosystem productivity and biodiversity is a recurrent topic in community ecology [START_REF] Dodson | The relationship in lake communities between primary productivity and species richness[END_REF][START_REF] Hayden | Climate and productivity shape fish and invertebrate community structure in subarctic lakes[END_REF]. For animal communities, the general pattern is that as productivity increases so does species richness [START_REF] Cusens | What is the form of the productivity-animal-species-richness relationship? A critical review and meta-analysis[END_REF]) but sometimes a unimodal relationship is observed. For instance, [START_REF] Jeppesen | Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient[END_REF] observed that fish species richness was unimodally related to TP, and peaked at 0.1-0.4 mg L -1 among 71 Danish lakes. In our study, there no significant relationship between TP, or any other variable related to eutrophication, and species richness. This result suggests that, at highest levels of eutrophication, inter-specific interactions and/or environmental harshness (e.g., hypoxia, toxic algal bloom) can lead to a decline in species richness (Thomas and Eckmann, 2007), resulting in the overall flat relationship we observed in our study. The range of TP concentrations in our lakes is 0.01-0.77, which is similar to the study of [START_REF] Jeppesen | Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient[END_REF] and covers the peak range they found. This could explain why no linear relationship between species richness and eutrophication was found but further studies are needed to assess the generality of this result. Moreover, it is worth remembering that our study was based on local species richness which is not necessarily a good proxy for the total number of species inhabiting a lake.

Total biomass is frequently used as a proxy of the productivity sustaining a community, and as such should increases with eutrophication. As expected, total biomass is positively, but weakly, correlated to total phosphorus in our study. This result is consistent with several other studies that have shown that in lakes eutrophication correlates positively with total density and biomass [START_REF] Olin | Fish community structure in mesotrophic and eutrophic lakes of southern Finland: the relative abundances of percids and cyprinids along a trophic gradient[END_REF][START_REF] Egertson | Relationship of fish catch and composition to water quality in a suite of agriculturally eutrophic lakes[END_REF][START_REF] Søndergaard | Water Framework Directive: ecological classification of Danish lakes[END_REF][START_REF] Mehner | Rapid recovery from eutrophication of a stratified lake by disruption of internal nutrient load[END_REF][START_REF] Hayden | Climate and productivity shape fish and invertebrate community structure in subarctic lakes[END_REF][START_REF] Yu | Changes in pelagic fish community composition, abundance, and biomass along a productivity gradient in subtropical lakes[END_REF].

Changes in trophic availability and resources weakly affects functional community structure

Some species and traits were expected to increase or decrease with increasing eutrophication due to accompanying changes in food resources availability [START_REF] Jeppesen | Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient[END_REF][START_REF] Mehner | Composition of fish communities in German lakes as related to lake morphology, trophic state, shore structure and human-use intensity[END_REF][START_REF] Yu | Changes in pelagic fish community composition, abundance, and biomass along a productivity gradient in subtropical lakes[END_REF]. In particular, it has been observed in previous studies that the abundance of zooplanktivorous fishes increases with eutrophication, probably as a consequence of a higher phytoplankton productivity sustaining higher zooplankton abundance in eutrophic lakes [START_REF] Bergstrand | Changes in the fish and zooplankton communities of Ringsjön, a Swedish lake undergoing man-made eutrophication[END_REF][START_REF] Jeppesen | Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient[END_REF]. However, in our study chlorophyll a concentration did not contribute much to the first axis of the RLQ analysis of feeding traits suggesting that an increase in primary productivity was not a major driver of community change in this respect. In the same vein, zooplanktivory was only slightly associated with the first axis.

Small zooplanktivorous species (Toxabramis swinhonis, Hemiculter leucisculus, Hemiculter bleekeri, Coilia nasus and Pseudolaubuca sinensis) are, as a whole, dominant in the investigated lakes (averaged 64.40% in abundance), particularly T.

swinhonis which was found to be the dominant species in many eutrophic lakes in the Yangtze River [START_REF] Yu | Changes in pelagic fish community composition, abundance, and biomass along a productivity gradient in subtropical lakes[END_REF]. However only a few of them (T. swinhonis and C. nasus) increased in abundance with eutrophication, suggesting that zooplanktivory is not systematically associated with eutrophication in our study, and, as a result, has a weak signature in a RLQ plot. One explanation could be that in addition to abiotic filters (resource abundance or availability), biotic filters (competition, predation) are also at play. Some small pelagic zooplanktivorous species could be prevented from increasing in abundance despite favorable conditions because of predation from pelagic piscivores or competition with other zooplanktivores.

Habitat degradation affects taxonomy and function of communities

Water clarity declines as eutrophication increases and turbidity has proven to be a strong factor of fish community structuring in lakes (Rodríguez and Lewis, 1997). The negative relationship observed between turbidity and functional dispersion (FDis)

suggests that species with particular biological traits are more abundant in turbid lakes, which is in line with a notable contribution of turbidity and Secchi depth to the RLQ analysis based on habitat use traits. In other words, fish communities are more homogeneous (less dispersed) in terms of functional traits in the most eutrophic lakes.

Fish species not strongly depending on vision for foraging, escaping predators or engaging in intraspecific interactions (e. g. courtship) should be favored in turbid waters.

However, among the traits considered in this study, only one, nocturnal activity (H.DA1), could confer such a direct advantage in turbid waters, and, indeed, is correlated positively, but slightly, to eutrophication.

Turbidity could have a more indirect and complex action and consequently could act as an environmental filter on traits not necessarily associated with visual dependency of the species [START_REF] Smith | Eutrophication of freshwater and marine ecosystems[END_REF]. For instance, in highly turbid lakes, a large amount of sediment and organic material could accumulate on demersal eggs and impair their development by asphyxiating them [START_REF] Smith | Eutrophication of freshwater and marine ecosystems[END_REF]. In the same vein, macrophytes become rare as turbidity increases and fish species (such as Carassius auratus, Cyprinus carpio, Hemibrabus maculatus, Paracanthobrama guichenoti, Tachysurus fulvidraco, Pseudorasbora parva) that use them for spawning, sheltering or foraging are expected to decline accordingly.

It is well known that eutrophication leads to degradation of the demersal zone by increasing the frequency of hypoxia events [START_REF] Argillier | Development of a fish-based index to assess the eutrophication status of European lakes[END_REF] and, indeed, our results suggest that species foraging or reproducing at the bottom of the lake decrease in abundance as eutrophication increases. Traits typical of demersal lacustrine fish species (Department of Ichthyology, Institute of Hydrobiology, CAS, 1976) are negatively related to eutrophication, such as "cylindrical body shape" (H.BS5), "top eye position" (H.EP1), "demersal eggs" (S.ET2) and "inferior mouth position" (F.MP1).

In addition, rheophilic species (H.HP2) display the same pattern of low abundance in eutrophic lakes. Those species that prefer running, well oxygenated, waters are typically intolerant to hypoxia.

A very specific change in habitat likely driven by eutrophication might account for the rarity of a particular group of fishes in highly eutrophic lakes. The females of Sarcocheilichthys nigripinnis, Acheilognathus macropterus, Acanthorhodeus chankaensis and Rhodeus ocellatus have an ovipositor which is used to deposit eggs inside bivalves and juveniles remain in the host until they can swim (Department of Ichthyology, Institute of Hydrobiology, CAS, 1976). Those species are negatively related to eutrophication and this could be explained by a negative relationship observed in the Yangtze River between the diversity of their hosts and eutrophication [START_REF] Zhang | Responses of the macroinvertebrate taxonomic distinctness indices of lake fauna to human disturbances in the middle and lower reaches of the Yangtze river[END_REF]. Another interesting pattern is that the periodic type of life history strategy [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF], long living and fecund, seems to be favored in eutrophic lakes, as exemplified by the positive relationship between body size and absolute fecundity with eutrophication. If this result is not a by-product of eutrophication selecting large species for other reasons (e. g. favoring piscivory), this suggests that eutrophication increases the unpredictability of young stages survivorship, a factor promoting the evolution of a periodic strategy [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF], a point deserving further investigation.

Role of lake morphology

A positive correlation between lake area and fish species richness has been frequently reported [START_REF] Dodson | The relationship in lake communities between primary productivity and species richness[END_REF][START_REF] Mehner | Rapid recovery from eutrophication of a stratified lake by disruption of internal nutrient load[END_REF], as expected for insular habitat in general. No such pattern has been observed in the present study, which is consistent with previous research on lakes of Yangtze River basin [START_REF] Yu | Changes in pelagic fish community composition, abundance, and biomass along a productivity gradient in subtropical lakes[END_REF].

However, our study is based on local species richness not the total number of species inhabiting a lake and our results do not exclude the existence of a species-area relationship at the latter scale. Because the investigated lakes have been disconnected

from the main river for about 50 years [START_REF] Liu | Estimation of minimum area requirement of riverconnected lakes for fish diversity conservation in the Yangtze River floodplain[END_REF], extinction prone species are more likely to be absent from small lakes because of their small population size. In particular species that cannot reach high densities, such as large or top-predator species, should be extirpated first from small, isolated habitats [START_REF] Krauss | Local species immigration, extinction, and turnover of butterflies in relation to habitat area and habitat isolation[END_REF]. Actually, in our study, it has been observed the opposite trend in which the abundance of large and piscivorous species tend to be negatively associated with lake area, but positively with eutrophication. A possible explanation could be that eutrophication, through increased lake productivity, contributes much more than lake area (ecosystem size) to population sizes. This result is coherent with the study of [START_REF] Jia | Understanding trophic structure variation in fish assemblages of subtropical shallow lakes: Combined effects of ecosystem size, productivity, and disturbance[END_REF] indicating that productivity was more important than ecosystem size in explaining the variance in mean trophic level in fish communities in the Yangtze River. Another important variable for explaining lacustrine fish community patterns is average or maximum depth of the lake. Despite the fact that all the lakes considered in this study are shallow (maximum water depth ranges from 0.8 m to 5.7 m), depth was nevertheless positively correlated to species richness. The effects of depth on ecosystem processes could be complex, particularly when acting in interaction with other variables, such as light penetration (Rodríguez and Lewis, 1997). However, it is generally assumed that habitat diversity increases with depth [START_REF] Zhao | Patterns of fish species richness in China's lakes[END_REF] and this could explain the observed positive relationship with species richness.

Geographic constraints on eutrophication induced traits filtering

In this study, we identified several filtering effects of eutrophication on traits related to habitat use and life history strategies, which adds to the few previous works that have attempted to link lacustrine fish community metrics or species traits to anthropogenic disturbances [START_REF] Jeppesen | Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient[END_REF]Egertson and Downing, 2003;[START_REF] Jeppesen | The impact of nutrient state and lake depth on topdown control in the pelagic zone of lakes: a study of 466 lakes from the temperate zone to the arctic[END_REF][START_REF] Søndergaard | Water Framework Directive: ecological classification of Danish lakes[END_REF][START_REF] Launois | Selecting fish-based metrics responding to human pressures in French natural lakes and reservoirs: towards the development of a fish-based index (FBI) for French lakes[END_REF][START_REF] Sagouis | Taxonomic versus functional diversity metrics: how do fish communities respond to anthropogenic stressors in reservoirs?[END_REF][START_REF] Jia | Understanding trophic structure variation in fish assemblages of subtropical shallow lakes: Combined effects of ecosystem size, productivity, and disturbance[END_REF][START_REF] Yu | Changes in pelagic fish community composition, abundance, and biomass along a productivity gradient in subtropical lakes[END_REF]. In Europe, probably the most investigated region with regard to fish-based assessment of lacustrine ecological status, few species traits have been successfully tested, most of them related to trophic ecology [START_REF] Ritterbusch | European fish-based assessment reveals high diversity of systems for determining ecological status of lakes[END_REF]. In this regard, and despite comparable ranges of eutrophication, some notable differences occur between China (our study) and Europe. First in contrast with Europe, eutrophication has a weak effect on trophic structure of communities in our study.

Second, some traits react to eutrophication in opposite directions in each region. It is the case for instance for large predatory or piscivorous fishes reported to be increasingly disfavored as eutrophication increases in European lakes [START_REF] Bergstrand | Changes in the fish and zooplankton communities of Ringsjön, a Swedish lake undergoing man-made eutrophication[END_REF][START_REF] Jeppesen | Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient[END_REF][START_REF] Sandström | Effects of eutrophication on young-of-the-year freshwater fish communities in coastal areas of the Baltic[END_REF][START_REF] Søndergaard | Water Framework Directive: ecological classification of Danish lakes[END_REF] [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF], not yet tested in a eutrophication context, are promising. For instance, three reproductive strategies have been described by these authors (opportunistic, periodic, equilibrium) and it could be interesting to study how they favor or not the persistence of species in eutrophic lakes.

In the context of environmental protection, informed decision-making criteria must be based on the biological composition and functional characteristics of the ecosystem [START_REF] Nandi | Nutrient condition and chironomid assemblages in Kolkata, India: assessment for biomonitoring and ecological management[END_REF]. Therefore, the identification of sensitive species and indicative functional traits is critical [START_REF] Petriki | A fish-based index for the assessment of the ecological quality of temperate lakes[END_REF]. Our study indicates that some demersal equilibrium species and their associated functional traits can serve as indicators of eutrophication-induced habitat degradation in lake management in the middle reaches of the Yangtze River. And some species increasing with trophic availability may reflect the extent of eutrophication.

Applying a life history strategy framework to analyze fish community responses to eutrophication

The main hypothesis of this study was that eutrophication negatively affected equilibrium species through decreased environmental stability and/or decreased resources limitation. Our results validated this hypothesis by showing that equilibrium species decreased in abundance with eutrophication and that the proportion of equilibrium species was negatively correlated with eutrophication. It was also shown that among the two other strategies that could potentially be favored by eutrophication, the periodic strategy was the most responsive. Equilibrium species as defined in this study are taxonomically diverse belonging to 7 families so the results are unlikely to be biased by phylogenetic relatedness. Moreover, using the demographic traits without a priori (without defining strategy groups beforehand) revealed that the abundance of species with traits associated with equilibrium strategy (high parental care and large egg diameter) were more negatively correlated with eutrophication. Not only the working hypothesis is validated but it also explains the main changes observed in species abundance. So, it is unlikely that an alternate life history pattern outperforms the one proposed in this study to account for the relationship between eutrophication and demographic traits. [START_REF] Zeug | Relationships between hydrology, spatial heterogeneity, and fish recruitment dynamics in a temperate floodplain river[END_REF] proposed that compared with rivers, lakes are more stable and more favorable to equilibrium species, and in particular those with parental care. For instance, in Africa, the proportion of guarder and bearer species is 60-90 % in the hydrologically stable ancient great lakes (e.g., Victoria Lake), 30-40% in more seasonal lentic habitats such as floodplains, and less than 30% in rivers [START_REF] Bruton | The proportion of different eco-ethological sections of reproductive guilds of fishes in some African inland waters[END_REF]. In average, the percentages observed in the present study (29.2 %) are similar to the ones observed for African seasonal lentic habitats which seems in line with the fact that the habitats are comparable in both regions (seasonal floodplains).

In the analysis of species abundance with different life history strategies, the decline of equilibrium species along the eutrophication gradient was paralleled by the opposite trend for periodic and opportunistic species. This is consistent with the fact that both periodic and opportunistic strategists are better suited to habitats with higher variability [START_REF] Winemiller | Life history strategies, population regulation, and implications for fisheries management[END_REF][START_REF] Mims | Life history theory predicts fish assemblage response to hydrologic regimes[END_REF]. [START_REF] Lin | Eutrophication as a driver of r-selection traits in a freshwater fish[END_REF] found that eutrophication can drive a species in lakes of the Yangtze River floodplain to have higher relative reproductive effort and smaller body size at the population level (life history tends to be more opportunistic). However, in a study on the response of marine fish communities to eutrophication, the abundances of opportunistic species first increased with the rise of chlorophyll a and then tended to be asymptotically stable [START_REF] Pecuchet | From traits to life-history strategies: Deconstructing fish community composition across European seas[END_REF]. It was observed in present study that eutrophication favors periodic species more than opportunistic species. In particular, the relative species richness of periodic strategists increases with eutrophication, while there is no apparent trend for opportunistic species. When looking at species abundances the pattern is weaker, but species with opportunistic life history traits (intermediate values along PCA1) tend to be less responsive to eutrophication than periodic strategists.

The opportunistic life history is favored in density-independent environments or in settings that experience maximum unpredictability [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF].

Higher relative reproductive effort allows opportunistic strategists to quickly occupy habitat after major environmental disturbance [START_REF] Winemiller | Factors driving temporal and spatial variation in aquatic floodplain food webs[END_REF] in particular when hydrological connectivity favors colonization from rivers [START_REF] Zeug | Relationships between hydrology, spatial heterogeneity, and fish recruitment dynamics in a temperate floodplain river[END_REF].

However, almost all of the lakes in this study are disconnected from the Yangtze River by dams or sluices [START_REF] Liu | Estimation of minimum area requirement of riverconnected lakes for fish diversity conservation in the Yangtze River floodplain[END_REF] and there is no advantage for opportunistic strategists in being a better colonist. In the studied lakes most opportunistic strategists were prey to periodic predators such as Culter alburnus, Chanodichthys mongolicus and Chanodichthys dabryi, so that some opportunistic species may be rare or absent in eutrophic lakes with high predation pressure by periodic predators.

Periodic strategists are large sized, late sexually mature, and have high absolute fecundity. When environmental conditions are unfavorable, they can endure years without good recruiting conditions, while very high recruitment success in favorable years compensates for many bad years [START_REF] Winemiller | Patterns of life-history diversification in North American fishes: implications for population regulation[END_REF]. In rivers, the key to successful recruitment of periodic species is that periods of mass reproduction coincide with seasons of abundant resources. The Yangtze is a seasonal river, offering a favorable background for periodic strategists (Wang et al., 2016). Moreover, the studied lakes are also seasonal with regard to temperature and primary productivity, with most periodic species breeding in spring and summer. Under the dual effects of high temperature season and eutrophication, nutrients are more easily converted into the primary productivity of lakes [START_REF] Guan | Eutrophication changes in fifty large lakes on the Yangtze Plain of China derived from MERIS and OLCI observations[END_REF], thus providing larvae with more abundant food resources through a bottom-up process. Eutrophication also increases the seasonality of lake primary productivity [START_REF] Wei | Characteristics of Phytoplankton Production in Wet and Dry Seasons in Hyper-Eutrophic Lake Taihu, China[END_REF].

Periodic species could be favored over opportunistic species if mortality of eggs and larvae increases at a higher rate than adult mortality [START_REF] Winemiller | Why do most fish produce so many tiny offspring?[END_REF].

It could be the case if hypoxia and/or toxic algal bloom are spatially localized such that the less mobile life stages cannot swim out of detrimental conditions (von [START_REF] Nordheim | Lethal effect of filamentous algal blooms on Atlantic herring (Clupea harengus) eggs in the Baltic Sea[END_REF]. In addition, eggs and larvae are generally more vulnerable to hypoxia than the other stages because they have higher demand in oxygen [START_REF] Müller | Fish habitat requirements as the basis for rehabilitation of eutrophic lakes by oxygenation[END_REF]). In the process of supplanting the dominance of equilibrium species, periodic strategists hold a greater advantage than opportunism strategists, probably because periodic strategists are in a more favorable position in competition for resources and in predator-prey relationships.

