T Silverthorn
email: teresa.silverthorn@inrae.fr

N López-Rojo
email: naiara.lopez-rojo@univ-grenoble-alpes.fr

Greenhouse gas dynamics in river networks fragmented by drying and damming

Keywords: Fragmentation, Catchment, Intermittent, Reservoir, Regulation

1. River fragmentation by drying and damming is occurring more frequently in the Anthropocene era, yet there is limited knowledge of how this fragmentation influences greenhouse gas (GHG) fluxes in river networks. River networks have the potential to be important sources of GHGs to the atmosphere through both similar and dissimilar mechanisms associated with temporary (drying) and permanent (damming) fragmentation.

2. We conducted a review of the literature and found 49, 43, and 6 studies about GHGs (CO2, CH4, and N2O) in rivers impacted by damming, drying, and their interaction, respectively.

3. We found research lacking in non-arid climates and in small water retention structures for studies regarding drying and damming, respectively. The major factors directly influencing GHG fluxes in river networks impacted by drying were sediment moisture, temperature, organic matter content and texture. In networks impacted by damming, the most influential factors were water temperature, dissolved oxygen, and phytoplankton chlorophyll a.

4. Based on our literature review and meta-ecosystem theory, we propose that the spatial distribution of fragmentation strongly influences GHG fluxes at the river network scale.

5. The actionable future research directions identified here will help to improve our understanding of the effects of fragmentation by drying and damming on GHG fluxes, with the potential to inform river management and climate change mitigation strategies.

Introduction

Rivers were long considered as passive pipes for carbon transport between the land and ocean in early conceptual models of the carbon cycle [START_REF] Wohl | Carbon dynamics of river corridors and the effects of human alterations[END_REF]. Now, we know that physical, chemical, and biological processes in river networks have an important influence on carbon dynamics [START_REF] Benstead | An expanded role for river networks[END_REF][START_REF] Cole | Plumbing the global carbon cycle: Integrating inland waters into the terrestrial carbon budget[END_REF][START_REF] Wohl | Carbon dynamics of river corridors and the effects of human alterations[END_REF].

River networks are areas of disproportionate biogeochemical exchange compared to their spatial extent in the landscape [START_REF] Allen | Global extent of rivers and streams[END_REF]. The river continuum concept describes the longitudinal gradient of physical conditions in unaltered rivers [START_REF] Vannote | The river continuum concept[END_REF]. It details the fate of organic matter along a river network, with high allochthonous inputs in headwater reaches, and increasing importance of autochthonous primary production and organic matter transport from upstream as stream size increases [START_REF] Vannote | The river continuum concept[END_REF].

However, river networks do not always follow this conceptual model, for example, due to alterations as a result of the increasing pressure of fragmentation of river networks in the Anthropocene era [START_REF] Fuller | Causes and consequences of habitat fragmentation in river networks[END_REF]. In this review, we define fragmentation at the landscape scale as the temporary (drying) or permanent (damming) change in hydrologic connectivity, disrupting the spatial flows of energy, materials, and organisms within a river network [START_REF] Fahrig | Effects of Habitat Fragmentation on Biodiversity[END_REF][START_REF] Fuller | Causes and consequences of habitat fragmentation in river networks[END_REF][START_REF] Loreau | Meta-ecosystems: A theoretical framework for a spatial ecosystem ecology[END_REF]. River networks are fragmented by both natural (e.g. drying, waterfalls, beaver dams) and anthropogenic agents (e.g. damming, water withdrawal, pollution) [START_REF] Fuller | Causes and consequences of habitat fragmentation in river networks[END_REF]. In this review, we will focus on drying and damming (referring to artificial in-stream barriers of all sizes), as they are two of the most widespread causes of fragmentation in river networks [START_REF] Belletti | More than one million barriers fragment Europe's rivers[END_REF][START_REF] Messager | Global prevalence of non-perennial rivers and streams[END_REF].

The occurrence of drying in river networks is increasing due to climate change, damming, and water abstraction [START_REF] Datry | Causes, Responses, and Implications of Anthropogenic versus Natural Flow Intermittence in River Networks[END_REF]. These waterways that periodically cease to flow, can be defined as intermittent rivers and ephemeral streams (hereafter, IRES) [START_REF] Datry | Intermittent Rivers: A Challenge for Freshwater Ecology[END_REF]. Recent estimates have quantified the extent of IRES which dry for at least one day of the year, as comprising 51-60% of the total global stream network [START_REF] Messager | Global prevalence of non-perennial rivers and streams[END_REF]. Fragmentation by drying can create different states in space and time: dry river beds, isolated pools, and lotic waters. These flow regime states, and the transitions between them (i.e. rewetting and drying), create unique environments that may promote the production and consumption of GHGs.

In parallel, a large majority of the earth's river systems are impacted by damming. A global analysis found that only 37% of rivers longer than 1000 km are free-flowing without barriers over their entire length [START_REF] Grill | Mapping the world's free-flowing rivers[END_REF]. Dams are a disruption to the river continuum, as defined in the serial discontinuity concept [START_REF] Stanford | The serial discontinuity concept of lotic ecosystems[END_REF], causing alterations in abiotic and biotic processes and patterns through the fragmentation of longitudinal, lateral, and vertical pathways [START_REF] Wang | Effects of dams on riverine biogeochemical cycling and ecology[END_REF]. Dams also alter river hydrology, sedimentation, nutrient biogeochemical cycling, reducing the connectivity of water, organisms, sediments, organic matter, nutrients and energy [START_REF] Grill | Mapping the world's free-flowing rivers[END_REF]. Damming can promote GHG emissions from both the reservoir surface, as well as degassing emissions at the reservoir outlet (i.e. dam spillways and turbines), downstream river emissions, and drawdown emissions from reservoir sediments that are periodically inundated and exposed to the atmosphere [START_REF] Deemer | Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis[END_REF][START_REF] Maavara | River dam impacts on biogeochemical cycling[END_REF].