This study expands the applicability of the triangular life history theoretical framework in two ways: by adding lakes to the aquatic ecosystems where it was successfully tested and by adding eutrophication to the human impacts for which it provides a pertinent assessment.

Existing researches on the application of life history theory frameworks have focused on rivers [START_REF] Olden | Life-history strategies predict fish invasions and extirpations in the Colorado River Basin[END_REF][START_REF] Tedesco | River hydrological seasonality influences life history strategies of Constance example revisited[END_REF][START_REF] Mims | Life history theory predicts fish assemblage response to hydrologic regimes[END_REF][START_REF] Mims | Fish assemblages respond to altered flow regimes via ecological filtering of life history strategies[END_REF][START_REF] Hitt | Fish life history trends indicate increasing flow stochasticity in an unregulated river[END_REF], floodplains [START_REF] Zeug | Relationships between hydrology, spatial heterogeneity, and fish recruitment dynamics in a temperate floodplain river[END_REF][START_REF] Dembkowski | Comparison of fish assemblages in two disjoined segments of an oxbow lake in relation to connectivity[END_REF]Miranda, 2011), oceans (Vila-Gispert et al., 2002;[START_REF] Pecuchet | From traits to life-history strategies: Deconstructing fish community composition across European seas[END_REF], and estuaries (Teichert et al., 2017). The framework has been particularly tested and validated in rivers with regard to hydrological variability. For instance, high hydrological variability and unpredictability re associated with the prevalence of opportunistic strategists [START_REF] Hitt | Fish life history trends indicate increasing flow stochasticity in an unregulated river[END_REF], basins with strong seasonality have a higher proportion of periodic species [START_REF] Tedesco | River hydrological seasonality influences life history strategies of Constance example revisited[END_REF], and flow stabilization favors the dominance of equilibrium strategy [START_REF] Mims | Fish assemblages respond to altered flow regimes via ecological filtering of life history strategies[END_REF]. In contrast, few studies focused on lakes and when they did it was to compare stable (lakes) with variable (rivers)

ecosystems [START_REF] Zeug | Relationships between hydrology, spatial heterogeneity, and fish recruitment dynamics in a temperate floodplain river[END_REF] but not to address patterns observed among lakes. To our knowledge, this study is the first to test the triangular life history strategy framework using lakes with contrasted environmental features. By showing that life history strategies predict how fish assemblages respond to lake eutrophication, we demonstrated that the triangular life history framework is pertinent over a large range of freshwater habitats, not just rivers.

The triangular life history strategy framework has proven useful to assess several anthropogenic disturbances impacting fish communities such as hydrological control [START_REF] Mims | Fish assemblages respond to altered flow regimes via ecological filtering of life history strategies[END_REF], land use change [START_REF] Manfrin | Effect of river restoration on life-history strategies in fish communities[END_REF], fishing [START_REF] Rose | Compensatory density dependence in fish populations: importance, controversy, understanding and prognosis[END_REF], exotic species invasion [START_REF] Olden | Life-history strategies predict fish invasions and extirpations in the Colorado River Basin[END_REF], and climate change [START_REF] Pecuchet | From traits to life-history strategies: Deconstructing fish community composition across European seas[END_REF]. In this study we showed that human driven eutrophication differentially affected species according to their life history strategy in the way predicted by the triangular life history strategy framework, reinforcing its usefulness as an ecological indicator in a large array of human perturbations. More generally, our study emphasizes the importance of species traits to assess, explain, and predict community responses to human and natural perturbations.

Conclusion

Based on the previous research, this chapter discusses the impact of eutrophication as a key environmental factor on the species abundance, taxonomic diversity, functional diversity, species structure, functional trait composition and special functional groups (life history strategies) of fish communities from the multi-dimensional level of fish communities. Among them, the taxonomic diversity, especially the difference in species richness, is difficult to be directly explained by the eutrophication gradient. However, species abundance and biomass were positively correlated with eutrophication, and functional diversity was negatively correlated. Given the strong response of functional diversity to eutrophication, this demonstrates that eutrophication affects fish communities through environmental filtering. After analyzing the composition of species structure and functional traits, this study found that eutrophication essentially affects species composition through trait filtering and thus affects fish community structure. From the perspective of the effect of environmental filtering on functional traits, eutrophication can increase the dominance of tolerant species of pelagic zooplanktivores traits that are more suitable for eutrophic turbid environment by affecting habitat quality and changing resource availability, while decrease the dominance of species relying on benthic habitats. In an overall analysis of species functional traits, we found that the composition of fish community life history strategies seemed to be altered by the direct impact of eutrophication, i.e., a significant decline in equilibrium species and an increase in periodic and opportunistic species. We therefore tested our hypotheses using a triangular continuous life history strategy framework, and our findings suggest that eutrophication negatively impacts equilibrium species by reducing environmental stability and reducing resource constraints. In the process of supplanting the dominance of equilibrium species, periodic strategists hold a greater advantage than opportunism strategists, probably because periodic strategists are in a more favorable position in competition for resources and in predator-prey relationships.

The application of the theoretical framework of life history strategies is also extended in the process of testing the hypotheses. In summary, this chapter mainly reveals the mechanism by which eutrophication affects the functional traits of fish communities through the environmental filtering process, emphasizing that functional structure fundamentally explains the response process of fish communities to environmental stress.

Chapter4 Environmental pressure mediates interspecies interactions to drive changes in fish community diversity

Introduction

Despite worldwide efforts to protect vulnerable species and habitats, global biodiversity loss is accelerating at an unprecedented rate [START_REF] Maxwell | Biodiversity: The ravages of guns, nets and bulldozers[END_REF]. Species extinctions and changes in community composition, both at global and regional scales, have brought widespread attention to the consequences of biodiversity loss on ecosystem integrity, function and services [START_REF] Tilman | Productivity and sustainability influenced by biodiversity in grassland ecosystems[END_REF]. Ecologists are therefore increasingly interested in research in the fields of ecology and conservation that explore the relationship between biodiversity and ecosystem function. The relationship between species diversity and ecosystem stability has been an ongoing debate in the field of ecology since the middle of the last century [START_REF] Macarthur | Fluctuations of animal populations and a measure of community stability[END_REF]. When ecosystem stability is defined as a trend of less fluctuation, species richness stabilizes an overall property of a community, such as productivity or total community abundance [START_REF] Tilman | Productivity and sustainability influenced by biodiversity in grassland ecosystems[END_REF]. High species richness at the community level should be accompanied by higher rates of ecosystem processes, often referred to as ecosystem functioning, broadly defined as energy transfer and material cycling [START_REF] Lawton | What do species do in ecosystems?[END_REF][START_REF] Chapin | Biotic control over the functioning of ecosystems[END_REF][START_REF] Ghilarov | Ecosystem functioning and intrinsic value of biodiversity[END_REF][START_REF] Hooper | Effects of biodiversity on ecosystem functioning: a consensus of current knowledge[END_REF]. Higher ecosystem functioning is generally thought to be the result of resource use complementarity among functionally diverse species (Petchey and Gaston, 2002a;[START_REF] Petchey | Integrating methods that investigate how complementarity influences ecosystem functioning[END_REF][START_REF] Hooper | Effects of biodiversity on ecosystem functioning: a consensus of current knowledge[END_REF][START_REF] Burkepile | Herbivore species richness and feeding complementarity affect community structure and function on a coral reef[END_REF]. Therefore, in essence, the functional diversity of the community can maximize the utilization of resources to achieve ecosystem functions, rather than the number of species themselves [START_REF] Hooper | Effects of biodiversity on ecosystem functioning: a consensus of current knowledge[END_REF]Mokany et al., 2008).

In fish communities, the simplest model for the relationship between abundance (or biomass) and species abundance is the "more individuals hypothesis" [START_REF] Srivastava | Why more productive sites have more species: an experimental test of theory using tree-hole communities[END_REF]. The hypothesis states that high productivity leads to higher overall densities and thus more species. But this hypothesis ignores the interactions among organisms, that is, competition and predation among species. However, due to competition, there are species in the biome that have a competitive advantage [START_REF] Tilman | Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly[END_REF]. Competition will cause dominant species to squeeze the ecological niches of other species, resulting in a decline in the number of species and a simplification of the community structure, and the rise of community individuals may also be limited to individual species with strong adaptability [START_REF] Hsu | Modulation of aggressive behaviour by fighting experience: mechanisms and contest outcomes[END_REF][START_REF] Laskowski | The making of winners (and losers): how early dominance interactions determine adult social structure in a clonal fish[END_REF].

In stable ecosystems, overpopulation of dominant species is usually inhibited, such as predators, resource constraints, or disturbance mechanisms, which can maintain community species diversity [START_REF] Caswell | Predator-mediated coexistence: a nonequilibrium model[END_REF][START_REF] Chesson | Mechanisms of maintenance of species diversity[END_REF][START_REF] Ritchie | Predator interactions, mesopredator release and biodiversity conservation[END_REF]. However, under the pressure of external disturbance, when the restriction is broken, the competitive advantage species maintain a higher competitive advantage.

This can negatively impact other competing species through population increases, thereby driving loss of local fish species richness and changes in community structure [START_REF] Nagelkerken | Species interactions drive fish biodiversity loss in a high-CO2 world[END_REF][START_REF] Ramírez-Ortiz | Reduced fish diversity despite increased fish biomass in a Gulf of California Marine Protected Area[END_REF].

The negative impacts of biodiversity loss on ecosystem function are often trophicmediated, with the loss of a few macrofauna species having similar impacts as the loss of many plant species [START_REF] Duffy | Biodiversity loss, trophic skew and ecosystem functioning[END_REF]. The decline of species richness with the increase of resource availability has been studied earlier in plant communities and has been widely verified, which is a common pattern in plant communities [START_REF] Stevens | Impact of nitrogen deposition on the species richness of grasslands[END_REF][START_REF] Fraser | Worldwide evidence of a unimodal relationship between productivity and plant species richness[END_REF][START_REF] Midolo | Impacts of nitrogen addition on plant species richness and abundance: A global meta-analysis[END_REF]. [START_REF] Band | Assessing the roles of nitrogen, biomass, and niche dimensionality as drivers of species loss in grassland communities[END_REF] put forward the "biomassdriven competition hypothesis" in their research on explaining the decline of species richness with the increase of resource availability in plant communities. And they observed a unimodal relationship between plant biomass and species richness, attributing this pattern to increased biomass leading to increased interspecific competition leading to a decline in species richness. This pattern has rarely been found in fish communities, but there is some evidence that it may exist in fish communities as well. [START_REF] Nagelkerken | Species interactions drive fish biodiversity loss in a high-CO2 world[END_REF] found that near a CO2 outlet on the seafloor, fish are rich in food resources due to seawater acidification. At the same time, the decline in the number of predators has led to a sharp increase in the number of a few competitive species, resulting in a significant decrease in competing species, resulting in a loss of local fish diversity. Ramírez-Ortiz et al. ( 2020) observed decreases in fish species richness, functional richness, and functional dispersion despite increases in fish biomass in marine protected areas in the Gulf of California. And they speculate that competition between species, habitat loss and continued fishing pressure may have contributed to this result.

According to the research in the previous chapters, under the influence of environmental filtering, multiple environmental pressures undoubtedly break resource constraints and change the interaction between species by filtering species traits. And we also observed that eutrophication increased the dominance of a few dominant species by changing resource availability, and increased fish biomass as a whole.

Although we did not observe a direct relationship between eutrophication or other environmental stresses and species richness, this supports our adoption of the "biomassdriven competition hypothesis" to explain the impact of environmental stresses on fish community diversity. Therefore, this study tested this hypothesis on the one hand by exploring the relationship between biomass and taxonomy and functional diversity. On the other hand, structural equation modeling was applied to explore the bottom-up effects of multiple environmental factors on fish communities through the availability of food resources. Thus, revealing the mechanism by which the multiple environmental pressures of lakes in the middle reaches of the Yangtze River mediate the interaction between species and drive the diversity of fish communities.

Materials and methods

Study area and sample setting

See Chapter 2.2.1 for the setting of the study area and sample points.

Sample and data collection

For the collection and acquisition methods of fish community data, fish functional traits, total phosphorus concentration, lake area and agricultural land data, see 2.2.2.

Phytoplankton biomass and zooplankton biomass in each lake were also investigated in this study at the same sampling sites as fish and water quality. The method of converting plankton and plant density into biomass according to cell volume was adopted, and the survey method of plankton density was carried out according to the standard specification [START_REF] Zhang | Handbook of Fishery Natural Resources Survey in Inland Waters[END_REF].

For the investigation of phytoplankton, when sampling, quantitative samples were collected from the measured water layer with 1000-2000 mL of water. Add 10-15 mL of Lugol's iodine solution (5%) per liter, shake well and fix, and concentrate to 30 mL after resting in the laboratory for 48 hours. During microscopic examination, take 0.1 mL sample, put it into a 0.1 mL counting frame, and cover it with a cover glass. Under the 200-400 times microscope, the visual field counting method or the whole slide counting method are used for quantitative detection. When counting under the microscope, each sample should not be less than 2 slides, and when the count values of the two slides are similar, take the average value as the measurement result. When the error of the two slides exceeds 15%, count the third slide, and take the average of the two slides with similar results (the error is less than 15%) as the measurement result.

For the survey of zooplankton, quantitative samples were collected in 20 L water samples in batches with a 5 L water collector. Then use No. 13 plankton mesh (pore size 0.112 mm) to filter and concentrate, put the filtrate into a 50 ml specimen bottle, wash the filter screen with filtered water for 3 times, and put the filtrate into a 50 ml specimen bottle. This is a quantitative sample of Cladocera and Copepods. Quantitative samples of phytoplankton for the quantification of protozoa, rotifers and nauplii. Add about 5 ml of 5% formaldehyde solution to the specimen bottle to kill and fix the zooplankton. During microscopic examination, the identification of protozoa, rotifers and nauplii is to use phytoplankton samples, suck out 0.1 mL samples, place them in a 0.1 mL counting frame, cover them with a cover slip, and count the entire slice under a medium-power microscope. Count two slides for each bottle of sample, and take the average value; for rotifers: use phytoplankton samples, suck out 1 mL of sample, place it in a 1 mL counting frame, and count the whole slide. Count two slides for each bottle of sample, and take the average value; for cladocera and copepods, use a 5 mL counting frame to divide the samples into several times and count them all. If there are too many individuals in the sample, dilute the sample to 30 mL or 50 mL, and use a 5 mL counting frame for whole slide counting. Two slides were counted per vial of sample.

Data analysis

In this study, regression analysis was used to investigate the relationship between fish biomass and fish community taxonomy and functional diversity, thereby testing the "biomass-driven competition hypothesis". Path analysis among four types of environmental factors, phytoplankton biomass, zooplankton biomass and fish biomass were conducted using structural equation modeling to reveal the bottom-up effects of nutrient-mediated environmental factors on fish biomass.

Relationship between fish biomass and biodiversity

In this study, regression analysis was used to analyze the fish biomass as a predictive variable, and species richness, Pielou evenness index, Simpson dominance index, function richness, function evenness, function dispersion and function redundancy were selected as response variables. Among them, fish biomass is represented by biomass per unit effort (BPUE, g net -1 h -1 ). The species diversity index is calculated using the function "diversity" of the R package "vegan" [START_REF] Oksanen | Community Ecology Package (R Package Version 2.6-2)[END_REF], and the functional diversity index is calculated using the function "dpFD" of the R package "FD" [START_REF] Laliberté | Measuring functional diversity from multiple traits, and other tools for functional ecology[END_REF].

After exploring the relationship between each index and fish biomass, the optimal regression model was selected, and quadratic regression was used for species richness, functional richness and fish biomass, and linear regression was used for other indexes and fish biomass. In regression analysis, all variables were ln transformed to make the data more fit for normal distribution.

Structural equation models reveal bottom-up influences

This study aims to explain the bottom-up effects of multiple environmental factors on fish biomass changes. As nutrient availability is involved, the complex relationship between environmental stress, nutrient status, various prey organisms and fish biomass needs to be analyzed. Such analyzes require statistical tools that can handle the increasing scale and resolution of available data, as well as the complexity of interactions between biodiversity, ecosystem function, and environmental drivers.

Structural equation modeling (SEM) is an appropriate statistical framework to account

for this complexity because it allows simultaneous testing of hypotheses for causal relationships between multiple driver and response variables [START_REF] Grace | Structural equation modeling and natural systems[END_REF][START_REF] Daam | Establishing causal links between aquatic biodiversity and ecosystem functioning: Status and research needs[END_REF].

This study examined the relationship between fish biomass and zooplankton biomass, phytoplankton biomass, and various environmental factors using structural equation modeling. Since assumptions about causality in structural equation modeling must be established a priori, we assumed that total biomass could be explained by both zooplankton biomass and phytoplankton biomass through trophic cascades. At the same time, in order to prevent the nutrient-mediated process from being fully considered in the system, we set the relationship between total phosphorus and fish biomass. In addition, in order to reflect the top-down effect, we set the effect of total phosphorus on phytoplankton biomass and the effect of phytoplankton biomass on zooplankton biomass. As in previous studies, we verified that land use, lake morphology, and lake proliferation patterns are more primary environmental pressures than eutrophication.

Therefore, we also set the impact of agricultural land, lake area and lake fishery production on total phosphorus. Since we wanted to study the bottom-up effects of nutrient-mediated environmental factors on fish biomass in this study, we did not set the impact of other environmental factors on fish biomass except total phosphorus. This effect was verified in previous studies, and most of these effects were achieved by increasing eutrophication levels.

In the structural equation model, the land use data were transformed by square root arcsine, and all other variables were transformed by ln to make the data more consistent with the normal distribution. Structural equation modeling was run using the function "psem" of the R package "piecewiseSEM" [START_REF] Lefcheck | piecewiseSEM: Piecewise structural equation modelling in r for ecology, evolution, and systematics[END_REF]. The species evenness of fish communities showed a significant downward trend with the increase of biomass. The species dominance of fish communities showed a significant upward trend with the increase of biomass.

Results

Relationship between fish biomass and biodiversity

Figure 4-3 Regression analysis between fish biomass and Simpson dominance index

All the variables are transformed by ln (). The functional evenness of fish communities showed a significant downward trend with the increase of biomass.

Figure 4-5 Regression analysis between fish biomass and functional evenness index

All the variables are transformed by ln (). The functional dispersion of fish communities showed a significant downward trend with the increase of biomass.

Structural equation models reveal bottom-up effects

Pathways in structural equation modeling can be roughly divided into three parts (Figure 45678). The first is the impact of the main variables selected from the three types of environmental variables of lake morphology, land use and proliferation mode, lake area, agricultural land and annual fishery production on the main indicator of eutrophication, total phosphorus. The second is the bottom-up effect of total phosphorus on fish biomass through phytoplankton biomass and zooplankton biomass. The third is the direct impact of total phosphorus on fish biomass. In the first part, the annual production of lake fishery has a significant and strong positive effect on total phosphorus (path coefficient is 0.6846, P<0.01). This is consistent with the results analyzed in the previous chapters, that is, the occurrence of lake eutrophication is mainly related to the lake fishery model. The direct positive effects of TP on phytoplankton biomass (path coefficient 0.4450, P<0.05) and phytoplankton biomass on zooplankton biomass (path coefficient 0.4153, P<0.05) were both significant in the second part. However, the direct impact of zooplankton and plant biomass on fish biomass was not significant. Although not significant, phytoplankton biomass had a negative effect on fish biomass (path coefficient of -0.2240, P=0.34), and zooplankton biomass had a positive effect on fish biomass (path coefficient of 0.1536, P=0.54). At the same time, total phosphorus also had a positive effect on zooplankton biomass (path coefficient was 0.3567, P=0.0538). Compared with the indirect effect of nutrient level on fish biomass, total phosphorus had a significant direct positive effect on fish biomass (path coefficient 0.4839, P<0.05). The R 2 of each response variable in the whole model was 0.44 for total phosphorus, 0.20 for phytoplankton biomass, 0.43 for zooplankton biomass, and 0.25 for fish biomass, suggesting that there are still some predictors or pathways not considered in the model. The solid line represents significant (P<0.05), while the dotted line represents insignificant

Discussion

This study aimed to explore the interspecies interactions, especially the impact of species competition on taxonomic and functional diversity, through the relationship between biomass and community diversity. On this basis, explore the top-down effects of multiple environmental pressures, especially trophic drivers, on fish community biomass. To summarize the results of the study, with the increase of fish biomass, all aspects of diversity index indicated the unity of community structure, implying the increase of interspecific competition. However, the species and functional richness showed a first increase, and then a downward trend after the biomass reached a certain level. Environmental stresses, especially rising eutrophication, have increased fish biomass, further increasing interspecific competition.

Biomass-driven competition hypothesis

Theoretical and ecological models suggest that higher levels of intraspecific competition and lower levels of interspecific competition (lower niche overlap) promote species richness in ecological communities [START_REF] Hutchinson | Homage to Santa Rosalia or why are there so many kinds of animals?[END_REF][START_REF] Macarthur | Limiting similarity convergence and divergence of coexisting species[END_REF][START_REF] Finke | Niche partitioning increases resource exploitation by diverse communities[END_REF]. Due to the existence of interspecific competition, there are species with competitive advantages in the biome [START_REF] Tilman | Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly[END_REF].

Competition will cause dominant species to squeeze the ecological niches of other species, resulting in a decline in the number of species and a simplification of the community structure, and the rise of community individuals may also be limited to individual species with strong adaptability [START_REF] Hsu | Modulation of aggressive behaviour by fighting experience: mechanisms and contest outcomes[END_REF][START_REF] Laskowski | The making of winners (and losers): how early dominance interactions determine adult social structure in a clonal fish[END_REF].

In stable ecosystems, the overgrowth of dominant species is usually inhibited, such as predators, resource constraints, or disturbance mechanisms, which can maintain community species diversity [START_REF] Caswell | Predator-mediated coexistence: a nonequilibrium model[END_REF][START_REF] Chesson | Mechanisms of maintenance of species diversity[END_REF][START_REF] Ritchie | Predator interactions, mesopredator release and biodiversity conservation[END_REF]. However, under the pressure of external disturbance, when the restriction is broken, the competitive advantage species maintain a higher competitive advantage.

This can negatively impact other competing species through population increases, thereby driving loss of local fish species richness and changes in community structure [START_REF] Nagelkerken | Species interactions drive fish biodiversity loss in a high-CO2 world[END_REF][START_REF] Ramírez-Ortiz | Reduced fish diversity despite increased fish biomass in a Gulf of California Marine Protected Area[END_REF]. This population surge due to competitive advantage of an individual species also results in an increase in total abundance, or total biomass, for the biome as a whole. It is accompanied by intensified competition among species and loss of species richness and diversity.

This phenomenon and theory have led ecologists to frequently find that increases in species abundance or biomass led to decreases in community richness and diversity.

This phenomenon was first systematically studied and explained in plant communities. [START_REF] Band | Assessing the roles of nitrogen, biomass, and niche dimensionality as drivers of species loss in grassland communities[END_REF] put forward the "biomass-driven competition hypothesis" in their research on explaining the decline of species richness with the increase of resource availability in plant communities. And they observed a unimodal relationship between plant biomass and species richness, attributing this pattern to increased biomass leading to increased interspecific competition leading to a decline in species richness.

This pattern has rarely been found in fish communities, but there is some evidence that this pattern may also exist in fish communities [START_REF] Nagelkerken | Species interactions drive fish biodiversity loss in a high-CO2 world[END_REF][START_REF] Ramírez-Ortiz | Reduced fish diversity despite increased fish biomass in a Gulf of California Marine Protected Area[END_REF]. And this pattern of total biomass increase leading to biodiversity decline has been attributed directly or indirectly to changes in interspecies interactions, possibly due to reduced predation or increased interspecific competition.

For this study, in fact, before this chapter, we have explained that eutrophication leads to a few traits being favored through the research on the filtering process of functional traits of environmental factors. This leads to increased dominance of a small number of functionally similar species, ultimately leading to functional homogenization of fish communities. Although we did not quantify the degree of interspecific competition in lakes of different trophic states, we observed an increase in total biomass and a decrease in functional dispersion with increasing eutrophication. The reason for the simultaneous biomass rise was due to the dominance of a few species, which further suggested that the interspecific competition in our fish communities was more intense with the level of eutrophication. Unfortunately, we did not observe a direct relationship between eutrophication and species richness.

Therefore, in this chapter, we propose a "biomass-driven competition hypothesis" that has been validated in plant communities based on previous findings. That is, the increase in the biomass of fish communities is essentially due to the increase in the abundance of a few competitively dominant species, which further drives the interspecific competition in the community, resulting in a decline in biodiversity and species richness. Because the connection between biodiversity and ecosystem function is realized essentially through functional diversity [START_REF] Hooper | Effects of biodiversity on ecosystem functioning: a consensus of current knowledge[END_REF]Mokany et al., 2008). Therefore, this study explored the relationship between biomass and community diversity from the perspective of taxonomic diversity and functional diversity. First of all, this study found a unimodal relationship between fish biomass, species richness, and functional richness, and the fitting degree was high. This is very similar to the pattern found by [START_REF] Band | Assessing the roles of nitrogen, biomass, and niche dimensionality as drivers of species loss in grassland communities[END_REF] in plant communities. At lower biomass levels, an increase in biomass resulted in an increase in richness due to less overlapping niches, resulting in less interspecific competition. This stage seems to be more in line with the "more individuals hypothesis" [START_REF] Srivastava | Why more productive sites have more species: an experimental test of theory using tree-hole communities[END_REF], that is, more abundance leads to an increase in species richness. However, after the community biomass reaches a certain level, the level of interspecific competition increases to the exclusion of competitive inferior species, resulting in a decline in species richness and functional richness. In addition, in order to verify that the increase of biomass does lead to the increase of the level of interspecies competition, species evenness, species dominance, functional evenness, functional dispersion and functional redundancy were used to reflect the intensity of interspecies competition in the community from various aspects.

The results were as expected. As the biomass increased, the community species and functional evenness and functional dispersion showed a significant linear decrease, which suggested the overlap of ecological niches in the community. The species dominance and functional redundancy of the community showed a significant linear increase, implying the increased dominance of a few species and the high redundancy of functions.

Overall, this study not only verified the causal relationship between biomass and richness in the "biomass-driven competition hypothesis", that is, the unimodal relationship between biomass and species richness and functional richness. It is also supplemented to verify that the increase in biomass does represent an increase in interspecific competition in the community, that is, interspecific competition drives the reduction of species and functional richness.

Nutrition-mediated loss of biodiversity

Although this study verified the main part of the "biomass-driven competition hypothesis", that is, community biomass caused the reduction of species and functional richness by driving the intensification of interspecific competition. But in fact, when [START_REF] Band | Assessing the roles of nitrogen, biomass, and niche dimensionality as drivers of species loss in grassland communities[END_REF] proposed this hypothesis, it was to explain the phenomenon that the species richness in plant communities decreased with the increase of resource availability. While the negative effects of biodiversity loss on ecosystem function are indeed often trophic-mediated, the loss of a few macrofauna species has similar effects as the loss of many plant species [START_REF] Duffy | Biodiversity loss, trophic skew and ecosystem functioning[END_REF]. The decline of species richness with the increase of resource availability has been studied earlier in plant communities and has been widely verified, which is a common pattern in plant communities [START_REF] Stevens | Impact of nitrogen deposition on the species richness of grasslands[END_REF][START_REF] Fraser | Worldwide evidence of a unimodal relationship between productivity and plant species richness[END_REF][START_REF] Midolo | Impacts of nitrogen addition on plant species richness and abundance: A global meta-analysis[END_REF]. This pattern has rarely been found in fish communities, but there is some evidence that it may exist in fish communities as well. [START_REF] Nagelkerken | Species interactions drive fish biodiversity loss in a high-CO2 world[END_REF] found that near a CO2 outlet on the seafloor, fish are rich in food resources due to seawater acidification. At the same time, the decline in the number of predators has led to a sharp increase in the number of a few competitive species, resulting in a significant decrease in competing species, resulting in a loss of local fish diversity.

Likewise, the positive correlation between eutrophication and fish biomass has been verified in previous chapters of this study. A positive correlation between eutrophication and some zooplankton-eating and piscivorous fishes was also confirmed.

Therefore, in this study, we hope to reveal how nutrients mediate biomass changes and thus drive diversity loss by exploring the direct or indirect effects of eutrophication on fish biomass through bottom-up trophic cascades. In structural equation modeling, a cascaded positive correlation among TP, phytoplankton biomass, and zooplankton biomass was confirmed. However, the effect of phytoplankton and zooplankton biomass on fish biomass was not significant. Although the direct negative effects of phytoplankton on fish biomass and the direct positive effects of zooplankton on fish biomass are ecologically significant. Phytoplankton blooms may lead to water hypoxia and harmful algal blooms, which may negatively affect fish communities, and a positive correlation between zooplankton as direct food for fish and fish biomass is expected.

However, the effect through the trophic cascade appears to be weak compared to the significant direct positive effect of TP on fish biomass. This aspect proves that nutrient availability does lead to the increase of fish biomass, but there seem to be some underlying processes that have not been revealed by our study.

It is worth noting that the construction of the structural equation model is a priori, that is, this study only constructed the part we are interested in, and did not incorporate all possible paths into the model. Examples include direct or indirect effects of land use, lake morphology and proliferation patterns on fish biomass. However, in previous studies, lake fishery proliferation intensity was confirmed to be positively correlated with fish biomass and was the main cause of lake eutrophication, but it was not considered in this study. Lake area and agricultural land use may also have an effect on fish biomass, although no correlation was found in previous direct correlation analyses.

Conclusion

The research in this chapter is based on the research in the previous chapter on the environmental filtering effect of multiple environmental pressures on fish communities in the lakes of the middle reaches of the Yangtze River, focusing on the role of species interactions on fish communities. The research mainly focuses on the mechanism by which environmental pressure mediates the interaction between species to drive the diversity change of fish community. Although no direct relationship between species richness and environmental factors was found in previous studies, the impact of environmental factors on fish community diversity could be explained by interspecies interactions. First of all, the relationship between biomass and species evenness, species dominance, functional evenness, functional dispersion, and functional redundancy, as well as previous studies, confirmed that the increase of community biomass was mainly caused by the increase of a few species, which increased the competition between species. Second, a unimodal relationship between fish biomass and species and functional richness was identified through a test of the "biomass-driven competition hypothesis". That is to say, when the biomass (the degree of interspecific competition) reaches a certain level, the richness will decrease. Finally, it was established that the process by which biomass drives competition leading to loss of community diversity is mediated by nutrient availability, with eutrophication being the main cause of biomass rise. This study reveals the mechanism by which multiple environmental factors mediate species interactions to drive changes in fish community diversity. On the one hand, it discusses the specific role of species interaction on the diversity of fish community construction in lakes in the middle reaches of the Yangtze River. On the other hand, environmental filtering is still the main factor mediating the process. This lays the foundation for future research on the relative importance of environmental filtering and inter-species interactions in fish community construction.

Chapter5 Key predictors of fish communities and their applications in ecosystem management

Introduction

With the development of ecological theory, and under the background of global change and the impact of rapid changes in biodiversity, predicting the diversity of biological communities and the distribution pattern of species has become one of the research hotspots in the field of theoretical and applied ecology [START_REF] Guo | A model study on fish distribution pattern and ecosystem structure and function in Chinese lakes[END_REF]. With the development of ecosystem management, it has been proven that the management of lake fish communities only focusing on one or more species of fish has certain limitations, and it needs to be managed at the level of the entire ecosystem. Therefore, how to recognize the relationship between fish communities and the environment is an important part of ecosystem management [START_REF] Cheng | The relationship between fish communities and main environmental factors in the Yangtze River Basin and its prediction model[END_REF]. Understanding species composition distributions and community diversity patterns in ecosystems is a necessary prerequisite for understanding ecosystem functions and processes, as well as conservation and spatial planning issues (Reiss et al., 2011). In the context of today's intensified global changes, ecologists' research on models applied to ecosystem management has not been limited to quantitatively describing the relationship between species and environmental factors. More attention is paid to the application of models to predict the distribution pattern of species composition, community structure and diversity. In ecosystem management, the key to effective management and conservation of biological resources is to identify key predictors in the process of predicting species distribution patterns and diversity on a larger scale.

Random Forest (RF) is an algorithm developed from Classification and Regression Trees (CART) and packaging methods [START_REF] Breiman | Random forests[END_REF]. The algorithm has increasingly prominent applications in remote sensing [START_REF] Lawrence | Mapping invasive plants using hyperspectral imagery and Breiman Cutler classifications (RandomForest)[END_REF], forestry [START_REF] Falkowski | Characterizing forest succession with lidar data: An evaluation for the Inland Northwest, USA[END_REF], ecology [START_REF] Cutler | Random forests for classification in ecology[END_REF][START_REF] Evans | Gradient modeling of conifer species using random forests[END_REF][START_REF] Murphy | Quantifying Bufo boreas connectivity in Yellowstone National Park with landscape genetics[END_REF], and climate change [START_REF] Prasad | Newer classification and regression tree techniques: bagging and random forests for ecological prediction[END_REF][START_REF] Rehfeldt | Empirical analyses of plant-climate relationships for the western United States[END_REF]. In ecological problems, an important application of random forest model is the evaluation of variable importance [START_REF] Cutler | Random forests for classification in ecology[END_REF]. Most statistical methods for regression and classification typically measure variable importance indirectly using criteria such as statistical significance and the Akaike information criterion [START_REF] Cutler | Random forests for classification in ecology[END_REF]. The random forest model takes a completely different approach. In the random forest model, the Gini coefficient is generally used to evaluate the importance of variables. The Gini coefficient can be interpreted as the ability of a variable to distinguish different groups [START_REF] Oh | Estimating neuronal variable importance with random forest[END_REF], which may be more suitable for small samples [START_REF] Archer | Empirical characterization of random forest variable importance measures[END_REF]. Gradient Forest (GF) is a highly flexible, nonparametric, and nonlinear method for quantifying the magnitude and pattern of changes in biodiversity and multispecies composition along gradients of environmental variables [START_REF] Ellis | Gradient forests: calculating importance gradients on physical predictors[END_REF][START_REF] Pitcher | Exploring the role of environmental variables in shaping patterns of seabed biodiversity composition in regional-scale ecosystems[END_REF]Fabrice et al., 2018). The method also assesses predictor importance and complements existing techniques such as variance partitioning among predictors.

Gradient forest models are an ensemble of random forest models [START_REF] Breiman | Random forests[END_REF].

Since gradient forest models are based on random forest models, this machine learning approach can capture very complex relationships between predictor and response variables [START_REF] Ellis | Gradient forests: calculating importance gradients on physical predictors[END_REF].

Initially, it was assumed that the response of the natural environment to gradual changes is stationary, but this is not the case (Scheffer et al., 2001;[START_REF] Scheffer | Catastrophic regime shifts in ecosystems: linking theory to observation[END_REF]. Many studies have shown that the perturbation responses of ecosystems and biomes to environmental gradients have threshold effects rather than simple linear relationships [START_REF] Groffman | Ecological thresholds: the key to successful environmental management or an important concept with no practical application?[END_REF][START_REF] Richardson | Estimating ecological thresholds for phosphorus in the Everglades[END_REF][START_REF] Majumder | Inferring critical thresholds of ecosystem transitions from spatial data[END_REF]. Thresholds are the points at which small changes in driving forces lead to disproportionate changes in ecosystems (King and Baker;2014). Beyond this threshold, ecosystems undergo major and even irreversible changes [START_REF] Sonderegger | Using SiZer to detect thresholds in ecological data[END_REF][START_REF] Scheffer | Early-warning signals for critical transitions[END_REF]. Such changes in lakes may have created strong selective pressures that resulted in large-scale changes in fish communities and ecosystems [START_REF] King | How novel is too novel? Stream community thresholds at exceptionally low levels of catchment urbanization[END_REF]. Threshold detection in fish communities and ecosystems has attracted much attention and considerable effort from ecologists. Threshold detection is widely used in the development of water quality standards, habitat protection, identification of species conservation goals, ecosystem restoration, and aquatic ecosystem management [START_REF] King | Integrating bioassessment and ecological risk assessment: an approach to developing numerical water-quality criteria[END_REF][START_REF] Richardson | Estimating ecological thresholds for phosphorus in the Everglades[END_REF][START_REF] Majumder | Inferring critical thresholds of ecosystem transitions from spatial data[END_REF]. The determination of ecological thresholds is of great significance for lake management and can provide early signals of ecosystem transformation [START_REF] Munson | Ecosystem thresholds, tipping points, and critical transitions[END_REF]. From the perspective of species and function conservation, ecological thresholds can provide important references for setting conservation goals [START_REF] Brejão | Threshold responses of Amazonian stream fishes to timing and extent of deforestation[END_REF][START_REF] Zhang | Threshold responses in the taxonomic and functional structure of fish assemblages to land use and water quality: A case study from the Taizi River[END_REF]. If management decision-makers can set management standards with reference to thresholds, management efficiency can be maximized and ecological risks can be avoided to the greatest extent [START_REF] Samhouri | Identifying thresholds for ecosystem-based management[END_REF]. In threshold detection, both sound choice of variables and model appropriateness can explain the presence and location of thresholds differently [START_REF] Dodds | Thresholds, breakpoints, and nonlinearity in freshwaters as related to management[END_REF][START_REF] King | Considerations for analyzing ecological community thresholds in response to anthropogenic environmental gradients[END_REF]. Threshold Indicator Taxa Analysis (TITAN) is an analytical method for identifying single and cumulative species responses to environmental variables that combines and extends change-point analysis and indicator species analysis. TITAN can determine the point and direction of change, and the extent to which each species responds along an environmental gradient.

In addition to detecting species-level thresholds, TITAN can also indicate communitylevel thresholds [START_REF] King | Considerations for analyzing ecological community thresholds in response to anthropogenic environmental gradients[END_REF].