Fragmentation by drying and damming occur both separately and in interaction, and they may have certain comparable effects, as well as interactive effects, on GHG fluxes from river networks. Reservoirs increase water residence time in rivers and transform lotic reaches into lentic waters [START_REF] Abril | Effects of water flow regulation on ecosystem functioning in a Mediterranean river network assessed by wood decomposition[END_REF][START_REF] Sabater | Alterations of the global water cycle and their effects on river structure, function and services[END_REF]. Below impoundments, flow rates may be reduced or the flow regime may change from perennial to temporary, leading to an increase in dry riverbed surface and modifications in ecosystem processes [START_REF] Abril | Effects of water flow regulation on ecosystem functioning in a Mediterranean river network assessed by wood decomposition[END_REF][START_REF] Bianchi | The experimental flow to the Colorado River delta: Effects on carbon mobilization in a dry watercourse[END_REF][START_REF] Datry | Causes, Responses, and Implications of Anthropogenic versus Natural Flow Intermittence in River Networks[END_REF]. In many cases, natural and anthropogenic drivers interact to cause intermittence [START_REF] Datry | Causes, Responses, and Implications of Anthropogenic versus Natural Flow Intermittence in River Networks[END_REF]. Alternatively, baseflow augmentation by reservoirs can create artificial permanent flow regimes downstream of reservoirs in reaches that previously may have been intermittent in nature [START_REF] Acuña | Managing temporary streams and rivers as unique rather than second-class ecosystems[END_REF]. These impacts are in many ways parallel to those posed by drying, where we observe a spatial and temporal mosaic of dry, lentic, and lotic reaches [START_REF] Datry | Intermittent Rivers: A Challenge for Freshwater Ecology[END_REF]. Global dry inland waters represent a significant source of CO2 and methane (CH4) emissions [START_REF] Keller | Global CO2 emissions from dry inland waters share common drivers across ecosystems[END_REF][START_REF] Marcé | Emissions from dry inland waters are a blind spot in the global carbon cycle[END_REF][START_REF] Paranaíba | Crosscontinental importance of CH4 emissions from dry inland-waters[END_REF]. Additionally, enhanced CO2 emissions have been observed upon the rewetting of riverbed sediments and leaf litter accumulated in dry reaches [START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF][START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF][START_REF] Schiller | Sediment respiration pulses in intermittent rivers and ephemeral streams[END_REF]. In lentic pools and impoundments, organic matter and sediment can also accumulate [START_REF] Wang | Effects of dams on riverine biogeochemical cycling and ecology[END_REF]. This accumulation coupled with increased water retention time, allows for an augmentation in water temperature and the creation of anaerobic conditions, which promote denitrification and methanogenesis, and the generation of GHGs [START_REF] Wang | Effects of dams on riverine biogeochemical cycling and ecology[END_REF]. Both drying [START_REF] Döll | How is the impact of climate change on river flow regimes related to the impact on mean annual runoff? A global-scale analysis[END_REF] and damming [START_REF] Zarfl | A global boom in hydropower dam construction[END_REF] are occurring with increasing frequencies, therefore, it is important to quantify their impacts on GHG fluxes in order to inform management decisions and improve global GHG budgets.

Our objective was to review the literature of the effects of fragmentation by drying and damming on GHG fluxes in river networks. We discuss the factors and processes affecting GHG fluxes at different hydrological states of flow regimes in IRES and dammed rivers: exposed sediments, lentic waters, and lotic waters. Flow regime states are described along a continuum of scale, from the reach, to the network, to the global scale. We then discuss the individual and interactive effects of drying and damming at the network scale. Finally, we propose four actionable future research directions.

Methods

We conducted a systematic search of studies addressing the effects of drying and damming on GHG fluxes from river networks. The search was primarily conducted using Web of Science for studies published from 1900 to January 2021, in addition to personal literature databases and reference lists of the identified primary studies and review papers.

Relevant papers published after our literature search were not included in the figures, but mentioned in-text where pertinent. We used a comprehensive combination of keywords to describe intermittent rivers, reservoirs, and GHGs (Table S1). For practicality, when the search returned over 1000 entries, we only considered the first 1000 entries, sorted in order of relevance. Articles were excluded from analysis if they were identified as aquatic but not a stream or river environment (e.g. wetland, pond).

We considered three of the principle GHGs responsible for the greenhouse effect: CO2, CH4, and N2O. Although they do not measure GHG fluxes directly, we considered studies of potential denitrification providing a proxy of N2O fluxes, and studies that indirectly estimated GHG fluxes from dissolved GHG concentrations in stream water. Given the vast body of research regarding GHG fluxes in reservoirs, we only considered studies that took measurements from the river upstream and/or downstream of the reservoir in addition to the reservoir itself. We additionally considered studies that compared GHG fluxes at the reservoir surface to nearby free-flowing rivers or lakes. Studies that did not report GHG fluxes clearly, included only a single day of data, or did not report fluxes on a per areal basis were not included in our final selection of studies reported in the figures and tables. In a few cases, mean GHG fluxes were not reported in text, thus they were extracted from the article figures using visual inspection. When the gas flux was reported in grams per area, we converted the value to a per molar basis (e.g. CO2-C, CH4-C, and N2O-N). For the 43 studies about rivers impacted by drying, 49 about damming, and 6 about their interactive effects, we collected a suite of qualitative and quantitative information. This included basic information about the study type and location, as well as information about the GHG measurements including which gases were measured, their fluxes and drivers (see Table S2 for the full list of information extracted from each study). Direct comparisons of GHG fluxes between studies are done with caution, appreciating the impacts of differences in spatial and/or temporal scales and sampling methodologies between studies.

Greenhouse gas fluxes in intermittent rivers

Recent advances in our knowledge of controls of GHG fluxes along with updated maps of inland waters have allowed for more well-informed estimates of GHG fluxes in freshwaters [START_REF] Drake | Terrestrial carbon inputs to inland waters: A current synthesis of estimates and uncertainty[END_REF][START_REF] Raymond | Global carbon dioxide emissions from inland waters[END_REF]. Streams and rivers in particular have been identified as global hotspots of CO2 evasion, with recent global CO2 estimates of 2.0 ± 0.2 Pg C y -1 [START_REF] Liu | The importance of hydrology in routing terrestrial carbon to the atmosphere via global streams and rivers[END_REF], with an additional 1.02 Pg CO2 eq. y -1 from CH4 and N2O [START_REF] Zheng | Global methane and nitrous oxide emissions from inland waters and estuaries[END_REF]. However, research in the field of IRES has historically fallen behind that of perennial systems, particularly with respect to studies examining biogeochemical cycling [START_REF] Leigh | Ecological research and management of intermittent rivers: An historical review and future directions[END_REF]. This lag in research may be due to the unique nature of intermittent ecosystems -not terrestrial, but not always aquatic -making them difficult to fit into the domain of aquatic or terrestrial ecology [START_REF] Arce | A conceptual framework for understanding the biogeochemistry of dry riverbeds through the lens of soil science[END_REF][START_REF] Steward | When the river runs dry: Human and ecological values of dry riverbeds[END_REF]. We found 43 studies examining GHG fluxes in IRES, with a peak of studies published within the last decade (Figure S1). Most of the studies were conducted in arid regions, where IRES are prevalent such as the Mediterranean, Australia, and Arizona, USA (Figure 1). The majority of studies that we identified were metabolism studies, which measure diel oxygen curves as a proxy for metabolism [START_REF] Hall | Chapter 34-Stream Metabolism[END_REF]. These metabolism studies were primarily reach-scale field studies during lotic and dry states, just in isolated pools, or lotic reaches within an intermittent river network. For those studies measuring in situ GHG efflux, most studies measured CO2 alone, some measured N2O alone, and a few measured several or all three major GHGs (Figure 2A). Studies most frequently examined all three of the major hydrological states: dry, isolated pools, and lentic waters. Some studies just looked at drying and rewetting, and others just measured fluxes in isolated pools or just sediment rewetting.

We found four global efforts examining GHG fluxes in IRES, however, the large majority of identified studies were at the reach scale, with only a few at the network scale (Figure 2B).

Greenhouse gas fluxes associated with river damming

There is a large body of literature about GHG fluxes from reservoirs, thus we restricted our review to studies which included measurements upstream and/or downstream of the reservoir, in addition to the reservoir itself. Forty-nine studies investigated GHG flux dynamics in river networks impacted by reservoirs. The vast majority of the studies were conducted in China, with studies also frequently occurring in Brazil, French Guiana, and the USA, with some global efforts (Figure 1). Most of the identified studies were conducted on what could be considered a large reservoir, with an average volume of 15.6 ± 18.0 km 3 and an average surface area of 769.9 ± 654.2 km 2 . The median catchment area for studies examining damming was 58,000 km 2 , compared to 505 km 2 for the studies examining drying (Figure S2). Over half of the identified studies were field studies, there were also several review studies, and some studies used modeling or lab incubations. There is more abundant research, and more even distribution of research on each gas for studies we identified pertaining to reservoirs than to IRES, indicating that the field of research about damming is at a more advanced stage (Figure 2A). With the exception of one study on floodplain sediments, all the studies on damming measured GHG fluxes at the air-water interface. Considering the limited number of studies addressing GHG fluxes from exposed reservoir sediments identified in our literature review, we do not extensively discuss them. Additionally, a few studies compared GHG fluxes during low and high water levels.

Greenhouse gas fluxes from dry sediments can exceed those of inundated ones

Carbon dioxide fluxes

The CO2 flux from exposed, dry sediments in IRES was consistently higher than fluxes from lotic waters in the same study; on average dry fluxes were nearly double the flux from lotic waters (Figure 3A, Table S3). This suggests a positive effect of drying on CO2 fluxes, possibly due to an increase of oxygen in the sediments promoting organic matter degradation when water is no longer limiting diffusion [START_REF] Kosten | Extreme drought boosts CO2 and CH4 emissions from reservoir drawdown areas[END_REF]. In a synthesis of CO2 fluxes from dry inland waters, dry sediment fluxes were higher than those from lentic waters, and more similar to fluxes reported from lotic waters [START_REF] Marcé | Emissions from dry inland waters are a blind spot in the global carbon cycle[END_REF]. In some cases, the CO2 flux from IRES dry sediments is comparable to riparian soil fluxes [START_REF] Bretz | Integrating Ecosystem Patch Contributions to Stream Corridor Carbon Dioxide and Methane Fluxes[END_REF][START_REF] Gómez-Gener | When water vanishes: Magnitude and regulation of carbon dioxide Emissions from dry temporary streams[END_REF][START_REF] Schiller | Carbon dioxide emissions from dry watercourses[END_REF]. Similar to observations in dry riverbeds, CO2 fluxes from reservoir drawdown areas are on average higher than from the reservoir surface [START_REF] Keller | Global CO2 emissions from dry inland waters share common drivers across ecosystems[END_REF], resulting in a 50% increase in the current global CO2 emission estimate from reservoirs when accounting for drawdown areas [START_REF] Keller | Global carbon budget of reservoirs is overturned by the quantification of drawdown areas[END_REF].

The factors that most influenced CO2 fluxes in IRES were sediment moisture, temperature, organic matter content, and texture (Figure 4). Sediment moisture strongly influences CO2 fluxes, as water stress can inhibit microbial activity [START_REF] Manzoni | Responses of soil microbial communities to water stress: Results from a meta-analysis[END_REF]. In a global analysis of CO2 fluxes in dry inland waters, soil moisture was the strongest predictor of CO2, followed by the interaction of organic matter content and moisture [START_REF] Keller | Global CO2 emissions from dry inland waters share common drivers across ecosystems[END_REF]. Thus, under moisture-limiting conditions, increasing the organic matter content or temperature of the sediments will not increase the CO2 flux, as the microbial activity is inhibited by the water stress [START_REF] Keller | Global CO2 emissions from dry inland waters share common drivers across ecosystems[END_REF]. However, the relationship of CO2 flux with sediment moisture is not always linear, as significantly higher microbial activity was observed between 20 and 40% water filled pore space at an Australian IRES [START_REF] Mcintyre | Rewetting and litter addition influence mineralisation and microbial communities in soils from a semi-arid intermittent stream[END_REF].

One study even reported a negative relationship between CO2 flux and sediment moisture in a Mediterranean IRES [START_REF] Gómez-Gener | Hot spots for carbon emissions from Mediterranean fluvial networks during summer drought[END_REF]. They noted that although higher water content may promote CO2 production by facilitating contact between microorganisms and the available substrate, it may at the same time act as a physical barrier to restrict the evasion of CO2 [START_REF] Gómez-Gener | Hot spots for carbon emissions from Mediterranean fluvial networks during summer drought[END_REF]. High sediment moisture content may also lead to anaerobic conditions, which inhibit the production of CO2 by aerobic respiration due to oxygen limitation [START_REF] Kosten | Extreme drought boosts CO2 and CH4 emissions from reservoir drawdown areas[END_REF]. Temperature stimulates the microbial activity of autotrophic and heterotrophic respiration in soils [START_REF] Raich | Interannual variability in global soil respiration, 1980-94[END_REF], with a similar effect in inland water sediments [START_REF] Lü | The effects of climate warming on microbe-mediated mechanisms of sediment carbon emission[END_REF]. A diurnal pattern was observed in high frequency CO2 measurements from exposed riverine sediments, suggesting that warming during the day exponentially increased fluxes [START_REF] Koschorreck | Temporal patterns and potential drivers of CO2 emission from dry sediments of a large river[END_REF]. Sediment organic matter content influences CO2 fluxes as organic matter provides a substrate for decomposition. However, the quantity of organic matter may not explain the whole story, since the quality of organic matter can vary widely, with a rapidly decomposable fraction with a turnover rate in days, to a more slowly decomposable fraction with a turnover rate in days to years [START_REF] Qiu | Effects of plant-derived dissolved organic matter (DOM) on soil CO2 and N2O emissions and soil carbon and nitrogen sequestrations[END_REF].

Sediment texture also plays an important role in CO2 fluxes, as soils with a finer texture can have higher organic matter content and moisture retention, as well as greater surface area for microbial colonization, creating more favourable conditions for respiration [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF]. For example, higher CO2 fluxes were reported in loam and sandy loam (490 and 333 mmol m -2 d -1 , respectively) than in sandy (77 mmol m -2 d -1 ) channels [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF].