Starting from key predictors of fish populations and lake ecosystem management, this study revolves around three objectives. [START_REF] Feng | Eutrophication induces functional homogenization and traits filtering in Chinese lacustrine fish communities[END_REF] Based on the prediction model, evaluate the importance of environmental factors to the fish communities of lakes in the middle reaches of the Yangtze River, so as to identify key predictors for subsequent studies to quantify the importance of environmental filtering in the process of community construction. ( 2) At the same time, discuss the application and importance of predictive models in the management of lake ecosystems, especially fish communities. (3) Use the ecological threshold detection model to identify the population and community level thresholds of fish community changes caused by important environmental factors, and provide an example for the management of lake ecosystems using statistical analysis and ecological data to formulate management reference standards.

Materials and methods

Study area and sample setting

See Chapter 2.2.1 for the setting of the study area and sample points.

Sample and data collection

For the collection and acquisition methods of fish community data, fish functional traits and environmental factors, see 2.2.2.

Data analysis

In this study, the random forest model and the gradient forest model were first used to evaluate the importance of environmental variables from the perspectives of community indicators and taxonomic and functional multivariate structures, thereby screening key predictors. Secondly, the prediction model is used to predict the fish community indicators, and its prediction performance and accuracy are evaluated, so as to explore the application of the prediction model in the management of lake ecosystems. Finally, on the premise of identifying the key predictors, the TITAN model was used to monitor the ecological thresholds from the taxonomic and community levels along the gradient of environmental factors, and the existence of the thresholds was compared based on species and functional groups, respectively.

Evaluation of the importance of environmental variables in predictive models

A random forest model was used to construct a prediction model for the fish community index of lakes in the middle reaches of the Yangtze River to assess the importance of environmental variables. The fish community index includes six indicators in terms of species abundance, taxonomic diversity, and functional diversity, namely, the number of catches per unit effort, the biomass of catches per unit effort, species richness, Shannon diversity index, functional richness, and functional dispersion. The predictor variables include 25 variables in four categories including eutrophication, land use, lake morphology and proliferation mode. There are 15 eutrophication indicators, including Secchi depth (SD, m), dissolved oxygen (DO, mg L -1 ), conductivity (Cond, μS cm -1 ), pH, turbidity (Turb, NTU), total phosphorus (TP, mg L -1 ), total nitrogen (TN, mg L -1 ), chlorophyll a (Chla, μg L -1 ), chemical oxygen demand (CODMn, mg L -1 ), ammonia nitrogen (NH4 + -N, mg L -1 ), nitrite nitrogen (NO2 --N, mg L -1 ) and nitrate nitrogen (NO3 --N, mg L -1 ). There are 5 land use indicators, including forest, grassland, agriculture, pond, and urban. There are 5 lake morphology indicators, including lake area (km 2 ), shoreline length (m), watershed area (km 2 ), maximum water depth (WD, m) and shoreline development index (SDI). There are 3 lake fishery proliferation mode indicators, including the annual yield per unit water area (yield, kg hm-2), the annual stocking amount per unit water area (stocking, kg hm-2)

and the input level of feed and fertilizer (input). In the random forest model, the indicator IncMSE (increase in mean squared error) was used to evaluate the relative importance of environmental variables in predicting fish indicators [START_REF] Kuhn | Building predictive models in R using the caret package[END_REF]. In all variables, percentage variables are transformed by square root arcsine, other variables except pH and input level are transformed by ln, and predictor variables and response variables are transformed. The random forest model was constructed using the function "randomForest" of the R package "randomForest" [START_REF] Liaw | Classification and regression by randomForest[END_REF]. The test of the significance of environmental variables in the model was carried out using the R package "rfPermute" [START_REF] Archer | rfPermute: Estimate Permutation p-Values for Random Forest Importance Metrics, R package version 2[END_REF].

The gradient forest model was used to construct a prediction model for the fish community species and functional traits of lakes in the middle reaches of the Yangtze River to assess the importance of environmental variables. The species composition of the fish community is expressed by the abundance of species, and the functional trait structure is expressed by the community weighted mean (CWM) index of functional traits. CWM values correspond to the average value of each trait weighted by the relative abundance of each species in the community [START_REF] Lavorel | Assessing functional diversity in the field-methodology matters![END_REF], capturing information about trait characteristics in the community [START_REF] Frainer | Shifts in the diversity and composition of consumer traits constrain the effects of land use on stream ecosystem functioning[END_REF].

For categorical traits, the CWM value represents the proportion of individuals belonging to a particular trait class. The CWM values use the function "functcomp" of the R package "FD" [START_REF] Laliberté | Measuring functional diversity from multiple traits, and other tools for functional ecology[END_REF]. Species abundance data were transformed using Hellinger transform, and CWM values were transformed using ln(n+1). The selection and transformation of environmental variables are the same as the random forest model. In the gradient forest model, the index R 2 weighted importance was used to evaluate the relative importance of environmental variables in predicting the structure of fish community species and functional traits. The gradient forest model was constructed using the R packages "extendedForest" [START_REF] Liaw | Classification and regression by randomForest[END_REF] and "gradientForest" [START_REF] Ellis | Gradient forests: calculating importance gradients on physical predictors[END_REF].

Using random forest model to predict community indexes

Three indicators of fish communities were selected, namely, biomass per unit effort, functional richness, and functional dispersion. A random forest model was used to construct a prediction model for these indicators, and the predictor variables were screened and the prediction accuracy was evaluated. First, on the basis of evaluating the importance of environmental variables, by performing ten-fold cross-validation, the environmental variables are selected according to the cross-validation curve, so as to construct the optimal prediction model. Evaluate the effectiveness of a predictive model by calculating the overall rate of explanation of the relative variance of the response variable by the predictor variables in the model. Also uses the degree of agreement between the observed and predicted values of the response variable to assess the predictive accuracy of the model.

Ecological threshold detection based on species and functional groups

In this study, the TITAN model was used to detect species-and functional-groupbased ecological thresholds for fish communities. Functional groups were assessed using functional groups (FG). The 34 fish species that occurred more than 5 times in all sampling points in this study were clustered into different functional groups based on 22 functional traits using hierarchical clustering analysis (UPGMA). Ultimately, all 34 species were assigned to 12 functional groups with 1 to 5 species (Appendix 3).

Functional groups were partitioned using the function 'dpFD' [START_REF] Laliberté | Measuring functional diversity from multiple traits, and other tools for functional ecology[END_REF] of the R package 'FD'. The abundance of species that occurred more than 5 times (34 species) was used to represent the species composition of the community. The total abundance of fish in each functional group (12 functional groups) was used to represent the functional composition of the community.

We used Threshold Indicator Taxa ANalysis (TITAN) [START_REF] King | Considerations for analyzing ecological community thresholds in response to anthropogenic environmental gradients[END_REF] to analysis the threshold responses in the taxonomic and functional composition of fish community to water quality (TP). TITAN is an analysis method to identify individual and cumulative fish species responses to environmental variables that combines and extends change-point analysis and indicator species analysis. TITAN can determine the point and direction of change, and degree of response of each species or FG along an environmental gradient. In addition to detecting taxon-level thresholds, TITAN can also indicate community-level thresholds [START_REF] King | Considerations for analyzing ecological community thresholds in response to anthropogenic environmental gradients[END_REF].

The indicator value (IndVal) of each predictor variable that maximizes association of individual species with one side of the partition, either positively or negatively was examined in TITAN (Dûfrene & Legendre, 1997). The significant indicator species was identified using bootstrapping [START_REF] King | Considerations for analyzing ecological community thresholds in response to anthropogenic environmental gradients[END_REF]. The response direction (positive or negative) of each specie to the predictor variable gradient was confirmed if the results showed both high purity and high reliability. High purity requires species frequency and abundance changes in the same direction more than 95% of 500 bootstrapped runs.

High reliability requires that at least 95% of the 500 bootstrapped runs are significantly different from randomly distributed data (P < 0.05). IndVal z-scores of each species indicate the magnitude of the response. The thresholds and confidence limits at community level was examined by the sum of IndVal z-scores (sumz). Peaks in the sumz along the predictor variable gradient were associated with the maximum decline among negative response species (thresholds of z-) or maximum increase among positive response species (thresholds of z+) of frequency and abundance [START_REF] King | Considerations for analyzing ecological community thresholds in response to anthropogenic environmental gradients[END_REF]. The uncertainty of community thresholds is estimated by the bootstrapped values of predictor variable that produce the greatest sum(z-) or sum(z+) values and quantiles (e.g., 5%, 95%) of these distributions serve as empirical confidence limits [START_REF] Qian | Two statistical methods for the detection of environmental thresholds[END_REF][START_REF] Toms | Piecewise regression: a tool for identifying ecological thresholds[END_REF]. Narrow confidence limits can be further evidence of the existence of a threshold in the community, while wide confidence limits indicate that other responses are more likely (e.g., linear, modal or random) [START_REF] King | Considerations for analyzing ecological community thresholds in response to anthropogenic environmental gradients[END_REF]. In TITAN analysis, taxa data need to be discrete abundance data rather than continuous trait or biomass data [START_REF] King | Considerations for analyzing ecological community thresholds in response to anthropogenic environmental gradients[END_REF]. Therefore, this study uses the abundance of functional groups and species for analysis. And rare species were excluded to eliminate outliers that could cause potential bias. The R package "TITAN2"

was used to construct and analyze the TITAN model [START_REF] Baker | TITAN2: Threshold Indicator Taxa Analysis[END_REF].

Results

Evaluation of the importance of environmental variables in predictive models

Figure 5-1 shows the evaluation results of environmental variable importance using the random forest model for NPUE as the response variable. Among all the predictor variables, turbidity, transparency, and total phosphorus related to eutrophication, proportion of grassland related to land use, and SDI coefficient related to lake morphology had relatively high importance. The turbidity was significant (P<0.05). The results of evaluating the importance of environmental variables with species richness as the response variable using the random forest model are shown in Figure 5-3. Among all predictor variables, dissolved oxygen and chlorophyll a related to eutrophication, maximum water depth related to lake morphology, SDI coefficient, and lake shoreline length had relatively high importance. The results of evaluating the importance of environmental variables with functional richness as the response variable using the random forest model are shown in Figure 5-5. Among all the predictor variables, pH, dissolved oxygen, chlorophyll a related to eutrophication, and maximum water depth related to lake morphology had relatively high importance. Among them, the maximum water depth and dissolved oxygen were significant (P<0.05). The results of evaluating the importance of environmental variables using the random forest model for functional dispersion as the response variable are shown in

Figure 5-6. Among all the predictor variables, chlorophyll a, turbidity, total phosphorus related to eutrophication, maximum water depth related to lake morphology, stocking amount related to proliferation pattern had relatively high importance. Among them, chlorophyll a was significant (P<0.05). The evaluation results of the importance of environmental variables using the gradient forest model to assess the multivariate structure of fish community species composition as the response variable are shown in Figure 5-7. Among all the predictor variables, electrical conductivity, total phosphorus related to eutrophication, watershed area related to lake morphology, stocking volume and production related to proliferation pattern had relatively high importance. The evaluation results of the environmental variable importance using the gradient forest model on the multivariate structure of fish community functional traits as response variables are shown in Figure 5-8. Among all the predictor variables, turbidity, transparency, and total phosphorus related to eutrophication, and the proportion of grassland related to land use, had relatively high importance.

Discussion

The main purpose of the research in this chapter is to identify key predictors of fish communities in the middle reaches of the Yangtze River with the help of predictive models, so as to further identify environmental factors that play an important role in environmental filtering and prepare for the quantification of environmental filtering. In The key environmental factors in the land use category are agricultural land, grassland, pond. The key factors in the lake morphology category are lake area, maximum water depth, and SDI index. The key factors in the proliferation pattern category are total annual production, annual stocking volume, and feed and fertilizer input levels. After our evaluation and screening, these environmental factors have been proved to represent the environmental filtering effect of environmental pressure on fish communities from all aspects of fish communities.

For eutrophication, previous studies have reported the key role of phosphorus in lake eutrophication [START_REF] Carpenter | Phosphorus control is critical to mitigating eutrophication[END_REF][START_REF] Schindler | Reducing phosphorus to curb lake eutrophication is a success[END_REF]. In the prediction model, total phosphorus showed an important role in the prediction of abundance, biomass, functional diversity, species composition and functional trait structure. And in the previous chapters, it was also revealed that total phosphorus plays an important role in trait filtering, especially life history strategy selection. Chlorophyll a is the best proxy indicator of lake primary productivity and can represent the level of phytoplankton productivity [START_REF] Huot | Does chlorophyll a provide the best index of phytoplankton biomass for primary productivity studies?[END_REF]. In the prediction model, chlorophyll a played an important role in the prediction of species richness, functional richness, taxonomic diversity and functional diversity. Turbidity reflects the visibility state of lake water, and the direct cause of eutrophication causing lakes to transform from a state dominated by submerged plants to a state dominated by phytoplankton is the increase in turbidity [START_REF] Scheffer | Alternative equilibria in shallow lakes[END_REF]. In addition, turbidity, unlike transparency, does not rely on human judgment in obtaining data. In the prediction model, turbidity showed an important role in the prediction of abundance, biomass, functional diversity and functional trait structure.

Regarding land use, the middle reaches of the Yangtze River have a high degree of agriculturalization, and the main land use type is agricultural land [START_REF] Dong | Carbon burial by shallow lakes on the Y angtze floodplain and its relevance to regional carbon sequestration[END_REF].

The research in the previous chapters shows that the main reason for the differences in land use types of the lakes in the middle reaches of the Yangtze River is the degree of agricultural expansion, and the proportion of forests has a strong negative correlation with the proportion of agricultural land. Therefore, in order to avoid collinearity, only agricultural land is selected as the key factor. In predictive models, agricultural land was shown to play an important role in predicting biomass, functional diversity, and species composition. Natural grasslands, as a natural vegetation around lakes, may provide diverse habitat environments when lake water levels change [START_REF] Giacomazzo | Linking fisheries to land use: How anthropogenic inputs from the watershed shape fish habitat quality[END_REF]. In predictive models, native grasslands played an important role in predicting abundance, species composition, and functional trait structure. The pond area in the middle reaches of the Yangtze River also has a larger proportion around the lake. This is related to the high-intensity freshwater aquaculture in the middle reaches of the Yangtze River in China, and these ponds are likely to be reclamated from lakes, which may affect fish communities through aquaculture wastewater discharge and changes in lake coastal habitats [START_REF] Wang | Freshwater aquaculture in PR C hina: trends and prospects[END_REF]. In predictive models, pond proportions were shown to play an important role in predicting both species richness and taxonomic diversity.

For lake morphology, it plays an important role in shaping the lake habitat complexity and fish community structure. Larger lake areas may indicate complex habitat types, so positive correlations between lake area and fish species richness have often been reported in studies [START_REF] Dodson | The relationship in lake communities between primary productivity and species richness[END_REF][START_REF] Mehner | Rapid recovery from eutrophication of a stratified lake by disruption of internal nutrient load[END_REF]. In the prediction model, lake area showed an important role in the prediction of both functional richness and species composition. Lake depth can significantly alter the composition of lake fish communities [START_REF] Garcia | Correlations between type-indicator fish species and lake productivity in German lowland lakes[END_REF][START_REF] Søndergaard | Water Framework Directive: ecological classification of Danish lakes[END_REF]. In the prediction model, lake depth showed an important role in the prediction of species richness, functional richness and functional diversity. The complexity of lake shorelines is often associated with the diversity of fish habitats [START_REF] Winfield | Fish in the littoral zone: ecology, threats and management[END_REF]. In the prediction model, the SDI index plays an important role in the prediction of species richness, species composition and functional trait structure.

Regarding the lake fishery proliferation model, most lake fisheries in China have unreasonable resource development and utilization. For example, lack of routine resource dynamic analysis, unrestrained overfishing has destroyed lake aquatic biological resources, depleted lake fish resources, and miniaturized fish community structure [START_REF] Qian | Current status of fishery resources and environmental monitoring in Poyang Lake[END_REF][START_REF] Liao | Current status and changes of fishery resources in Dongting Lake[END_REF]. Unreasonable fish stocking and fishing management schemes, lack of reasonable assessment and planning of the fishery carrying capacity of the water ecosystem, lead to changes in the fish community and population structure, and at the same time led to changes in the structure of other aquatic organisms in the lake [START_REF] Zhang | Effects of fish stocking on lake ecosystems in china[END_REF]. In the high-density lake fishery with intensive and semi-intensive methods as the main methods, large inputs of feed and fertilizers lead to serious eutrophication of lake water and reduced transparency [START_REF] Cui | Fishery resources and environmental protection of lakes in the Yangtze River Basin[END_REF]. In the prediction model, the stocking amount, yield, and feed and fertilizer input of lake fisheries all played an important role in the prediction of biomass, functional diversity, species composition, and functional trait structure.

In this study, a random forest model was used to construct a prediction model for some indicators of fish communities. Generally speaking, the model has a certain prediction fitness and accuracy after optimization. The random forest model has advantages over the simple linear regression model in terms of data requirements. For example, it is a non-parametric model that does not require any assumptions about the distribution type of the predictor or response variable, provides sophisticated methods to deal with missing data, and can use intuitive charts to explain complex and abstract results, etc. Not only that, random forest models also improve the predictive ability of some tree models by reducing variation and bias [START_REF] Elith | A working guide to boosted regression trees[END_REF] [START_REF] Breiman | Random forests[END_REF]. However, due to the small amount of data in this study, the training set and data set of the model were not actually divided, but all the data was used for model construction. Therefore, the small amount of data may be a limitation of constructing the prediction model in this study. At the same time, it is worth noting that in Chapter 3, the functional dispersion and eutrophication indicators were used to construct a multiple linear regression model, and this simple prediction model also achieved good results. Therefore, the random forest model did not provide the expected prediction fitness in terms of prediction, although it played an important role in the identification of key predictors.

Application of threshold detection of key environmental factors in ecosystem management

As an extension of the application of key environmental factors in ecosystem management, this study took total phosphorus as an example to conduct taxonomic and community-level threshold detection based on the species and functional group structure of fish communities. The results we obtained have very specific managerial implications. From the threshold detection results at the species composition level, this study identified the total phosphorus threshold when important changes in species abundance in different corresponding directions occurred. This is important for developing strategies for key indicator species in management. It is worth noting that the results of positive and negative response species obtained in the threshold analysis are highly consistent with the results obtained in Chapter 3. Benthic and equilibrium species were negatively correlated with total phosphorus. However, some species that obtained more food resources due to increased food availability were positively correlated with total phosphorus, including both smaller opportunistic species and larger periodic species. Threshold analysis, however, provided more information than previous analyzes had. For example, the ecological thresholds of negative responding species are often lower, while the ecological thresholds of positive responding species are distributed on the total phosphorus gradient. This demonstrates that at lower concentrations the populations of the negatively responding species have begun to degenerate, which is also reflected at the threshold at the community level. In addition to this, the TITAN analysis also provides the magnitude and confidence intervals of species threshold responses, which indicate the extent and pattern of threshold responses of a certain species. Comparing the threshold analysis results based on species composition and functional group structure, we found that the ecological thresholds at the community level of the two showed that the threshold of species composition was lower than the threshold of functional group structure in both positive and negative corresponding directions. And the thresholds based on the functional group structure all have wide confidence intervals. This reflects that although a single species may exhibit large variations with environmental disturbances. However, for the functional structure of the entire fish community, due to functional redundancy, that is, different species have similar functions, it is possible that the overall function of the fish community did not show obvious degradation immediately.