There has been some research examining the fluxes of CO2 from both sediments and organic matter in IRES upon rehydration due to a rain event, with rewetting fluxes at least an order of magnitude higher than dry sediment fluxes in the same study (Table S4). In soils, pulses of CO2 after rehydration have been termed the "Birch effect", wherein the decomposition of labile soil organic matter, nitrogen mineralization, as well as CO2 flux are enhanced upon rewetting [START_REF] Birch | The effect of soil drying on humus decomposition and nitrogen availability[END_REF]. Sediment texture can play an important role in rewetting fluxes, CO2 fluxes were significantly higher and more variable in sites with fine sediment texture than more coarse textures [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF]. Carbon dioxide flux had a significant positive correlation with percent clay content, soil nitrogen, ammonium and nitrate content [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF]. The rewetting of leaf litter may also contribute to rewetting CO2 pulses, however, sediments emitted CO2 an order of magnitude higher than leaf litter, considering an equivalent surface area [START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF][START_REF] Schiller | Sediment respiration pulses in intermittent rivers and ephemeral streams[END_REF].

Only one study measured CO2 fluxes from dry riverbeds at the network scale, two studies were conducted at the global scale, and the majority of studies conducted at the reach scale. The study conducted at the network scale measured in situ CO2 and CH4 fluxes across two IRES networks impacted by drying and damming in Spain during the summer drought period [START_REF] Gómez-Gener | Hot spots for carbon emissions from Mediterranean fluvial networks during summer drought[END_REF]. The CO2 flux from dry riverbeds (209 mmol CO2 m -2 d -1 ) was similar to that from lotic waters (120 mmol CO2 m -2 d -1 ), but significantly higher than from impounded waters (36.6 mmol CO2 m -2 d -1 ) and isolated pools (17.2 mmol CO2 m -2 d -1 ).

They were not able to observe spatial variations along a longitudinal gradient from the headwaters to the river mouth for dry riverbed CO2 efflux, since the dry sites were all located in the headwaters [START_REF] Gómez-Gener | Hot spots for carbon emissions from Mediterranean fluvial networks during summer drought[END_REF]. The magnitude of CO2 fluxes from lotic waters has been found to be higher in headwaters than the mainstem due to greater inputs of terrestrially derived CO2 and organic carbon [START_REF] Hotchkiss | Sources of and processes controlling CO2 emissions change with the size of streams and rivers[END_REF]. However, it remains unknown whether or not this phenomenon would also hold true for dry rivers at the network scale.

Methane fluxes

Few studies measured CH4 fluxes, all of which reported relatively low rates of CH4 emission from dry riverbeds, < 0.6 mmol m -2 d -1 (Table S3, Figure 3A). The small number of studies limits our ability to draw conclusions, and demonstrates the need for more measurements of CH4 fluxes in IRES. A recent study of forest drainage ditches found similarly low dry condition CH4 fluxes (on average a mean flux of 0), which were significantly lower than lotic fluxes [START_REF] Peacock | Significant Emissions From Forest Drainage Ditches-An Unaccounted Term in Anthropogenic Greenhouse Gas Inventories[END_REF]. Likewise, CH4 fluxes from reservoir drawdown areas are presently considered of minor importance compared to total aquatic CO2 and CH4 fluxes from reservoirs [START_REF] Keller | Global carbon budget of reservoirs is overturned by the quantification of drawdown areas[END_REF][START_REF] Paranaíba | Crosscontinental importance of CH4 emissions from dry inland-waters[END_REF].

However, some studies have demonstrate the potential for sediment in drawdown areas to be CH4 sources [START_REF] Amorim | Methane diffusive fluxes from sediment exposed in a Brazilian tropical reservoir drawdown zone[END_REF][START_REF] Kosten | Extreme drought boosts CO2 and CH4 emissions from reservoir drawdown areas[END_REF][START_REF] Paranaíba | Crosscontinental importance of CH4 emissions from dry inland-waters[END_REF][START_REF] Serça | Nam Theun 2 Reservoir four years after commissioning: Significance of drawdown methane emissions and other pathways[END_REF]. The observed CH4 fluxes are likely due to the stimulation of the microbial community upon rewetting [START_REF] Conrad | Response of the methanogenic microbial communities in Amazonian oxbow lake sediments to desiccation stress[END_REF]. Methane fluxes from exposed sediments in both reservoirs and IRES should not be ignored in global GHG budgets for inland waters.

Several factors control CH4 fluxes in IRES: sediment temperature, moisture and organic matter content (Figure 4). As discussed in section 3.2.1 for CO2 fluxes, sediment temperature is associated with the stimulation of many biogeochemical processes. In soils, for example, temperature has a positive relationship with CH4 production [START_REF] Dalal | Greenhouse gas fluxes from natural ecosystems[END_REF].

Similarly to CO2, sediment moisture controls CH4 fluxes through a balance between the stimulation of microbial metabolism and the limitation of gas diffusivity [START_REF] Gómez-Gener | Hot spots for carbon emissions from Mediterranean fluvial networks during summer drought[END_REF][START_REF] Paranaíba | Crosscontinental importance of CH4 emissions from dry inland-waters[END_REF]. Methane production occurs primarily through methanogenesis under anaerobic conditions due to submergence or water logging [START_REF] Dalal | Greenhouse gas fluxes from natural ecosystems[END_REF]. We expect sediment organic matter content to drive carbon fluxes, as it provides substrate for decomposition [START_REF] Dalal | Greenhouse gas fluxes from natural ecosystems[END_REF]. Surprisingly, a global study of CH4 fluxes from dry inland waters found a negative relationship between CH4 fluxes and organic matter content [START_REF] Paranaíba | Crosscontinental importance of CH4 emissions from dry inland-waters[END_REF]. However, they note that this relationship may be more complex than it seems, with the quality of the organic matter playing an important role in particular in dry sediments, since the more frequently a sediment is exposed to the atmosphere the less labile its organic matter tends to be [START_REF] Serrano-Silva | Methanogenesis and methanotrophy in soil: A review[END_REF].

Nitrous oxide fluxes

There is limited research about N2O fluxes from dry riverbeds of IRES (Table S3). We found a singular in situ reach-scale study measuring N2O fluxes from dry riverbeds, which reported relatively low fluxes, driven primarily by sediment texture [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF].

Several other studies were conducted in the laboratory using sediment and/or water incubation assays to measure denitrification. To our knowledge there are no network or global scale studies of N2O fluxes from dry riverbeds. Likewise for reservoir drawdown areas, the few studies suggest that reservoir sediments can be sources of N2O upon drying [START_REF] Jin | Enhanced greenhouse gas emission from exposed sediments along a hydroelectric reservoir during an extreme drought event[END_REF].

Nitrous oxide production was found to be highest in the recently exposed reservoir sediments, and was associated with a temporary increased in organic matter processing due to increased oxygen availability upon drying [START_REF] Jin | Enhanced greenhouse gas emission from exposed sediments along a hydroelectric reservoir during an extreme drought event[END_REF].

Based on the few field and lab studies, sediment texture and moisture may be important influences on N2O fluxes. Finer sediment texture was associated with higher N2O fluxes, likely due to the higher nutrient and moisture content creating more favourable conditions for denitrification and/or nitrification, when compared to coarser textured sediments [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF]. Additionally, sediment moisture had a significant positive relationship with N2O fluxes, but only at the sandy sites [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF]. Similarly, in a laboratory study, N2O production was inhibited as soil moisture decreased [START_REF] Gómez | The effects of drying on sediment nitrogen content in a Mediterranean intermittent stream: A microcosms study[END_REF]. In soils, N2O is produced mainly through the processes of nitrification and denitrification, which are considered to be at their optimum at 50-60%, and above 60% waterfilled pore space, respectively [START_REF] Dalal | Greenhouse gas fluxes from natural ecosystems[END_REF].

Following the rewetting of sediments in an IRES, the N2O flux was two orders of magnitude higher than from dry sediments [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF]. Similar to CO2, N2O rewetting fluxes were significantly higher for finer sediment texture, indicating that higher soil nutrient concentration and enhanced moisture retention likely allowed for conditions favourable for denitrification and nitrification [START_REF] Gallo | Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways[END_REF]. In wet-dry cycles in sediments of aquatic ecosystems, N2O pulses are observed following sediment drying and rewetting, with peak fluxes observed when nitrification and denitrification are coupled under conditions with intermediate oxygen availability [START_REF] Koschorreck | Nitrogen turnover in drying sediments of an Amazon floodplain lake[END_REF][START_REF] Pinto | Effects of dry-wet cycles on nitrous oxide emissions in freshwater sediments: A synthesis[END_REF]. There is a clear need for more measurements of CH4 and N2O fluxes upon sediment rewetting, particularly given the high global warming potential of these two gases [START_REF] Myhre | Anthropogenic and Natural Radiative Forcing[END_REF].

Lentic waters can promote greenhouse gas fluxes from IRES and reservoirs

Carbon dioxide fluxes

The research on CO2 fluxes from isolated pools in IRES is limited, with no clear trend in fluxes when compared to lotic states (Table S5). While increasing attention is being paid to IRES by researchers, the research on isolated pools in IRES still remains limited [START_REF] Bonada | Conservation and Management of Isolated Pools in Temporary Rivers[END_REF]. Temporary, isolated pools are transitional environments between lotic and dry states, characterized by low dissolved oxygen concentrations, high temperature, and an accumulation of organic matter [START_REF] Bonada | Conservation and Management of Isolated Pools in Temporary Rivers[END_REF]. The abundance of organic matter within isolated pools tends to be high due to evaporation and accumulation of riparian vegetation inputs, fueling microbial respiration and leading to high production and thus concentration of CO2 [START_REF] Granados | The interruption of longitudinal hydrological connectivity causes delayed responses in dissolved organic matter[END_REF]. Despite the high CO2 production, there may be relatively low emission due to limited gas exchange in the stagnant waters (Gómez-Gener et al., 2015;[START_REF] Granados | The interruption of longitudinal hydrological connectivity causes delayed responses in dissolved organic matter[END_REF]. As isolated pools can be present for only short periods of time in IRES, during the transition between lotic and dry states, they are methodologically difficult to measure, and the amount of available data is limited.

In contrast, there is a larger body of knowledge about CO2 fluxes from reservoirs.

Carbon dioxide flux from the downstream river were on average three times higher than from the reservoir surface and 3.8 times higher than from the upstream river (Figure 3B, Table S6). This trend may indicate relatively high production of CO2 in reservoirs, but low diffusion rates because of 1) lower concentration in the surface water layer and/or 2) minimal turbulence, which tends to increase going downstream, thereby releasing the CO2 accumulated in the deepest part of the reservoir to the atmosphere.

Water temperature, retention time, dissolved oxygen and phytoplankton chlorophyll a concentration were key factors influencing CO2 flux in reservoirs and their associated rivers (Figure 4). Carbon dioxide fluxes increase with warmer water temperature; for example tropical reservoirs typically emit GHGs at greater rates than high latitude temperate and boreal reservoirs, likely due to higher rates of biological activity stimulated by warm water temperature and higher flooded biomass in tropical regions [START_REF] Wang | Effects of dams on riverine biogeochemical cycling and ecology[END_REF]. Increased water residence time is associated with increased interaction between organic substrates and microorganisms, allowing for decomposition of organic matter and CO2 production [START_REF] Gómez-Gener | Effect of small water retention structures on diffusive CO2 and CH4 emissions along a highly impounded river[END_REF]. Carbon dioxide fluxes from reservoirs typically had a negative relationship with dissolved oxygen and chlorophyll a concentrations. This is likely because the increased water residence time and nutrient accumulation characteristic of reservoirs are associated with increased phytoplankton production [START_REF] Li | Methane and CO2 emissions from China's hydroelectric reservoirs: A new quantitative synthesis[END_REF]. With the photosynthetic activity of the phytoplankton, chlorophyll a concentrations increase as CO2 in the water is fixed, reducing dissolved CO2 concentrations, and producing more dissolved oxygen [START_REF] Qin | CO2 and CH4 partial press and flux across water-air interface in the downstream of Jinsha River, China[END_REF].

Methane fluxes

Research about CH4 fluxes from isolated pools in IRES is limited (Gómez-Gener et al., 2015), in contrast to the large body of research on reservoirs. Mean CH4 efflux was not significantly different between isolated pools, lotic waters, and dry riverbeds, across two intermittent river networks in Spain, with nearly negligible average fluxes (mean <0.3 mmol CH4 m -2 d -1 ) recorded [START_REF] Gómez-Gener | Hot spots for carbon emissions from Mediterranean fluvial networks during summer drought[END_REF]. Impounded waters in the same river networks had notable CH4 efflux (mean 13.8 mmol CH4 m -1 d -1 ). The global GHG flux from reservoirs was recently estimated at 1,076 Tg CO2 eq. y -1 , with CH4 as the dominant contributor (748 Tg CO2 eq. y -1 ) [START_REF] Harrison | Year-2020 Global Distribution and Pathways of Reservoir Methane and Carbon Dioxide Emissions According to the Greenhouse Gas From Reservoirs (G-res) Model[END_REF]. Ebullitive (bubbling) CH4 emissions, can make up a large proportion of total CH4 emissions from reservoirs [START_REF] Deemer | Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis[END_REF].