Our study demonstrates that the detection of ecological thresholds is of great significance for understanding the mechanism of environmental filtering on fish communities and in the management of lake ecosystems and fish communities.

Determining ecological thresholds is important for lake management and can provide early signals of ecosystem transformation [START_REF] Munson | Ecosystem thresholds, tipping points, and critical transitions[END_REF]. Nowadays, people have fully recognized the necessity of biological indicators in the evaluation of surface water ecosystems health, and the identification of thresholds is conducive to the classification of the evaluation system (European Commission, 2000). Threshold detection is widely used in the formulation of water-quality criteria, habitat protection, determination of species protection targets, ecosystem restoration, and aquatic ecosystem management [START_REF] King | Integrating bioassessment and ecological risk assessment: an approach to developing numerical water-quality criteria[END_REF][START_REF] Richardson | Estimating ecological thresholds for phosphorus in the Everglades[END_REF][START_REF] Majumder | Inferring critical thresholds of ecosystem transitions from spatial data[END_REF]. From the perspective of species and functional conservation, ecological thresholds can provide an important reference for the setting of conservation goals [START_REF] Brejão | Threshold responses of Amazonian stream fishes to timing and extent of deforestation[END_REF][START_REF] Zhang | Threshold responses in the taxonomic and functional structure of fish assemblages to land use and water quality: A case study from the Taizi River[END_REF]. Ecosystem resilience refers to the intrinsic ability of an ecosystem to maintain its critical functions after experiencing various disturbances. The premise of maintaining ecosystem resilience is to ensure that the threshold of regime shift is not crossed [START_REF] Sasaki | Perspectives for ecosystem management based on ecosystem resilience and ecological thresholds against multiple and stochastic disturbances[END_REF][START_REF] Karr | Thresholds in Caribbean coral reefs: implications for ecosystem-based fishery management[END_REF]. If management decision-makers can set management standards with reference to thresholds, the efficiency of management will be maximized, and ecological risks can be avoided as much as possible [START_REF] Samhouri | Identifying thresholds for ecosystem-based management[END_REF].

Conclusion

The research in this chapter is an extension of the research on the mechanism of environmental filtering on fish community structure from the perspective of ecosystem and fish community management. Twelve key predictors of eutrophication, land use, lake morphology, and proliferation patterns were determined from the levels of fish community abundance, taxonomic diversity, functional diversity, species composition, and functional traits through the prediction model. This provides key indicators for subsequent chapters to quantify the environmental filtering effect of environmental stress on fish communities. In addition, the study in this chapter evaluates the prediction performance of the prediction model based on the construction of the prediction model.

However, because the data set is relatively small for the prediction model, although the obtained prediction model has the prediction ability, the prediction fitness and accuracy are not very ideal. In addition, based on the ecological threshold test of key factors, this study further supplements and expands the mechanism of environmental filtering on fish communities, and at the same time reflects the significance of ecological threshold tests in the management of lake ecosystems and fish communities.

Chapter6

The relative importance of environmental filtering and interspecific interactions in fish community assembly

Introduction

Community assembly is crucial to explain species coexistence mechanisms and biodiversity maintenance, and it is also one of the hot issues in current community ecology research [START_REF] Kraft | Community assembly, coexistence and the environmental filtering metaphor[END_REF][START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF]. Research theories on the mechanism of community assembly emerge in endlessly and are still under development, but there are currently two main theories to explain the mechanism of community assembly. One holds that deterministic processes such as niche differentiation among co-occurrence species dominate community assembly, the niche theory [START_REF] Chase | Towards a really unified theory for metacommunities[END_REF]. The other holds that dispersal and stochastic processes are the key to the coexistence of species and the maintenance of biodiversity in a community, that is, the neutral theory [START_REF] Hubbell | The unified neutral theory of biodiversity and biogeography[END_REF].

The neutral theory believes that diffusion and stochastic processes play a decisive role in community assembly. Under the neutral effect (neutral theory), species in the community are composed randomly, and there is no regular relationship between the coexistence patterns of species [START_REF] Hubbell | The unified neutral theory of biodiversity and biogeography[END_REF]. Based on the niche theory, community assembly is considered to be a process in which species in a regional species pool are selected into local communities through multi-layer environmental filtering and interactions between organisms, and the two provide opposing forces [START_REF] Keddy | Assembly and response rules: two goals for predictive community ecology[END_REF][START_REF] Zobel | The relative of species pools in determining plant species richness: an alternative explanation of species coexistence?[END_REF]. Specifically, the environmental filtering effect selects species with similar functional traits and similar adaptability to local environmental conditions, that is, similar ecological niches, to form the same community, so that species exhibit aggregation and dispersal or trait convergence [START_REF] Prinzing | The niche of higher plants: evidence for phylogenetic conservatism[END_REF]Weiher and Keddy, 2001). Organismal interactions lead to competitive exclusion between species with similar ecological niches, resulting in species segregation or trait divergence [START_REF] Weiher | Assembly rules, null models, and trait dispersion: new questions from old patterns[END_REF][START_REF] Webb | Phylogenies and community ecology[END_REF]. Niche theory clarifies that both environmental filtering and interbiotic interactions (such as interspecies competition and predator-prey relationships) play an important role in niche differentiation during community assembly [START_REF] Silvertown | Plant coexistence and the niche[END_REF].

With the progress of research on the mechanism of community assembly, ecologists tend to think that the niche theory and neutral theory of community assembly are not absolutely opposite. The two describe the characteristics of communities from different perspectives, and the compatibility and complementarity between these two theories can better reveal the mechanism of community assembly [START_REF] Tilman | Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly[END_REF][START_REF] Chase | Towards a really unified theory for metacommunities[END_REF][START_REF] Leibold | Coexistence of the niche and neutral perspectives in community ecology[END_REF]. Specifically, the stochastic process shown in the species coexistence pattern may be the result of the combined effect of deterministic processes. That is, the isolation and dispersal caused by the interaction between organisms and the aggregation and dispersal caused by environmental filtering are two opposite effects, which eventually lead to the stochastic dispersal mode of species coexistence, but the deterministic effect still plays a major role in the process of community assembly [START_REF] Heino | Untangling the assembly of littoral macroinvertebrate communities through measures of functional and phylogenetic alpha diversity[END_REF].

In niche theory, the relative importance of environmental filtering and interactions between organisms to community assembly has been widely discussed. The community assembly rule based on the niche theory considers that community assembly is a filtering process in which species in a regional species pool enter a local area through multi-layer environmental filtering and interactions between organisms [START_REF] Diamond | Ecology and evolution of communities[END_REF]. Environmental filtering and interactions between organisms together constitute the two opposing driving forces of community assembly [START_REF] Geheber | The effect of spatial scale on relative influences of assembly processes in temperate stream fish assemblages[END_REF].

If the distribution model of co-occurrence species in the community shows convergence of traits relative to the null model, then environmental filtering is the main driving force of community assembly; otherwise, if it shows divergence of traits, interactions between organisms play a dominant role [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF]. In addition, the relative importance of these two opposing driving forces in the assembly process of different biomes is different, and there is a scale correlation. For example, environmental filtering is the main driving force determining plant community assembly at regional scales, while interactions between organisms play a role at smaller scales [START_REF] Götzenberger | Ecological assembly rules in plant communities-approaches, patterns and prospects[END_REF]. However, in some animal community studies, the opposite conclusion was drawn, and the study found that the interaction between organisms played a decisive role in the process of animal community assembly [START_REF] Trisos | Unraveling the interplay of community assembly processes acting on multiple niche axes across spatial scales[END_REF]Fitzgerald et al., 2017a).

Regarding the research on the mechanism of fish community assembly, environmental filtering and interactions between organisms are considered to directly affect the functional traits of fish, thereby driving the formation of fish communities and maintenance of diversity. Therefore, the species-based functional trait method is considered by ecologists to be an important means to study the assembly mechanism of fish communities (Stuart-Smith et al., 2013;[START_REF] Troia | Functional strategies drive community assembly of stream fishes along environmental gradients and across spatial scales[END_REF][START_REF] Drew | Community assembly of coral reef fishes along the Melanesian biodiversity gradient[END_REF][START_REF] Mou-Tham | Assembly rules of fish communities in Tuamotu archipelago atoll lagoons: The case of Fangatau, a lagoon dominated by giant clam habitats[END_REF]. The functional traits of fish mainly include:

habitat selection, feeding nutrition, life history strategy and locomotion behavior (Stuart-Smith et al., 2013;[START_REF] Buisson | Toward a loss of functional diversity in stream fish assemblages under climate change[END_REF][START_REF] Escobar-Toledo | Temporal shifts in functional traits of the fish community in Terminos Lagoon (Mexico) in three periods (1980, 1998 and 2011)[END_REF][START_REF] Fitzgerald | Using trophic structure to reveal patterns of trait-based community assembly across niche dimensions[END_REF][START_REF] Colin | Functional diversity measures revealed impacts of non-native species and habitat degradation on species-poor freshwater fish assemblages[END_REF]Peoples and Midway, 2018;Wilkinson et al., 2018). In general, environmental factors such as water quality, substrate, habitat structure, land use, hydrological characteristics, and climatic factors, as well as disturbances from human activities, have important effects on fish functional traits [START_REF] Teresa | Trait-based metrics as bioindicators: Responses of stream fish assemblages to a gradient of environmental degradation[END_REF][START_REF] Córdova-Tapia | The role of environmental filtering in the functional structure of fish communities in tropical wetlands[END_REF]. At the same time, fish functional traits are also affected by biological factors such as interspecific competition, predator-prey interactions, and food resources [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF][START_REF] Geheber | The effect of spatial scale on relative influences of assembly processes in temperate stream fish assemblages[END_REF][START_REF] Fitzgerald | Using trophic structure to reveal patterns of trait-based community assembly across niche dimensions[END_REF]. [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF] studied the assembly mechanism of fish communities in American rivers using species-based functional traits and found that 75%-85% of river fish communities are determined by non-stochastic patterns.

Among them, environmental filtering plays a decisive role, followed by predator-prey interactions (predator-prey interactions), while interspecific competition plays the least role. Fish community assembly mechanisms are also affected by temporal and spatial scales. Fitzgerald et al. (2017a) used fish functional traits and functional diversity to study the assembly of fish communities in the main tributaries of the Amazon Basin, and found that two key drivers of fish community assembly will change with seasonal changes. [START_REF] Geheber | The effect of spatial scale on relative influences of assembly processes in temperate stream fish assemblages[END_REF] found that environmental filtering plays a decisive role in the assembly process of river fish communities, and its role is enhanced with the expansion of the spatial scale. In China, research on the assembly mechanism of freshwater fish communities started relatively late. The vast majority of fish community studies still focus on species surveys, taxonomic diversity, and resource dynamic analysis, and there are few reports explaining the lake fish community assembly mechanism from the perspective of ecological functions.

This study aims to reveal the fish community assembly machine in lakes in the middle reaches of the Yangtze River. In particular, the relative importance of two opposing forces based on niche theory, namely environmental filtering and interbiotic interactions (including interspecific competition and predator-prey interactions), in the process of community assembly. First, the co-occurrence pattern of species in fish communities was simulated and analyzed, and the species dispersal pattern was judged as aggregated dispersal, random dispersal or isolated dispersal. Then, based on the research in the previous chapters, the environmental filtering effect was quantified from the relationship between environmental pressure and species, and the interspecific competition and predator-prey interaction were quantified from species functional traits.

Finally, we analyze the relative importance of environmental filtering, interspecific competition, and predator-prey interactions on species co-occurrence patterns, the process of community assembly.

Materials and methods

Study area and sample setting

See Chapter 2.2.1 for the setting of the study area and sample points.

Sample and data collection

For the collection and acquisition methods of fish community data, fish functional traits and environmental factors, see 2.2.2.

Data analysis

First, this study used a null model to simulate and analyze the species cooccurrence pattern of fish communities, and judged whether the species dispersal pattern was aggregated dispersal, stochastic dispersal or isolated dispersal. Then, based on the research in previous chapters, environmental filtering was quantified from the 12 key environmental factors identified in Chapter 5 and the relationship between species, and interspecific competition and predator-prey interactions were quantified from species functional traits. Finally, regression models were used to analyze the relative importance of environmental filtering, interspecific competition, and predatorprey interactions on species co-occurrence patterns, that is, the community assembly process.

Null model and species co-occurrence model

Null model simulation was used to analyze whether the fish community exhibited a non-stochastic co-occurrence pattern. The null model quantifies co-occurrence patterns between species pairs under the specific assumption that species interactions do not exist during community assembly (Gotelli and Graves, 1996). The C-score index is used to quantify the co-occurrence relationship between paired species. The range of C-score is between 0 and 1, C-score=0 means complete overlap, and C-score=1 points for complete separation. Choose two null models: FF (fixed row-fixed column) and FE (fixed row-equiprobable column) to calculate C-score. A fixed row-fixed column (FF) model, which preserves the total occurrence of species within each watershed and the total abundance of species between sites. A fixed row-equiprobable column (FE) model that preserves differences in total occurrence of species within watersheds but preserves differences in richness between sites within watersheds. The FF and FE models represent ecologically plausible species colonization processes with a low probability of Type I error [START_REF] Gotelli | Null model analysis of species co-occurrence patterns[END_REF]. The standard effect size (SES) was used to evaluate the C-score standard deviation between the measured value and the simulated value based on the group, SES=(meanobs-meansim)/SDsim. In order to rule out the possible impact of rare and widespread species on the assessment of species co-occurrence patterns. We assembled models in parallel considering all species while only considering species that occur more than 2 times and less than 24 times in lakes (total number of lakes 26, i.e., 2<number of occurrences<N-2). In order to judge the species co-occurrence pattern, quantify interspecies competition. The 8 variables included 6 functional traits, maximum standard body length and parental care index in life history strategy, diet and trophic level in feeding nutrition, habitat preference and vertical position in habitat utilization. In addition, two variables, family and genus to which species belong, were used to represent phylogenetic differences. The Gower's coefficient of dissimilarity among the eight variables for each species was then calculated to quantify interspecies competition using functional and phylogenetic dissimilarity between each species pair.

The quantification of predator-prey interactions was based on species feeding habits, body size differences between species pairs, and species food records in the literature, using 0 and 1 for quantification. 0 means that there is no predatory relationship between species pairs, and 1 means that there is a predatory relationship. The Gower dissimilarity coefficient was calculated using the function "gowdis" of the R package "FD" [START_REF] Laliberté | Measuring functional diversity from multiple traits, and other tools for functional ecology[END_REF].

The relative importance of environmental filtering and interspecies interactions in community assembly

After completing the quantification of species co-occurrence patterns (SES), environmental filtering (ENV), interspecies competition (COMP) and predator-prey interactions (PRED). We obtained species-pair distance matrices for quantifying the four processes separately. We used a matrix of SES values for the co-occurrence of species pairs as response variables, and a dissimilarity matrix for environmental filtering, a dissimilarity matrix for interspecies competition, and a matrix representing relative importance in the model, only slightly greater than that of interspecies interactions, and the relative importance of interspecies competition was greater than that of predator-prey interactions. Both ENV and PRED were positively correlated with SES, that is, the more similar the environmental requirements, functions and phylogeny among species, the closer the inter-species co-occurrence pattern was to inter-species aggregation. importance in the model and is much greater than that of interspecies interactions, and the relative importance of interspecies competition is greater than that of predator-prey interactions. Both ENV and PRED were positively correlated with SES, that is, the more similar the environmental requirements, functions and phylogeny among species, the closer the inter-species co-occurrence pattern was to inter-species aggregation. However, the SES value of the more widely distributed species pairs (occurrence times ≥N-2) is basically zero, tending to random distribution. This part of randomly scattered species has little effect on the results, while species aggregated among species has a greater impact on the results. The degree of functional and phylogenetic divergence between these species pairs is diverse, and there is no consistent rule. However, the similarity of environmental requirements is extremely high, so there is a higher degree of interpretation and a positive correlation trend between SES and ENV. In this study, two null models were used to study the species co-occurrence patterns of fish communities in lakes in the middle reaches of the Yangtze River.

Although there are some differences in the judgment of the co-occurrence mode of species in the community by the two null models, the species distribution tends to be random in general. We then explored the relative importance of environmental filtering, the role of interspecies competition, and predator-prey interactions on species cooccurrence patterns during community assembly. The results showed that the role of environmental filtering had the highest significant relative importance, the role of competition between species was less important, but still significant, and the interaction between predator-prey had little effect on community assembly.

This study quantifies environmental filtering based on differences in species responses to actual environments, and quantifies competition and predator-prey interactions between species pairs within fish communities based on species' functional traits. The above three predictor variables were used to build a model to predict the cooccurrence pattern of species in fish communities, so as to realize the quantitative study of the relative importance of environmental filtering and biological interactions in fish community assembly.

6.4.1 Mechanism of fish community assembly in lakes in the middle reaches of Yangtze

River

In this study, co-occurrence patterns between species pairs were quantified by SES values in the null model. The results of the FF model showed that the fish community species in lakes in the middle reaches of the Yangtze River showed a stochastic dispersal pattern. However, the FE model shows that the SES value is significantly less than zero, which belongs to the aggregated dispersal, but it does not reach the degree of over aggregation. This result seems to indicate that seemingly stochastic processes play a greater role in the assembly of fish communities in lakes in the middle reaches of the Yangtze River. This clearly contradicts our hypothesis that a pair of counterforces in the deterministic process, environmental filtering and interspecies interactions, play a more dominant role in community assembly. This pattern, in which random processes play a major role in community assembly, has been extensively reported in other groups of organisms in different ecosystems, such as macroinvertebrates in lakes [START_REF] Heino | Untangling the assembly of littoral macroinvertebrate communities through measures of functional and phylogenetic alpha diversity[END_REF] and aquatic plants [START_REF] Liu | Contrasting patterns and drivers in taxonomic versus functional diversity, and community assembly of aquatic plants in subtropical lakes[END_REF]. Birds, vegetation, and macroinvertebrates in wetland ecosystems [START_REF] Daniel | Stochastic and deterministic processes drive wetland community assembly across a gradient of environmental filtering[END_REF]. Bacterial and archaea communities in Marine ecosystems [START_REF] Wang | Community assembly of bacteria and archaea in coastal waters governed by contrasting mechanisms: A seasonal perspective[END_REF]. Soil animal community [START_REF] Caruso | Relative role of deterministic and stochastic determinants of soil animal community: a spatially explicit analysis of oribatid mites[END_REF]. Bacteria and fungi in terrestrial ecosystems [START_REF] Powell | Deterministic processes vary during community assembly for ecologically dissimilar taxa[END_REF] and mosses and vascular plant communities (Maren et al., 2018). However, if the community assembly process is only understood from neutral theory, the role of stochastic processes in fish community assembly is likely to be overestimated. With the development of community assembly mechanism research, ecologists tend to believe that the niche theory and neutral theory of community assembly are not absolutely opposite [START_REF] Tilman | Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly[END_REF][START_REF] Chase | Towards a really unified theory for metacommunities[END_REF][START_REF] Leibold | Coexistence of the niche and neutral perspectives in community ecology[END_REF]. Specifically, the randomness exhibited by species co-occurrence patterns may be the result of a combination of deterministic processes. That is, the isolation and dispersal caused by the interaction between organisms and the aggregation and dispersal caused by environmental filtering are two opposite effects, which eventually lead to the stochastic dispersal mode of species co-occurrence, but the deterministic effect actually still plays a major role in the process of community assembly [START_REF] Heino | Untangling the assembly of littoral macroinvertebrate communities through measures of functional and phylogenetic alpha diversity[END_REF]. While in the null model, the phenomenon that the FE model tends to classify species cooccurrence patterns more clustered than the FF model has been reported in previous studies. Although why lifting the constraints on abundance across different sites in simulations would lead to lower levels of aggregation among species requires further research [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF].