Reservoirs increase water residence time, trapping sediments and organic matter [START_REF] Crawford | Basin scale controls on CO2 and CH4 emissions from the Upper Mississippi River[END_REF][START_REF] Wang | Effects of dams on riverine biogeochemical cycling and ecology[END_REF]. The resulting bottom anoxia can lead to conditions favourable for methanogenesis in reservoir sediments, generating CH4 [START_REF] Wang | Effects of dams on riverine biogeochemical cycling and ecology[END_REF]. On average, CH4 fluxes were an order of magnitude higher from the downstream reaches than from the reservoir surface and upstream reaches (Figure 3B). However, in individual studies the results were variable (Table S6), with observed CH4 fluxes at the reservoir surface that were greater or lower than upstream [START_REF] Yang | Spatial and seasonal variability of diffusive methane emissions from the Three Gorges Reservoir[END_REF][START_REF] Yang | Contrasting methane emissions from upstream and downstream rivers and their associated subtropical reservoir in eastern China[END_REF] and downstream waters [START_REF] Guérin | Methane and carbon dioxide emissions from tropical reservoirs: Significance of downstream rivers[END_REF][START_REF] Zhao | Spatial and temporal patterns of greenhouse gas emissions from Three Gorges Reservoir of China[END_REF]. Methane production may not translate directly to CH4 efflux, as the upwardly diffusing CH4 can be converted to CO2 by methanotrophic bacteria [START_REF] Wang | Effects of dams on riverine biogeochemical cycling and ecology[END_REF]. However, when water is released from the bottom of the dam, CH4 emissions can be very high [START_REF] Wang | Effects of dams on riverine biogeochemical cycling and ecology[END_REF]. As such, CH4 emission rates can be high directly after the dam outflow, and decline further downstream [START_REF] Guérin | Methane and carbon dioxide emissions from tropical reservoirs: Significance of downstream rivers[END_REF].

Water temperature, air temperature, chlorophyll a, dissolved organic carbon, and dissolved oxygen controlled CH4 fluxes from reservoirs and their associated rivers (Figure 4).

Water and air temperature control biological activity, in the case of CH4 production, the activity of methanogenic bacteria. As such, in temperate northern hemisphere climates, maximum CH4 flux can be observed in the summer when the water and air temperatures are at their highest [START_REF] Bastien | CO2 and CH4 diffusive and degassing emissions from 2003 to 2009 at Eastmain 1 hydroelectric reservoir, Quebec, Canada[END_REF]. Methane fluxes are positively correlated to phytoplankton chlorophyll a concentrations [START_REF] Descloux | Methane and nitrous oxide annual emissions from an old eutrophic temperate reservoir[END_REF], which can reflect the trophic status of a reservoir. A global review found an order of magnitude higher CH4 flux from eutrophic than oligotrophic reservoirs [START_REF] Deemer | Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis[END_REF], where the high nutrient concentrations, low dissolved oxygen concentrations, and high water temperatures likely stimulate CH4 production. When other environmental conditions are favourable, increased carbon content in the sediment can provide substrate to stimulate microbial activity and promote the release of CH4 in reservoirs [START_REF] Yang | Water-air interface greenhouse gas emissions (CO2, CH4, and N2O) emissions were amplified by continuous dams in an urban river in Qinghai-Tibet Plateau, China[END_REF]. The influence of allochthonous carbon inputs on CH4 production may be especially important in small, temperate reservoirs with lower water residence times [START_REF] Chanudet | Emission of greenhouse gases from French temperate hydropower reservoirs[END_REF]. Dissolved oxygen concentrations have a negative relationship with CH4 fluxes, as anaerobic conditions in sediments promote methanogenesis.

Moreover, lower dissolved oxygen concentrations can prevent the oxidation of CH4 during its transport through the water column from the sediments [START_REF] Yang | Water-air interface greenhouse gas emissions (CO2, CH4, and N2O) emissions were amplified by continuous dams in an urban river in Qinghai-Tibet Plateau, China[END_REF]. The large number of studies focusing on reservoirs have allowed researchers to identify the key factors influencing CH4 fluxes, however to reach this level of knowledge with IRES, more data are needed.

Nitrous oxide fluxes

We did not identify a single study of N2O fluxes from isolated pools of IRES, while several studies examined N2O fluxes from reservoirs and their associated rivers (Figure 3, Table S5 andS6). Global N2O fluxes from reservoirs was estimated at 31.7 Tg of CO2 equivalents per year [START_REF] Deemer | Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis[END_REF], which is notable, but a smaller contribution than CO2 (328) and CH4 (748) [START_REF] Harrison | Year-2020 Global Distribution and Pathways of Reservoir Methane and Carbon Dioxide Emissions According to the Greenhouse Gas From Reservoirs (G-res) Model[END_REF]. There was no clear trend in the spatial variation of N2O fluxes (Figure 3B, Table S6) with some studies reporting the highest average fluxes at the upstream river [START_REF] Cheng | Distribution and emission of N2O in the largest river-reservoir system along the Yellow River[END_REF], downstream river [START_REF] Yang | Contrasting methane emissions from upstream and downstream rivers and their associated subtropical reservoir in eastern China[END_REF], and reservoir surface [START_REF] Yang | Water-air interface greenhouse gas emissions (CO2, CH4, and N2O) emissions were amplified by continuous dams in an urban river in Qinghai-Tibet Plateau, China[END_REF]. Anoxic conditions in reservoir sediments can lead to conditions favourable for denitrification, generating N2O emissions [START_REF] Wang | Effects of dams on riverine biogeochemical cycling and ecology[END_REF]. Thus, in theory, if water is then released from the bottom of the reservoir to the downstream river, it can result in a large release of N2O downstream.

Nitrate concentrations, dissolved oxygen, and water temperature were cited as influencing N2O fluxes from reservoirs and their associated rivers (Figure 4). Nitrous oxide fluxes are a function of both nitrification and denitrification rates, which are controlled by nitrogen and oxygen availability [START_REF] Liang | Control of the Hydraulic Load on Nitrous Oxide Emissions from Cascade Reservoirs[END_REF]. Denitrification in particular is limited by nitrate availability [START_REF] Baulch | Nitrogen enrichment and the emission of nitrous oxide from streams[END_REF], explaining why nitrate concentrations had a positive relationship with reservoir N2O fluxes. Dissolved oxygen concentrations are particularly important in controlling N2O production in reservoirs and their associated rivers. However, the relationship varies depending on whether nitrification or denitrification dominate N2O production, as nitrification is an aerobic process while denitrification is an anaerobic process [START_REF] Liang | Control of the Hydraulic Load on Nitrous Oxide Emissions from Cascade Reservoirs[END_REF]. A dissolved oxygen concentration that optimizes the co-occurrence of nitrification and denitrification will result in the highest yield of N2O, for example in the oxycline [START_REF] Liang | Control of the Hydraulic Load on Nitrous Oxide Emissions from Cascade Reservoirs[END_REF]. When nutrient availability is not limiting, warmer water temperatures can stimulate the microbial activity contributing to the production of N2O [START_REF] Beaulieu | Nitrous Oxide Emissions from a Large, Impounded River: The Ohio River[END_REF].

Spatial and temporal patterns of drying and damming at the network scale

River networks are meta-ecosystems made up of an aquatic continuum embedded within a terrestrial matrix [START_REF] Battin | Biophysical controls on organic carbon fluxes in fluvial networks[END_REF]. A meta-ecosystem is defined as a set of ecosystems connected by spatial flows of energy, materials, and organisms across ecosystem boundaries [START_REF] Loreau | Meta-ecosystems: A theoretical framework for a spatial ecosystem ecology[END_REF]. Meta-ecosystem theory, an extension of metapopulation and metacommunity theories, is useful in conceptualizing the effects of river network fragmentation on biogeochemical and ecological processes [START_REF] Loreau | Meta-ecosystems: A theoretical framework for a spatial ecosystem ecology[END_REF], such as those leading to the production and consumption of GHGs. In river networks, energy, materials, and organisms flow by-way-of hydrological connectivity in three spatial dimensions: longitudinal, lateral, and vertical [START_REF] Cid | From meta-system theory to the sustainable management of rivers in the Anthropocene[END_REF]. In fragmentation by drying and damming, the interruption of surface flow disrupts hydrological connectivity in one or more of the three dimensions, creating heterogeneous mosaics of habitat patches with varying conditions [START_REF] Boulton | Chapter 2.3-Hydrological Connectivity in Intermittent Rivers and Ephemeral Streams[END_REF][START_REF] Boulton | Chapter 2.3-Hydrological Connectivity in Intermittent Rivers and Ephemeral Streams[END_REF][START_REF] Cid | From meta-system theory to the sustainable management of rivers in the Anthropocene[END_REF]. This spatial variability also varies temporally within and among years [START_REF] Costigan | Fundamental spatial and temporal disconnections in the hydrology of an intermittent prairie headwater network[END_REF], further contributing to the high habitat heterogeneity in IRES. Compared to the temporary nature of fragmentation by drying, reservoirs typically represent a permanent discontinuity feature within the network altering the fluxes of water, sediment, materials (including organic matter) and organisms [START_REF] Grill | Mapping the world's free-flowing rivers[END_REF]. We propose that the spatial distribution of fragmentation at the network scale is a major factor influencing the magnitude and dynamics of GHG fluxes. More specifically, the network location of the fragmentation (i.e. upstream, downstream) and the type of fragmentation (i.e. drying, damming), and their interaction play an important role in GHG dynamics at the river network scale.

The location of fragmentation along a river network is an important consideration for determining its ecological impacts [START_REF] Jacquet | Disturbance-driven alteration of patch connectivity determines local biodiversity recovery within metacommunities[END_REF][START_REF] Schmera | Does isolation influence the relative role of environmental and dispersal-related processes in stream networks? An empirical test of the network position hypothesis using multiple taxa[END_REF]. Intermittent reaches can occur anywhere in a river network, from headwaters to the mainstem [START_REF] Datry | Broad-scale patterns of invertebrate richness and community composition in temporary rivers: Effects of flow intermittence[END_REF]. Network scale drying patterns vary due to differences in climate, geology, vegetation, and human interference [START_REF] Shanafield | An overview of the hydrology of non-perennial rivers and streams[END_REF]. For example, in temperate regions intermittent reaches are often in headwaters [START_REF] Kampf | Managing nonperennial headwater streams in temperate forests of the United States[END_REF], although central and downstream drying reaches are also common [START_REF] Arscott | Aquatic invertebrate community structure along an intermittence gradient: Selwyn River, New Zealand[END_REF][START_REF] Datry | Benthic and hyporheic invertebrate assemblages along a flow intermittence gradient: Effects of duration of dry events[END_REF].

In arid regions, intermittent reaches can occur anywhere and often entire river networks are intermittent [START_REF] Welter | The influence of storm characteristics on hydrological connectivity in intermittent channel networks: Implications for nitrogen transport and denitrification[END_REF]. Network scale patterns of drying can have important implications on biogeochemical cycling due to the disparate conditions observed along a river network from the headwaters to the estuarine zone [START_REF] Allen | River ecosystem conceptual models and non-perennial rivers: A critical review[END_REF][START_REF] Larned | Emerging concepts in temporary-river ecology[END_REF]. 20 Within a river network, we expect high GHG fluxes from dry sediments at upstream reaches (Figure 5A), where allochthonous organic matter inputs to the stream bed tend to be greater due to a higher edge to area ratio of riparian tree cover [START_REF] Richardson | A synthesis of the ecology of headwater streams and their riparian zones in temperate forests[END_REF].

For example, the greatest amount of terrestrial organic matter input in dry channels was reported from first-order, forested, temperate IRES when sampling 212 streams across nearly all continents [START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF]. Moreover, the relatively stronger land-water connectivity in headwaters compared to the mainstem can supply terrestrially derived carbon inputs to stream under lotic conditions [START_REF] Hotchkiss | Sources of and processes controlling CO2 emissions change with the size of streams and rivers[END_REF], and it may be an important source of carbon during drying events [START_REF] Gómez-Gener | Drought alters the biogeochemistry of boreal stream networks[END_REF]. The shading effects of riparian cover in headwaters can also buffer against the negative impacts of sediment desiccation on decomposer communities and associated GHG production [START_REF] Schreckinger | Attributes of drying define the structure and functioning of microbial communities in temperate riverbed sediment[END_REF].

However, the lower sediment temperature near the headwaters and coarser sediment texture may conversely limit CO2 fluxes. We could expect the highest CH4 and N2O fluxes from dry river reaches in the mainstem, where finer sediments tend to accumulate, retaining moisture and nutrients [START_REF] Saxton | Soil water characteristic estimates by texture and organic matter for hydrologic solutions[END_REF], along with the diminished effect of riparian shading creating warmer sediment temperatures stimulating microbial activity [START_REF] Raich | Interannual variability in global soil respiration, 1980-94[END_REF].

Nevertheless, these proposed network scale patterns in dry sediment GHG fluxes still need to be tested.