Environmental filtering plays an important role in the assembly of fish communities, and this pattern has been proven to exist at different landscape scales, such as rivers [START_REF] Peres-Neto | Patterns in the co-occurrence of fish species in streams: the role of site suitability, morphology and phylogeny versus species interactions[END_REF][START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF] and lakes [START_REF] Jackson | Null models and fish communities: evidence of nonrandom patterns[END_REF][START_REF] Castillo-Escrivà | Spatial and environmental analysis of an ostracod metacommunity from endorheic lakes[END_REF][START_REF] Maloufi | Environmental heterogeneity among lakes promotes hyper β-diversity across phytoplankton communities[END_REF]. It is well known that most organisms cannot establish and survive in all environments [START_REF] Kraft | Community assembly, coexistence and the environmental filtering metaphor[END_REF], and fish communities are limited by environmental conditions. Lake ecosystems in particular are threatened by multiple anthropogenic stressors such as climate change, land use intensification, eutrophication, acidification, water abstraction, water level regulation, morphological changes and invasive species [START_REF] Dudgeon | Freshwater biodiversity: importance, threats, status and conservation challenges[END_REF][START_REF] Smol | Under the radar: long-term perspectives on ecological changes in lakes[END_REF]. An important role of environmental filtering for the assembly of lake biomes can thus be expected [START_REF] Heino | Lakes in the era of global change: moving beyond single-lake thinking in maintaining biodiversity and ecosystem services[END_REF]. In this study, environmental filtering also had the greatest relative importance during the assembly of fish communities in lakes in the middle reaches of the Yangtze River. Specifically, species that showed greater similarity in environmental requirements (i.e., eutrophication, land use, lake morphology, proliferation patterns) tended to be more positively associated with co-occurrence patterns, i.e., aggregated dispersal. Notably, environmental filtering plays a more important role in the null model that includes all species (rare and widespread). After digging deeper into the data, we found that this is due to the fact that rare species tend to exhibit a highly aggregated dispersal pattern, and their environmental requirements are often very similar, which further illustrates the important role of environmental filtering in the community assembly of this study.

The important role of species interactions in the assembly of lake biomes has also been frequently reported [START_REF] Englund | Predation leads to assembly rules in fragmented fish communities[END_REF][START_REF] García-Girón | Biotic interactions hold the key to understanding metacommunity organisation[END_REF], unlike river ecosystems where environmental filtering plays a decisive role. [START_REF] Peres-Neto | Patterns in the co-occurrence of fish species in streams: the role of site suitability, morphology and phylogeny versus species interactions[END_REF][START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF]. In this study, interspecies competition significantly affected the species co-occurrence patterns in lake fish communities, and its relative importance was second only to environmental filtering. Especially in the model analysis of removing rare and widespread species, it is almost equivalent to environmental filtering.

However, the mode of interspecies competition being weaker than environmental filtering still exists in our study. It can also be seen from the results that the overall fish community is aggregated from the FE model. Relatively speaking, the environmental filtering effect may play more roles as a counterforce to the competition among species.

Species are expected to be segregated because of interspecies competition [START_REF] Weiher | Assembly rules, null models, and trait dispersion: new questions from old patterns[END_REF][START_REF] Webb | Phylogenies and community ecology[END_REF]. In terms of the characteristics of biological groups, weaker competition between fish populations may generally exist, mainly due to the wide range of food habits of fish [START_REF] Shurin | All wet or dried up? Real differences between aquatic and terrestrial food webs[END_REF]. And as an ectothermic animal, its demand for energy is relatively moderate [START_REF] Gotelli | Species co-occurrence: a meta-analysis of JM Diamond's assembly rules model[END_REF], so the intensity of competition is lower than that of a constant temperature animal of the same weight [START_REF] Peters | The ecological implications of body size[END_REF]. From the perspective of our null model, which is based on the absence/presence data of species, it measures the co-occurrence pattern of species and is not based on species abundance. Species with competing relationships are more prone to changes in relative abundance, often not enough to cause interspecies isolation, and species isolation often occurs on a large environmental gradient [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF]. The pressure gradient hypothesis supports that in the presence of more severe environmental changes or human disturbance, the impact of interspecies competition on communities is weakened [START_REF] Power | Biotic and abiotic controls in river and stream communities[END_REF]. The spatial scale of the study could also explain the lack of evidence for a role for competition in species aggregation, perhaps more in the microenvironment [START_REF] Taylor | Abundance and distribution within a guild of benthic stream fishes: local processes and regional patterns[END_REF][START_REF] Holomuzki | Biotic interactions in freshwater benthic habitats[END_REF]. Our study is based on the regional scale of lakes in the middle and lower reaches of the Yangtze River. Notably, in our regression models, interspecies competition had the opposite effect on interspecies co-occurrence patterns as expected and did not result in species segregation. Our study is based on the assumption that the functional traits and phylogenetic indicators we choose to measure interspecies competition are not affected by environmental filtering. Indeed, our chosen indicators, despite their maximal efforts to capture competing interplaying biological and spatial factors in an ecological sense, are still inevitably associated with environmental filtering. Because both fish function and species classification reflect the specificity of species adaptation to the environment [START_REF] Tilman | Forecasting agriculturally driven global environmental change[END_REF].

Predator-prey interactions played little role in community assembly in this study, although there are many reports in freshwater ecosystems suggesting negative predatorprey correlations, all based on changes in population abundance [START_REF] Gilliam | Structure of a tropical stream fish community: a role for biotic interactions[END_REF][START_REF] Englund | Predation leads to assembly rules in fragmented fish communities[END_REF]. At the same time some reports have suggested that the relatively low abundance of predators may explain why the effects of predator-prey interactions were not observed [START_REF] Grossman | Assemblage organization in stream fishes: effects of environmental variation and interspecific interactions[END_REF]. This is the same situation as in this study, that is, the prey species dominate. Also, our null model is based on species cooccurrence patterns rather than abundance. Since predation rarely results in species extinction, this may be even more detrimental to the observed effects of predator-prey interactions on community assembly.

Comparison with other studies on the assembly mechanism of fish communities

In order to gain a deeper understanding of the fish community assembly patterns found in this study in lakes in the middle reaches of the Yangtze River. This study contrasts our findings with other studies that have used similar approaches to explore the relative importance of environmental filtering and interactions between species. In this way, the theoretical system and method framework of global biome assembly will be enriched, and it will also provide a scientific basis for the management and scientific protection of lake fish communities in the Yangtze River Basin.

Focusing on our study area, lakes in the Yangtze River Basin, although there are few related studies, [START_REF] Jia | Unravelling fish community assembly in shallow lakes: Insights from functional and phylogenetic diversity[END_REF] explored the mechanism of fish community assembly in shallow lakes in the same area using functional and phylogenetic diversity. [START_REF] Jia | Unravelling fish community assembly in shallow lakes: Insights from functional and phylogenetic diversity[END_REF] also used the null model versus species co-occurrence patterns to quantify the importance of deterministic and stochastic processes in fish community assembly. Their results show that deterministic and stochastic assemblage processes occur simultaneously, and that stochastic processes account for the majority of fish community assembly in most of the lakes studied. Like our study, they also realized that such results may be the result of a combination of deterministic processes. And since more communities were shown to be over-aggregated than over-segregated in their study, [START_REF] Jia | Unravelling fish community assembly in shallow lakes: Insights from functional and phylogenetic diversity[END_REF] speculated that environmental filtering might play a more important role. But because the study did not quantify species interactions and environmental filtering, interpretation of the results is purely hypothetical. And it found no environmental factors that could well explain its results.

Generally speaking, there are many studies on the mechanism of fish community assembly in river ecosystems around the world. [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF] studied the assembly mechanism of fish communities in American rivers using species-based functional traits and found that 75%-85% of river fish communities are determined by non-stochastic patterns. Among them, environmental filtering plays a decisive role, followed by predator-prey interactions (predator-prey interactions), while interspecific competition plays the least role. Their research is the same as this study in that they also observed that interspecific competition tends to cause interspecies aggregation.

Although [START_REF] Giam | Environment and predation govern fish community assembly in temperate streams[END_REF] attributed this phenomenon mainly to the influence of weaker interspecific competition, we believe that the correlation between selected functional traits and the environment cannot be ignored. Their findings differ from this study in that our study found that interspecific competition was second only to environmental filtering, possibly because our study was a lake ecosystem and the study scale was small. Fitzgerald et al. (2017a) used fish functional traits and functional diversity to study the assembly of fish communities in the main tributaries of the Amazon Basin, and found that two key drivers of fish community assembly will change with seasonal changes. This demonstrates that environmental gradients affect the strength of interactions between species. [START_REF] Geheber | The effect of spatial scale on relative influences of assembly processes in temperate stream fish assemblages[END_REF] found that environmental filtering plays a decisive role in the assembly process of river fish communities, and its role is enhanced with the expansion of the spatial scale. This highlights that the assembly mechanisms of fish communities are influenced by spatial scales.

In terms of the research methods adopted, this study used a species pair cooccurrence method based on the differences in species function, phylogeny and environmental requirements. Because our goal is to reveal community assembly mechanisms by linking different types of species interactions to the degree of species co-occurrence in species pairs. Another alternative method is to regard different lake fish communities as metacommunities, and judge the assembly mode of each metacommunity [START_REF] Heino | Elements of metacommunity structure and community-environment relationships in stream organisms[END_REF][START_REF] Tonkin | Elements of metacommunity structure of river and riparian assemblages: Communities, taxonomic groups and deconstructed trait groups[END_REF]. Mechanisms of fish community assembly for each metacommunity can also be assessed by using the null model to test for phylogenetic or functional trait dispersion [START_REF] Peres-Neto | Patterns in the co-occurrence of fish species in streams: the role of site suitability, morphology and phylogeny versus species interactions[END_REF][START_REF] Troia | Functional strategies drive community assembly of stream fishes along environmental gradients and across spatial scales[END_REF]. And environmental filtering can be quantified based on the specific environment [START_REF] Montaña | Intercontinental comparison of fish ecomorphology: null model tests of community assembly at the patch scale in rivers[END_REF]. This differs from our approach, which is based on species-pair co-occurrence patterns and the similarity of traits between species.

Conclusion

This chapter, as the core chapter of this thesis, explains the key issues in the mechanism of fish community assembly in lakes in the middle reaches of the Yangtze River by quantifying the inter-species co-occurrence patterns, environmental filtering, inter-species competition and predator-prey interactions. Firstly, it was verified that neutral theory and niche theory are not absolutely opposite in explaining fish community assembly, but compatible and complementary. Although fish communities in lakes in the middle reaches of the Yangtze River tend to be more stochastic dispersal, this pattern is caused by the two interactive forces of environmental filtering and interspecies interactions. We also determined the relative importance of environmental filtering in the process of fish community assembly in lakes in the middle reaches of the Yangtze River. While explaining the relatively low importance of inter-species interactions, it reveals that the fish community assembly process in this region is greatly influenced by environmental factors. In comparing with other relevant research, explore ways to improve and expand future research. This study enriches the theoretical system and method framework of global biome assembly, and also provides a scientific basis for the management and scientific protection of lake fish communities in the Yangtze River Basin.

Chapter7 General conclusions and perspectives

Research conclusions

The middle reaches of the Yangtze River is one of the most concentrated areas of lakes in China, forming a typical river-lake complex ecosystem and forming a unique and rich freshwater fish fauna. However, in the past few decades, the lakes in the middle reaches of the Yangtze River have been disturbed by factors such as water pollution, land use change, habitat fragmentation, and overstocking, and the composition and structure of fish communities have undergone significant changes. In recent years, China has fully implemented the Yangtze River ecological environment protection and restoration strategy, and the lake recuperation system has been gradually improved.

Since then, lakes in the middle reaches of the Yangtze River have entered a new historical period of ecological restoration and succession, which provides a valuable historical opportunity and an ideal ecosystem for studying the mechanism of fish community assembly in the process of ecological restoration and succession of lakes.

Clarifying the fish community assembly mechanism in the process of lake ecological restoration succession has important theoretical significance for the maintenance of Build a prediction model to identify key predictors and explore the application of prediction models and ecological threshold detection in lake ecosystem management.

(5) To explore the main driving forces in the mechanism of fish community construction and to quantify the relative importance of environmental filtering and inter-species interactions in community assembly. The main conclusions drawn from this study are as follows:

(1) The four categories of environmental pressures involved in this study (eutrophication, land use, lake morphology, and proliferation patterns) can comprehensively and effectively explain the heterogeneity of environment and human disturbance among lakes. They are plausible as major drivers of environmental screening. Second, we validated that eutrophication, as a secondary disturbance, is influenced by other environmental factors and broadly acts on multiple dimensions of fish community structure. Therefore, eutrophication has been identified as a key factor in the study of the relationship between lake environmental pressure and fish community structure in the middle reaches of the Yangtze River, which is conducive to revealing its specific impact on fish communities.

(2) This study reveals that eutrophication affects fish communities through environmental filtering, which originally affects species composition through trait filtering, thereby affecting fish community structure. It is emphasized that functional structure fundamentally explains the response process of fish communities to environmental stress. Specifically, from the perspective of habitat and feeding nutrition, eutrophication increases the dominance of tolerant species in pelagic zooplanktivorous species by affecting habitat quality and changing resource availability. From the perspective of habitat utilization, eutrophication reduces the species dominance of benthic life. From the perspective of life history strategy, we applied the triangular continuous life history strategy framework to conduct research, and the results showed that eutrophication negatively impacted equilibrium species by reducing environmental stability and reducing resource limitation. In displacing the species dominance of the equilibrium, periodic strategists have a greater advantage than opportunistic strategists, possibly because periodic strategists are in a better position in resource competition and predator-prey relationships.

(3) In this study, no direct relationship between species richness and environmental factors was found, but it was determined that the increase in community biomass was mainly due to the increase in the dominance of a few species. This implies intensified competition among species, while the impact of environmental factors on the diversity of fish communities can be explained by inter-species interactions. Based on this, this study proposes the "biomass-driven competition hypothesis". It was verified that the rise of fish community biomass intensified interspecies competition, and there were unimodal relationships between fish biomass and species and functional richness. And it is determined that the process of biomass-driven competition leading to the loss of community diversity is mediated by nutrient availability, and environmental pressures such as eutrophication are the main reason for the increase in biomass. This process reflects the joint influence of environmental filtering and inter-species interactions on fish community structure and diversity.

(4) Starting from ecosystem and fish community management, this study expands the research on the mechanism of environmental filtering on fish community structure.

Twelve key predictors in four categories of eutrophication, land use, lake morphology, (5) This study sheds light on key issues in the mechanism of fish community assembly in lakes in the middle reaches of the Yangtze River by quantifying interspecies co-occurrence patterns, environmental filtering, interspecies competition, and predatorprey interactions. Firstly, it was verified that neutral theory and niche theory are not absolutely opposite in explaining fish community assembly, but compatible and complementary. Although fish communities in lakes in the middle reaches of the Yangtze River tend to stochastic dispersal, this pattern is due to the interaction of environmental filtering and inter-species interactions. And it is determined that the environmental filtering effect is relatively more important in the process of fish community assembly in lakes in the middle reaches of the Yangtze River. While explaining the relatively low importance of inter-species interactions, it reveals that the fish community assembly process in this region is greatly influenced by environmental factors.

To sum up, this study starts from the multiple environmental pressures faced by lakes in the middle reaches of the Yangtze River, and identifies key environmental factors that have important impacts on fish communities. And reveal the impact of its mediated environmental filtering and inter-species interactions on the species composition, functional structure and diversity of fish communities. At the same time, the feasibility of applying the prediction model based on key environmental factors and the ecological threshold detection model to lake ecosystem and fish community management was explored and evaluated. Finally, based on the quantification of species co-occurrence patterns, environmental filtering, interspecies competition, and predatorprey interactions, the relative importance of environmental filtering and interspecies interactions in fish community assembly was revealed from a mechanistic perspective.

This study enriches the theoretical system and method framework of global biological community assembly, and also provides a scientific basis for the management and scientific protection of lake fish communities in the Yangtze River Basin.

Research prospects

Based on the multiple environmental pressures faced by the lakes in the middle reaches of the Yangtze River, this study systematically studied the mechanism and relative importance of environmental filtering and inter-species interactions in the process of fish community assembly. The application of quantitative analysis-based ecological prediction models in lake ecosystem management is also explored. However, under the current background of global changes, as well as the domestic Yangtze River ecological environment protection and restoration strategy, measures such as the "Yangtze River fishing ban for ten years" have been further promoted, and at the same time, with the continuous innovation of ecological theories. We realize that there is a lot of room for improvement in future research, whether it is based on improving the methods and ideas adopted in this study, or based on the enlightenment of the protection of lake ecosystems and fish communities in the Yangtze River Basin, the whole country, and the world. Therefore, we put forward some prospects for this study from two aspects: the improvement of research methods and the enlightenment of ecosystem management.

Based on improvements of research method ideas:

(1) The environmental pressure and human disturbance faced by the lake ecosystem are very complex. In this study, when dealing with the complexity among environmental pressures, we chose to identify key environmental factors. It is equivalent to finding a "spokesperson" for the complicated environmental pressure.

However, this will inevitably ignore some potential effects. Future research may try to quantify the explanatory power and interaction of different environmental pressures on fish community changes.

(2) This study mainly focuses on the functional structure of fish communities, so the functional traits of species are crucial research tools. However, the functional traits in this study are basically obtained based on the existing literature records, and such data obviously ignore the possible population differences between different lakes.

Moreover, the proportion of continuous data is small, and some categorical variables may confound the functional characteristics of species. For example, qualitative description methods such as omnivorous and piscivorous are far less ecologically meaningful than the information provided by the results of intestinal content analysis.

(3) This study hardly involved phylogenetic diversity, mainly because the research scale was limited to lakes in the middle reaches of the Yangtze River, and there was little difference in fish fauna among different lakes. But phylogenetic diversity may provide more information from different dimensions.

(4) This study attempts to accurately quantify interspecies competition, both at the lake level and at the species pair level. From the lake level we employed a series of taxonomic and functional diversity indices, and the species pair level quantified based on similarity between species. However, these methods are indirect and cannot achieve pure and direct quantification of the effect of competition.

(5) This study focuses on the impact of environmental pressure on fish communities, that is, it mainly studies the bottom-up effect of the ecosystem. No studies have been conducted on the top-down effects of fish populations on the environment, such as water quality. However, how to quantify the upward and downward effects separately will be a difficult point.

(6) Based on the co-occurrence patterns and dissimilarity between species pairs is only one of the research methods for the mechanism of fish community assembly. There are other methods that may provide more open ideas, such as the study of metacommunities, or the study of species and function turnover and nesting effects based on β-diversity.

(7) This study hopes to identify a pair of opposing forces in the deterministic process of community assembly, that is, environmental filtering that lead to trait convergence or interspecies aggregation, and interbiotic interactions that lead to trait divergence or interspecies segregation. However, functional traits that quantify interactions between organisms are not completely independent of the effects of environmental factors. Therefore, it may be possible to try to build a model based on all deterministic processes that act on community assembly, without artificially defining it as an environmental filtering or an interaction between organisms, and split this pair of reactionary components through a statistical model.

Based on the enlightenments of ecosystem management:

(1) This study found that environmental filtering has a high relative importance in the assembly of fish communities in lakes. This shows that environmental pressure and human disturbance are indeed a great threat to lake ecological communities and fish community diversity, so this warns us that controlling human disturbance is the focus of lake ecosystem management.

(2) The key to the restoration of ecosystems and fish communities lies in the restoration of habitats. Our analysis of the functional structure of fish communities shows that due to habitat loss and simplification, the functional structure of fish communities tends to be homogeneous, which is the root cause of diversity loss.

(3) The research scale should be expanded. On the one hand, it should be extended spatially to the research on the laws of lake ecosystems and the mechanism of fish community construction in the whole country and even the world, and even the differences between different ecosystems, such as lakes and rivers. In this way, geographical restrictions can be broken and universal laws in the process of fish community assembly can be discovered.

(4) The expansion of the spatial research scale is also reflected in the analysis of subtle changes in small-scale areas, such as a certain lake. Factors affecting fish community assembly may have their specificity, complexity, and variability, and combining different scales may reveal changes affecting the relative importance of different driving forces in the process of fish community assembly.

(5) The extension of the time scale also has practical significance. For example, in the "10-Year Fishing Ban of the Yangtze River", the impact on the ecosystem and fish communities must continue to change over time, and different characteristics will be reflected in different stages.

(6) This study does not directly discuss the importance of river-lake connectivity in the process of lake fish community assembly. Because all the lakes studied are riverlake isolated lakes. However, landscape connectivity undoubtedly affects lake fish community assembly. Especially in the lakes studied in this study, the species diversity is low and the problem of single species is common. Studying the role of river-lake connectivity on fish community assembly may provide new ideas for fish community reconstruction.

(7) Research results should be more linked to ecosystem management practices.

For example, the identification of indicator species and traits, the screening of key environmental factors, and the detection of community ecological thresholds are of great significance to the management of lake ecosystems.

2. 1

 1 Introduction .................................................................................................... 2.2 Materials and methods ................................................................................... 2.2.1 Study area and sample setting ............................................................. 2.2.2 Sample and data collection ................................................................. 2.2.3 Data analysis ....................................................................................... 2.3 Results ............................................................................................................ 2.3.1 Spatial pattern of environmental factors ............................................. 2.3.2 Internal relationship of environmental factors .................................... 2.3.3 Influencing factors of fish community ................................................ 2.4 Discussion .................................................................................................... 2.4.1 Spatial patterns of multiple environmental pressures ....................... 2.4.2 Key environmental factors affecting fish communities .................... 2.5 Conclusion ................................................................................................... Chapter3 Response of fish community taxonomic and functional structure to eutrophication ............................................................................................................ 3.1 Introduction .................................................................................................. 3.2 Materials and methods ................................................................................. 3.2.1 Study area and sample setting ........................................................... 3.2.2 Sample and data collection ............................................................... 3.2.3 Data analysis ..................................................................................... 3.3 Results .......................................................................................................... 3.3.1 Correlation between eutrophication indicators ................................. 3.3.2 Response of fish community index to eutrophication....................... 3.3.3 Response of taxonomic composition of fish community to eutrophication ............................................................................................ 3.3.4 Response of functional traits of fish community to eutrophication .. 3.3.5 Response of life history strategies of fish community to eutrophication .................................................................................................................... Résumé Le cours moyen du Yangtsé est un écosystème typique de plaine d'inondation, mêlant cours principal du fleuve et une grande densité de milieux lacustres (une des plus forte de Chine). Il abrite une faune en poissons d'eau douce riche et unique. Durant les dernières décades, les lacs du cours moyen du Yangtsé ont été soumis à de nombreuses perturbations anthropiques induisant des impacts notables sur les communautés de poissons. Une meilleure connaissance des mécanismes d'assemblage des communautés est d'une grande importance pour préserver ou restaurer la biodiversité de ces milieux. Cette étude porte sur 26 lacs peu profonds du cours moyen du Yangtsé. Il est montré que les diverses perturbations humaines ont modifié la structure et la diversité taxonomique et fonctionnelle des peuplements via des processus de filtres environnementaux et d'interactions biotiques. Une analyse quantitative des patrons de co-occurrence des espèces permet de préciser la contribution des filtres environnementaux, de la compétition interspécifique et des relations proies-prédateurs dans les processus d'assemblage des communautés. Cette étude contribue à une meilleure connaissance des règles d'assemblage des communautés de poissons, ainsi qu'à définir les bases pour une gestion et une protection scientifiquement raisonnée des poissons lacustres du Yangtsé.

Figure 1 - 1

 11 Figure 1-1 Flow chart of the present study

Figure 2 - 1

 21 Figure 2-1 Map of the study region, the middle Yangtze River, China, and the locations of

  NH4 + -N, NO2 --N and NO3 --N were determined by Nessler's reagent colorimetric, and ultraviolet spectrophotometric screening methods, respectively. A known volume of pooled water sample was filtered through WHATMAN GF/C glass-fiber filters for Chla determination, which was measured spectrophotometrically following 90% acetone extraction. CODMn was determined by potassium permanganate and sodium oxalate titration method(APHA, 1998). Water quality variables were averaged per lake.Land use data were acquired by the GIS program ArcGIS (version 10.7) to analyze the percentages of different land use/cover types in the entire lake basin topography.

  ) Analyze the gradient pattern of the four environmental factors of eutrophication, land use, lake morphology, and proliferation mode in the lakes in the middle reaches of the Yangtze River.[START_REF] Feng | Application of artificial spawning substrates to support lacustrine fish recruitment and fisheries enhancement in a Chinese lake[END_REF] Analyze the correlation among the four types of environmental factors and find out their internal links. (3) Select the appropriate fish community structure measurement index, and identify the key environmental factors that have the most influence on the fish community through the correlation between environmental factors and fish community indicators. All analyzes in this study were performed under R version 4.2.2 (R Core Team, 2022).2.2.3.1 Pattern analysis of environmental factorsThe spatial pattern of environmental pressure of lakes in the middle reaches of the Yangtze River was analyzed from the perspective of four environmental factors: eutrophication, land use, lake morphology, and proliferation mode. Firstly, for each type of environmental factors, hierarchical clustering (Hierarchical Clustering) is used for cluster analysis, and the lakes are divided into different clusters according to the results.The principal component analysis (PCA) was used to reduce the dimensionality of each type of environmental factors. At the same time, the results of clustering are combined

Figure 2 - 2

 22 Figure 2-2 Cluster analysis results based on lake eutrophication

Figure 2 - 3

 23 Figure 2-3 Results of principal component analysis of lake eutrophication index

(

  compared with Group1 and Group4), the results are shown in Figure2-4. For other physical and chemical indicators of water bodies and different forms of nitrogen elements, except for dissolved oxygen and pH, there were no obvious differences among different groups (Figure2-5). Dissolved oxygen and pH showed an increasing trend from Group1-Group4.

Figure 2 - 4

 24 Figure 2-4 Differences in main eutrophication indicators between lakes

Figure 2 - 6

 26 Figure 2-6 Cluster analysis results based on lake land use

Figure 2 - 7

 27 Figure 2-7 Results of principal component analysis of lake land use

Figure 2 - 8

 28 Figure 2-8 Differences in land use types between lakes

Figure 2 - 9

 29 Figure 2-9 Cluster analysis results based on lake morphology

Figure 2 -

 2 Figure 2-10 Results of principal component analysis of lake morphology

Figure 2 -

 2 Figure 2-11 Differences of morphological parameters between lakes

Figure 2 -

 2 Figure 2-12 Cluster analysis results based on lake fishery model

Figure 2 -

 2 Figure 2-13 Results of principal component analysis of lake fishery model

Figure 2 -

 2 Figure 2-14 Differences of fishery model between lakes

  eutrophication. The correlation between land use indicators and eutrophication indicators is shown in Figure 2-15.

Figure 2 -

 2 Figure 2-15 The correlation between eutrophication index and land use index

Figure 2 -

 2 Figure 2-16 The correlation between eutrophication index and lake morphology index

  eutrophication of the water body, and the fishery proliferation model is the most relevant environmental factor to eutrophication in this study. The correlation between proliferation pattern indicators and eutrophication indicators is shown in Figure 2-17.

Figure 2 -

 2 Figure 2-17 The correlation between eutrophication index and lake fishery index

Figure 2 -

 2 Figure 2-18 The correlation between fish community index and eutrophication index

Figure 2 -

 2 Figure 2-19 The correlation between fish community index and land use index

Figure 2 -

 2 Figure 2-20 The correlation between fish community index and lake morphology index

Figure 2 -

 2 Figure 2-21 The correlation between fish community index and lake fishery index

  functional structure of fish in 26 shallow lakes along the middle reaches of Yangtze River basin, China. Present study further investigates the response of fish occurrence and abundance to eutrophication with different life history strategies and traits. The aims of this study are to Ⅰ) uncover the effect of eutrophication on the taxonomic structure of fish community; Ⅱ) figure out how eutrophication is associated with changes in the structure of functional traits in fish communities; Ⅲ) identify species and traits indicator of eutrophication; Ⅳ) by comparison with studies from other parts of the world, assess whether a general trait filtering by eutrophication exists; Ⅴ) in the framework of life history strategies, test the hypothesis that as eutrophication increases, occurrence and/or abundance of equilibrium species should decline.

Figure 3 - 1

 31 Figure 3-1 The division of three groups of life history strategies

Figure 3 - 3

 33 Figure 3-3 Correlation circle for 14 environmental variables

Figure 3

 3 Figure3-4) plus a large herbivorous species feeding on macrophytes

Figure 3 -

 3 Figure 3-4 RDA fish species abundance and environmental variables

Figure 3 - 5

 35 Figure 3-5 Ordinations of RLQ scores of environmental variables and species traits

Figure 3 - 6

 36 Figure 3-6 Ordinations of RLQ scores of species and lakes

Figure 3 - 7

 37 Figure 3-7 The relationships between species percentage of three life history strategies and

Figure 3 - 8

 38 Figure 3-8 Difference in eutrophication correlation coefficient of species among life history

Figure 3 - 9

 39 Figure 3-9 Principal Component Analysis of life history traits among all studied species

Figure

  Figure 3-10 The relationships between coordinates of life history PCA on first two axes and

Figure 4 -

 4 Figure 4-1 shows the regression analysis results between BPUE and species

Figure 4 - 1

 41 Figure 4-1 Regression analysis between fish biomass and species richness

Figure 4 - 2

 42 Figure 4-2 shows the regression analysis results between BPUE and Pielou

Figure 4 - 2

 42 Figure 4-2 Regression analysis between fish biomass and Pielou evenness index

Figure 4 -

 4 Figure 4-3 shows the regression analysis results between BPUE and Simpson

Figure 4 -

 4 Figure 4-4 shows the regression analysis results between BPUE and functional

Figure 4 - 4

 44 Figure 4-4 Regression analysis between fish biomass and functional richnessAll the variables are transformed by ln ().

Figure 4 -

 4 Figure 4-5 shows the regression analysis results between BPUE and functional

Figure 4 -

 4 Figure 4-6 shows the regression analysis results between BPUE and functional

Figure 4 - 6

 46 Figure 4-6 Regression analysis between fish biomass and functional dispersion indexAll the variables are transformed by ln ().

Figure 4 -

 4 Figure 4-7 shows the regression analysis results between BPUE and functional

Figure 4 - 7

 47 Figure 4-7 Regression analysis between fish biomass and functional redundancy indexAll the variables are transformed by ln ().

Figure 4 - 8

 48 Figure 4-8 Structural equation models reveal the bottom-up effects of environmental stress

Figure 5 - 1

 51 Figure 5-1 Evaluation of the importance of variables in RF of NPUE

Figure 5 - 2

 52 Figure 5-2 Evaluation of the importance of variables in RF of BPUE

Figure 5 - 3

 53 Figure 5-3 Evaluation of the importance of variables in RF of species richness

Figure 5 - 4

 54 Figure 5-4 Evaluation of the importance of variables in RF of Shannon diversity index

Figure 5 - 5

 55 Figure 5-5 Evaluation of the importance of variables in RF of functional richness

Figure 5 - 6

 56 Figure 5-6 Evaluation of the importance of variables in RF of functional dispersion

Figure 5 - 7

 57 Figure 5-7 Evaluation of the importance of variables in GF of species composition

Figure 5 - 8

 58 Figure 5-8 Evaluation of the importance of variables in GF of functional traits

Figure 5 - 9

 59 Figure 5-9 Prediction accuracy of the optimal RF for BPUE

Figure 5 -

 5 Figure 5-10 Prediction accuracy of the optimal RF for functional richness

Figure 5 -

 5 Figure 5-11 Prediction accuracy of the optimal RF for functional dispersion

Figure 5 -

 5 Figure 5-12 Ecological threshold detection at the level of species composition

Figure 5 -

 5 Figure 5-13 Ecological threshold detection at the level of functional groups

Figure 5 -

 5 Figure 5-14 Community level ecological threshold detection based on species composition

  this study, random forest models were used to assess the importance of environmental factors in terms of abundance, richness, and diversity of taxonomic structure and functional composition. At the same time, the gradient forest model was used to evaluate the importance of environmental factors from the perspective of multivariate structure of species composition and functional traits. Combining the above results and the research in the previous chapters, we identified variables that have important impacts on fish communities in the four categories of environmental pressures: eutrophication, land use, lake morphology, and proliferation patterns. Since the prediction of fish communities based on environmental factors has important practical significance in ecosystem management, we used the prediction model after variable screening and model optimization to predict biomass, functional richness, and functional dispersion. And the fitness and accuracy of the model were evaluated. After identifying key environmental predictors, one of its important applications in ecosystem management is the detection of ecological thresholds. We tested the ecological threshold of total phosphorus from the perspective of species composition, functional group structure and community level, and successfully obtained some results that are helpful for the optimization of ecosystem management.5.4.1 Identification of key predictors of fish communities and application of prediction modelsFor the identification of key predictors of fish communities, the integrated random forest model assesses taxonomic structure and functional composition in terms of abundance, richness, diversity and the gradient forest model evaluates environmental factors from the perspective of multivariate structure of species composition and functional traits. We determined 3 environmental factors for each category from the four categories of environmental pressures: eutrophication, land use, lake morphology, and proliferation mode, a total of 12 environmental factors. The key environmental factors in the eutrophication category are total phosphorus, chlorophyll a, and turbidity.

Figure 6 - 1 Figure 6 - 2

 6162 Figure 6-1 The relative importance of all predictive variables in the "FF All Species" model

Figure 6 - 3 Figure 6 - 4

 6364 Figure 6-3 The relative importance of all predictive variables in the "FF 2<OCC<N-2"

Figure 6 - 5

 65 Figure 6-5 The relative importance of all predictive variables in the "FE All Species" model

Figure 6 - 7 Figure 6 - 8

 6768 Figure 6-7 The relative importance of all predictive variables in the "FE 2<OCC<N-2"

Figure

  Figure 6-9 Regression relationship between SES and ENV in the "FF All Species" model

  lake biodiversity in the Yangtze River Basin, and can also provide technical support for the evaluation of the effect of watershed ecological restoration measures. In this study, 26 shallow lakes in the middle reaches of the Yangtze River were taken as the research object, and the lake environmental factors and fish community data were obtained through field surveys and other methods to carry out related research. The main research revolves around the following contents and ideas. (1) Elaborate the spatial pattern of environmental factors based on the impact of multiple environmental pressures faced by the lake ecosystem on fish communities. (2) To reveal the impact mechanism of key environmental factors on the species composition and functional structure of fish communities. (3) Proposed and verified the hypothesis that environmental pressure mediates interspecies interactions to drive changes in fish community diversity. (4)

  and proliferation pattern were identified by constructing predictive models, which provide key indicators for quantifying environmental filtering. In addition, the research in this chapter evaluates the prediction performance of the prediction model based on the construction of the prediction model. However, because the data set is small for the prediction model, the obtained prediction model has predictive ability, but the model fitness and accuracy are not ideal. As a supplement to the prediction model, this study carried out an ecological threshold test based on key factors, and further supplemented and expanded the mechanism of environmental filtering on fish communities. At the same time, it reflects the important significance of prediction model and ecological threshold test in the management of lake ecosystem and fish community.

  tropical riverine fishes.Oecologia, 156(3), 691-702. Teichert, N., Pasquaud, S., Borja, A., Chust, G., Uriarte, A., & Lepage, M. (2017).Living under stressful conditions: Fish life history strategies across environmental gradients in estuaries.Estuarine, Coastal and Shelf Science, 188, 18-26. Ter Braak, C. J., & Verdonschot, P. F. (1995). Canonical correspondence analysis and related multivariate methods in aquatic ecology. Aquatic sciences, 57(3), 255-289. Teresa, F. B., & Casatti, L. (2012). Influence of forest cover and mesohabitat types on functional and taxonomic diversity of fish communities in Neotropical lowland streams. Ecology of Freshwater Fish, 21(3), 433-442. Teresa, F. B., & Casatti, L. (2017). Trait-based metrics as bioindicators: responses of stream fish assemblages to a gradient of environmental degradation. Ecological Indicators, 75, 249-258. Tesfaye, M., Dufault, N. S., Dornbusch, M. R., Allan, D. L., Vance, C. P., & Samac, D.A. (2003). Influence of enhanced malate dehydrogenase expression by alfalfa on diversity of rhizobacteria and soil nutrient availability. Soil Biology andBiochemistry, 35(8), 1103-1113. Thiel, R., Sepulveda, A., Kafemann, R., & Nellen, W. (1995). Environmental factors as forces structuring the fish community of the Elbe Estuary. Journal of fish Biology, 46(1), 47-69. Thomas, G., & Eckmann, R. (2007). The influence of eutrophication and population biomass on common whitefish (Coregonus lavaretus) growth-the Lake

  Longgen and other judges for their valuable suggestions in my opening and mid-term assessment meetings. Thanks to Xiong Xiong, Han Jian, Wang Zhicong, Li Tianli, and Zhang Zhimin for their help in the cooperation project! At the same time, I would like support that I have the confidence and energy to move on! Having you in my life makes me full of longing and hope for the future

Table 2 -1 Name, code, maximum water depth and lake area of study lakes.

 2

	Yanzhi Lake	YZH	5.7	2.30

Lake

Code Maximum water depth (m) Lake area (km 2 )

Table 2 -2 The species investigated in this study

 2

	Family	Species	Code	Family	Species	Code
	Engraulidae	Coilia nasus*	C.nas	Gobionidae	Sarcocheilichthys nigripinnis* S.nig
	Salangidae	Protosalanx hyalocranius	P.hya	Gobionidae	Squalidus argentatus*	S.arg
	Salangidae	Neosalanx tangkahkeii	N.tan	Gobionidae	Saurogobio dabryi*	S.dab
	Xenocyprididae	Opsariichthys bidens	O.bid	Gobionidae	Abbottina rivularis*	A.riv
	Xenocyprididae Mylopharyngodon piceus Xenocyprididae Squaliobarbus curriculus	S.cur Acheilognathidae	Rhodeus ocellatus*	R.oce
	Xenocyprididae	Pseudolaubuca sinensis*	P.sin	Acheilognathidae	Rhodeus sinensis	R.sin
	Xenocyprididae	Hemiculter leucisculus*	H.leu	Cyprinidae	Cyprinus carpio*	C.car
	Xenocyprididae	Hemiculter bleekeri*	H.ble	Cyprinidae	Carassius auratus*	C.aur
	Xenocyprididae	Toxabramis swinhonis*	T.swi	Catostomidae	Myxocyprinus asiaticus	M.asi
	Xenocyprididae	Culter alburnus*	C.alb	Cobitidae	Misgurnus anguillicaudatus* M.ang
	Xenocyprididae Chanodichthys erythropterus*	C.ery	Siluridae	Silurus asotus*	S.aso
	Xenocyprididae	Chanodichthys mongolicus*	C.mon	Bagridae	Tachysurus fulvidraco*	T.ful
	Xenocyprididae	Chanodichthys dabryi*	C.dab	Bagridae	Tachysurus nitidus	T.nit
	Xenocyprididae	Parabramis pekinensis*	P.pek	Bagridae	Pelteobagrus eupogon	P.eup
	Xenocyprididae Megalobrama amblycephala* M.amb	Ictaluridae	Ictalurus punctatus +	I.pun
	Xenocyprididae	Xenocypris hupeinensis*	X.hup Hemiramphidae	Hyporhamphus intermedius*	H.int
	Xenocyprididae	Xenocypris macrolepis	X.mac	Sinipercidae	Siniperca chuatsi*	S.chu
	Xenocyprididae	Xenocypris davidi*	X.dav	Sinipercidae	Siniperca knerii	S.kne
	Xenocyprididae	Plagiognathops microlepis	P.mic	Odontobutidae	Odontobutis obscurus*	O.obs
	Xenocyprididae	Pseudobrama simoni*	P.sim	Odontobutidae	Micropercops swinhonis	M.swi
	Xenocyprididae Hypophthalmichthys nobilis* # H.nob	Gobiidae	Rhinogobius giurinus*	R.giu
	Xenocyprididae Hypophthalmichthys molitrix* # H.mol	Osphronemidae	Macropodus ocellatus	M.oce
	Gobionidae	Hemibrabus maculatus*	H.mac	Channidae	Channa argus*	C.arg
	Gobionidae	Paracanthobrama guichenoti*	P.gui Mastacembelidae	Sinobdella sinensis	S.sin
	Gobionidae	Pseudorasbora parva*	P.par

# M.pic Acheilognathidae Acheilognathus macropterus* A.mac Xenocyprididae Ctenopharyngodon Idella* # C.ide Acheilognathidae Acanthorhodeus chankaensis* A.cha * Species found in more than 3 lakes. + Introduced species. # Stocked species.

Table 2

 2

	-3 and Appendix

includes functional traits such as relative eye position, habitat preference, body shape, gregariousness, diurnal activity patterns, and vertical position. Traits values per species were obtained by consulting published literature, books, scientific reports, expert knowledge, field observations and online databases (Appendix 2)

Table 2 -3 The functional species traits used in the study

 2

	Traits	Code	Functional category	Type	Units
	Maximum standard length	S_SL	Life history strategy	Numeric	Maximum standard body length (cm).
	Longevity	S_L	Life history strategy	Numeric	Maximum life span (years).
	Growth parameter K	S_K	Life history strategy	Numeric	Parameter K of Von Bertalanffy growth formula (years -1 )
	Vulnerability	S_V	Life history strategy	Numeric	Vulnerability to fishing. Range 10-87.
	Resilience	S_R	Life history strategy	Ordinal	Resilience level of fish populations. Range 1-4. Very low, low, medium, high.
	Maturity age	S_MA	Life history strategy	Numeric	Mean age at maturation (years).
	Absolute fecundity	S_AF	Life history strategy	Numeric	Total number of oocytes per ovary (oocytes).
	Maturity standard length	S_ML	Life history strategy	Numeric	Mean standard length at maturation (cm).
	Egg diameter	S_ED	Life history strategy	Numeric	Mean diameter of mature (fully yolked) ovarian oocytes (mm).
	Egg type	S_ET	Life history strategy	Factor	Drifting (ET1), demersal (ET2), adhesive (ET3), pelagic (ET4).
	Parental care index	S_PC	Life history strategy	Ordinal	Parental care level. From low to high. Range 0-4.
	Feeding habit	F_FH	Feeding	Factor	Predator sit and wait (FH1), predator active search (FH2), grazer (FH3), scraper (FH4), substrate explorer (FH5), surface feeder (FH6).
	Trophic guild	F_TG	Feeding	Factor	Herbivore (TG1), algivore (TG2), detritivore (TG3), omnivore (TG4), zooplanktivore (TG5), zoobenthivore (TG6), piscivore (TG7).
	Trophic level	F_TL	Feeding	Numeric	Trophic position in the food chain. Range 2-4.5.
	Relative mouth size	F_MS	Feeding	Ordinal	Relative mouth size. From small to large. Range 1-5.
	Mouth position	F_MP	Feeding	Factor	Inferior (MP1), inferior-terminal (MP2), terminal (MP3), terminal-superior (MP4), superior (MP5).
	Relative eye position	H_EP	Habitat use	Factor	Top (EP1), lateral-top (EP2), lateral (EP3), lateral-bottom (EP4).
	Habitat preference	H_HP	Habitat use	Factor	Eurytopic (HP1), rheophilic (HP2), limnophilic (HP3), potamodromous (HP4).

Table 3 -1 Results of multiple regression models between community metrics and environmental variables

 3

	Response variable	Coefficient SE	t	Pr (> | t |)	R 2	% R 2
	Species richness				0.035	0.2572
	Intercept	-2.97E-16	0.17	0.000	1.000
	Water depth	0.50	0.18	2.759	0.012		45.60
	Chlorophyll a	0.36	0.18	2.034	0.055		23.43
	NO2 --N	-0.38	0.19	-2.062	0.052		18.28
	Lake area	0.25	0.17	1.466	0.158		12.69
	Total biomass				0.033	0.2287
	Intercept	9.13E-16	0.17	0.000	1.000
	Total phosphorus	0.52	0.21	2.571	0.017		61.42
	Dissolved oxygen	0.63	0.36	1.778	0.089		25.19
	pH	-0.60	0.37	-1.597	0.125		13.39
	Functional dispersion				0.003	0.4955

Table 3 -2 Environmental variables significance of the fish community-environment RDA

 3

	Variables	F	Pr (>F)
	Total nitrogen	1.50	0.131
	Total phosphorus	3.38	0.001
	CODMn	1.15	0.328
	Chlorophyll a	2.12	0.019
	Secchi depth	1.07	0.364
	Conductivity	1.43	0.150
	NO3 --N	1.77	0.050
	NH4 + -N	1.13	0.334
	NO2 --N	1.02	0.408
	Turbidity	1.11	0.340
	Dissolved oxygen	0.70	0.774
	pH	0.68	0.789
	Water depth	0.77	0.694
	Lake area	1.71	0.069

Table 3 -3 Results of multiple linear regression models between species eutrophication correlation coefficient and the coordinates of the species on the all axes (PCA1-PCA8) of the PCA analysis of life history traits

 3

	Response variable	Coefficient SE	t	p (> | t |) R 2
	Eutrophication correlation coefficient			0.053	0.1641

  whereas they display the opposite pattern in the present study. A possible reason is that, unlike perch, pike, and trout in European lakes, in the Yangtze Basin some predatory fishes, such as the

	piscivorous carps (Culter alburnus, Chanodichthys erythropterus, Chanodichthys
	mongolicus, Chanodichthys dabryi), can forage efficiently at low light intensities in
	turbid and dim environments (Department of Ichthyology, Institute of Hydrobiology,
	CAS, 1976).
	So far, possible general fish community patterns consistently observed in China

(present study) and other regions (mostly Europe) are as follows. Along a eutrophication gradient total biomass of the community increases and fish species having a benthic life stage are disadvantaged and functional homogeneity increases.

Note that the last relationship has been observed in only two studies, ours

[START_REF] Sagouis | Taxonomic versus functional diversity metrics: how do fish communities respond to anthropogenic stressors in reservoirs?[END_REF]

, using different metrics, and therefore deserves further investigation. Our results suggest that in addition to those related to habitat use or trophic ecology, new metrics based on the life history framework proposed by

Table 5 -1 Community-level ecological threshold detection results

 5

	Gradient	Threshold	Taxonomic 5%	95%	Threshold	Functional 5%	95%
	TP (μg L	-1 )
	TITAN sum (z-)	147.93	15.00	194.77	162.66	27.44	307.02
	TITAN sum (z+)	162.66	109.07	285.76	255.36	31.38	285.79

  . However, it is worth noting that in the random forest model, usually one-third of the data is removed from the bootstrap training samples and is not used for model construction. These removed data are called the test set of the model, and the other two-thirds of the data are called the training set. The training set is used to build the model, and these test sets are used as validation samples to evaluate the error of the model validation

  hesitation. And every day since then, Professor Li has always cared and taught me with such enthusiasm and kindness. As the last student of Professor Li, I am truly grateful and fortunate! Guides are important, but I am also grateful to have someone who can help me overcome difficulties on the road of scientific research. Thanks to Associate Professor Wang Qidong for his dedication to me! Rather than saying that he is my supervisor, he is more like a comrade in arms fighting with me. What he taught me is the attitude of facing difficulties and the attitude of overcoming difficulties. It is not terrible to encounter difficulties, there will always be a solution. This is the wealth I got from him. At the same time, Professor Wang's keen intuition and active thinking in the face of scientific research also benefited me a lot. It can be said that he paved the way for me on the road of scientific research, allowing me to have a clearer understanding of the road ahead, allowing me to realize the significance of research and find the direction of research. Therefore, I would like to thank Professor Wang for his hard work on me! My two French supervisors, Professor Sovan LEK and Professor Bernard HUGUENY, are the nobles of my research career! Thanks to Professor Sovan for giving me the opportunity to study in Toulouse University, France for a year. Although he has already retired, he still provides me with all possible help. He and Mrs. Sithan Lek-Ang always invite me to their home as a guest, entertain me warmly, provide convenience for my life, chat with me, and introduce many novel things. Whenever I talk about my scientific research progress, Professor Sovan can always provide me with some valuable but to-the-point suggestions, which are very helpful to broaden my horizons and develop my thinking. Professor Bernard is undoubtedly a scientist who should be greatly respected. Although he has rich experience and honors, he does not change his original intention and meticulousness when dealing with scientific research. It can be said that his teaching has polished and refined my scientific research ability. It is not even an exaggeration to say that he made me feel that the data processing that used to be boring has become an art, and the boring writing in the past has really risen to creation. And Bernard's teaching to me is entirely out of selfless dedication and rigorous attitude towards science. His spirit is worth my lifelong study. So express my sincere thanks to them. I sincerely thank Professor Liu Jiashou of the fishery ecology group of IHB for his help and support in my studies. Professor Liu is knowledgeable, unique, humorous, open-minded, simple and tolerant, and is a role model for me to learn from! I also thank Professor Zhang Tanglin, Professor Guo Chuanboyan, Associate Professor Ye Shaowen, and Associate Professor Li Wei for their advice, help and encouragement. Thanks to Ms. Yuan Jing, Mr. Chen Xinnian, and Ms. Peng Yumei for their help and convenience in my study and life. I would like to thank Professor Xie Shouqi, Professor Xu Jun, Professor Chen Yushun, Professor Chen Jun, Professor Duan Ming, Professor Guo

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (No. 2018YFD0900701), National Natural Science Foundation of China (No. 32002396), Youth Innovation Promotion Association CAS (No. 2022344). We appreciate the Hubei Provincial Department of Agriculture and to thank Song Dan, Du Xue, Lian Yuxi, Tang Jianfeng, and Qin Xiangchao for their concern and help! Thanks to Associate Professor Pablo TEDESCO of UPS's EDB Laboratory for his care and help in my life and research. Thanks to Professor Thierry OBERDORFF for his suggestion on my paper submission. Thanks to Professor Sébastien BROSSE and Associate Professor Lisa JACQUIN for their suggestions on my thesis research. Thanks to Ms. Dominique PANTALACCI from the Graduate School for helping me with my PhD registration and other work! Thanks to Li Chang, Ren Yan, Cai Xingwei, Huang Geng, Liao Chuansong, Yu Jixin, Wang Rui, Xiong Ying, Yuan Ting, Jin Shiyu, Dong Xianghong, Chen Xiaohang, Qian Jing, Xiong Mantang, Qu Xiao, Li Ruojing, Xiang Tao , Xiao Lingjun, Zhao Siqi, and Guo Chao have helped me in my study and life. At the same time, I would like to thank my classmates Maizhan, Li Shiqi, Huang Feng, and Wang Puze for traveling with me and helping me. Thanks to the younger brothers and sisters, Tan Lirong, Gong Kun, Feng Yuming, Li Junfeng, Wang Jianzhi, Rong Kuan, Shi Lei, Li Wang, Wang Jiacheng, Liu Qiao, Ji Xiang, Xu Bo, Yang Lei, Wu Jiajun, Wen Feng, Ke Jie, Zhang Hang, Ma Yuxing, Guo Qianqian and others helped and accompanied me. Thanks to my friends at IHB, Lin Yiwen, Lei Lei, Gao Guangbin and others for spending time with me! Thanks to Wang Jing and Zhou Yuxiang for working in the group, not only have I gained helpers but also friends! Special thanks to Deng Wenbo and Li Haoxuan for traveling with me and suffering! Especially thanks to my good friends, Zhang Yinzhe and Li Fan for their help with my experiment sampling, as well as the delicious food and jokes I

the t-test was used to test whether the mean SES value of all species pairs in each model was 0. The null model was assembled using the Pairs program (version 1.0) and the Cscore and SES between species pairs were obtained [START_REF] Ulrich | Pairs -a FORTRAN program for studying pair wise species associations in ecological matrices[END_REF].

Quantification of environmental filtering and interactions between species

In this study, 3 variables were selected from each of the four categories of environmental pressures: eutrophication, land use, lake morphology, and proliferation mode. A total of 12 variables were used to quantify environmental filtration. See 5.4.1 for the specific selection process and reasons. The key factors in the eutrophication category are total phosphorus, chlorophyll a, and turbidity. The key factors in the land use category are agricultural land, grassland, pond. The key factors in the lake morphology category are lake area, maximum water depth, and SDI index. The key factors in the proliferation pattern category are total annual production, annual stocking volume, and feed and fertilizer input levels. After determining the environmental variables, first use the Pearson correlation coefficient between the abundance data of each fish in each lake and the data of 12 environmental variables to calculate. See the calculation of "response to eutrophication index" in 3.2.3.4 for specific methods and meanings. Then a set of 12 correlation coefficients were obtained between each species, and then the Gower's dissimilarity coefficient between the correlation coefficients of each species was calculated. Environmental filtering was thus quantified using the dissimilarity in response to the environment between each species pair.

In this study, eight species functional and phylogenetic variables were used to the predator-predation relationship between species pairs as predictor variables.

Construct a multiple regression based on the distance matrix, and the multiple regression is constructed using a linear model. To improve the comparability of regression coefficients within and between models, the response and predictor variables were standardized (zero mean and unit variance). Multiple regression models based on distance matrices were assembled and tested using the "MRM" function of the R package "ecodist" [START_REF] Goslee | The ecodist package for dissimilarity-based analysis of ecological data[END_REF]. To calculate the relative importance of the predictors, the relative importance of the predictors was calculated by the "lmg" method using the "calc.relimp" function of the R package "relaimpo" [START_REF] Grömping | Relative importance for linear regression in R: the package relaimpo[END_REF].

Multiple regression relationships were visualized using the "scatterplot3d" function in the R package "scatterplot3d" [START_REF] Ligges | Scatterplot3d-an r package for visualizing multivariate data[END_REF].

Results

Patterns of species co-occurrence in fish communities

In this study, four null models were constructed, namely the FF model and FE model based on all species, and the FF model and FE model based on species with more than 2 occurrences and less than N-2 species. See Table 6-1 for the test results of whether the mean of the SES values of the four models is zero. Among the results of all FF models, the results of the null model SES mean test are equal to zero, indicating that the co-occurrence mode of fish community species in lakes in the middle reaches of the Yangtze River based on the FF null model is stochastic dispersal. In all FE models, the mean value of SES was significantly less than 0, indicating that the species cooccurrence pattern was aggregated dispersal. Although the SES value does not meet the judgment criteria of [START_REF] Kembel | Picante: R tools for integrating phylogenies and ecology[END_REF] for overdispersed and overclustered communities, that is, the SES value is greater than 1.96 (p>0.975) or less than -1.96 (p>0.025). The relative importance of all predictor variables in the model "FF All Species" is shown in Figure 6-1, and a visualization of the regression model except for predatorprey interactions is shown in Figure 6-2. The role of environmental filtering has the highest relative importance in the model and is much greater than that of interspecies interactions, and the relative importance of interspecies competition is greater than that of predator-prey interactions. Both ENV and PRED were positively correlated with SES, that is, the more similar the environmental requirements, functions and phylogeny among species, the closer the inter-species co-occurrence pattern was to inter-species aggregation.

promise: integrating species traits to transform descriptive community ecology into a predictive science. Freshwater Science, 32(2), 531-547.

Vila-Gispert, A., Moreno-Amich, R., & Garcia-Berthou, E. (2002). Gradients of life-

Appendix1 Description of functional trait

The 22 functional species traits used in this study, with a brief description of their ecological significance.

Appendix2 Values of functional traits of species

The species codes and the value of 22 functional traits for the 53 species studied. 3 functional categories (life history strategy, feeding and habitat use) were considered.

The species code and the value of 11 life history strategy. * Species found in more than 3 lakes (36 species). + Introduced species (1 species). # Stocked species (4 species).

The species code and the value of 11 feeding and habitat use traits.

Appendix3 Results of functional group division of species

Result from cluster analysis indicating species assignment into functional groups.

The image of each fish is displayed after the fish code. The species code is in Appendix2.

Rural Affairs for providing the convenience and kindly assistance in field sampling.

During the six years of PhD study, I have experienced too many growth and tests.

At the moment when I was about to finish my studies, every word and every emotion that all the teachers, classmates, friends and relatives who supported me said to me and gave me appeared in my mind.