As with drying, the spatial position of a reservoir is important for determining GHG flux magnitudes at the river network scale (Figure 5B). We expect reservoirs located in downstream reaches to have higher CO2 and CH4 fluxes associated with their larger size and the warmer water temperatures when compared to shaded and higher elevation upstream reaches. We expect higher biological activity and chlorophyll a levels in warmer waters [START_REF] Sarkar | Effect of climato-environmental parameters on chlorophyll a concentration in the lower Ganga basin, India[END_REF], leading to potentially high production and emission of CO2. However, if there are significant reservoirs upstream, this may reduce the amount of organic matter transported downstream [START_REF] Stanford | Revisiting the serial discontinuity concept[END_REF], thereby diminishing the emission potential despite the more favourable environmental conditions for GHG production. At the network scale, CO2 evasion from lakes was controlled by residence time and size, with CO2 fluxes increasing with the stream order of their network position, suggesting that larger lakes downstream are more efficient emitters of terrestrial CO2 [START_REF] Brinkerhoff | Lake morphometry and river network controls on evasion of terrestrially sourced headwater CO2[END_REF]. We expect to observe a similar trend in reservoirs, where a reservoir on large order river, with no upstream reservoirs, might have a higher magnitude of GHG fluxes than a reservoir located in the headwaters, considering two reservoirs with similar physical characteristics.

We may expect to see higher N2O fluxes at reservoirs in downstream reaches compared to upstream ones. Point sources of nitrogen entering the river may accumulate in downstream reaches, where the often sparse riparian cover has a reduced capacity of nitrate removal when compared to upstream reaches (Wherry et al., 2021). Application of the nutrient spiralling concept at the network scale found that nitrate was recycled more intensively in higher order streams where mass flux was greater (Ensign & Doyle, 2006). In addition, the generally warmer water temperatures in downstream reaches may further promote N2O production [START_REF] Beaulieu | Nitrous Oxide Emissions from a Large, Impounded River: The Ohio River[END_REF]. Thus, although complex and understudied, we propose that the network position of reservoirs has an important influence on the magnitude of GHG fluxes.

Interactive effects of drying and damming at the network scale

Drying and damming present complex and interactive fragmentation effects on a river network. The interactive effect of drying and damming at the network scale, may result in more organic matter processed in dry reaches and reservoirs than in lotic reaches [START_REF] Acuña | The effects of alterations in temperature and flow regime on organic carbon dynamics in Mediterranean river networks[END_REF]. According to our review, dry reaches and reaches downstream of reservoirs have a higher on average GHG flux potential than lotic reaches (Figure 3). Only six studies examining the interactive effects of drying and damming to different degrees were found in our search. The importance of the spatial distribution of drying and damming together along a river network is an important, but heavily understudied research area. In a Mediterranean river network impacted by drying and damming, there was both an increase an decrease in the frequency of low-flow events at two different reaches downstream of reservoirs, but an overall decrease in the frequency of high flow events [START_REF] Aristi | Flow regulation by dams affects ecosystem metabolism in Mediterranean rivers[END_REF]. They further found a consistent increase in all organic matter variables below reservoirs, which was possibly related to materials originating from the reservoir (e.g. re-suspended organic matter or outflow plankton) [START_REF] Aristi | Flow regulation by dams affects ecosystem metabolism in Mediterranean rivers[END_REF]. Similarly, an investigation of small reservoirs along an intermittent Mediterranean river network, found that reservoirs had higher microbial community processing of organic carbon, and they subsidized downstream lotic reaches, thereby increasing production and transformation of organic carbon along the river network [START_REF] Proia | Microbial carbon processing along a river discontinuum[END_REF]. In disagreement with the aforementioned studies, a simulation of organic carbon dynamics in a Mediterranean river network found lower sediment carbon and respiration rates downstream of reservoirs, due to the accumulation of carbon in reservoirs via sedimentation as a result of high water residence times [START_REF] Acuña | The effects of alterations in temperature and flow regime on organic carbon dynamics in Mediterranean river networks[END_REF]. They further found that increasing flow variability increased the amount of carbon processed within a river network, while reservoirs had a counteractive effect on extreme flow events due to their flow regulation capacity (e.g. reservoir water retention was not markedly reduced during droughts) [START_REF] Acuña | The effects of alterations in temperature and flow regime on organic carbon dynamics in Mediterranean river networks[END_REF]. These differences may be attributed to contextdependent reservoir (e.g. age, depth, water residence time) or river network (e.g. geology) characteristics [START_REF] Poff | How dams vary and why it matters for the emerging science of dam removal: An ecological classification of dams is needed to characterize how the tremendous variation in the size, operational mode, age, and number of dams in a river basin influences the potential for restoring regulated rivers via dam removal[END_REF]. For example, if the water is released from the top or the bottom of the reservoir, where the latter is more likely to be enriched in nutrients and organic matter [START_REF] Nürnberg | Lake responses to long-term hypolimnetic withdrawal treatments[END_REF].

With the interactive effect of drying and damming at the network scale, we expect more organic matter to be processed in dry reaches and reservoirs than in lotic reaches. The potential additive effect would therefore result in more GHGs emitted to the atmosphere compared to an equivalent, unfragmented, river network (Figure 5C). However, upstream drying and reservoirs may also hold back organic matter from downstream reaches [START_REF] Stanford | Revisiting the serial discontinuity concept[END_REF], resulting in lower downstream GHG fluxes. We could also expect the temporal nature of GHG fluxes to be more variable in fragmented river networks, as upon the rewetting of previously dry reaches, the accumulated organic matter can be transported downstream [START_REF] Datry | A global analysis of terrestrial plant litter dynamics in non-perennial waterways[END_REF], where it would then be stored and processed in a reservoir.

In addition, there may be a potential synergistic effect as the presence of a reservoir can reduce flow rates, or, particularly if water is abstracted from the reservoir, create new intermittent reaches downstream [START_REF] Bianchi | The experimental flow to the Colorado River delta: Effects on carbon mobilization in a dry watercourse[END_REF][START_REF] Zhou | Climate change may accelerate the decline of desert riparian forest in the lower Tarim River, Northwestern China: Evidence from tree-rings of Populus euphratica[END_REF]. As a result, organic matter processing rates and associated GHG fluxes in the river network are augmented due to the creation of these new dry and lentic states. However, in other cases, the reservoir baseflow may turn previously intermittent reaches perennial [START_REF] Acuña | Managing temporary streams and rivers as unique rather than second-class ecosystems[END_REF], resulting in a compensatory effect on GHG fluxes due to the replacement of dry reaches with lotic ones.

The interactive effects of drying and damming are complex and context dependant. With much of the current literature on this topic focused on the Mediterranean, we need an increased geographic diversity in studies to test our conceptual models of dry and damming interactive effects.

Future research directions

We uncovered several research gaps in geography, spatial scale, type of gas measured, and hydrological state measured. We additionally identified a major knowledge gap on the interactive effects of drying and damming. Here, we propose four actionable future research directions as identified in our review of the literature on the effects of fragmentation by drying and damming on GHG fluxes.

1. Firstly, we identified an information gap in terms of the geographic distribution, spatial sampling scale, and GHG measured by studies. Most of the studies about IRES were conducted in the Mediterranean, Australia, and southern United States. This narrow spatial focus omits the world's temperate regions, which also include IRES [START_REF] Messager | Global prevalence of non-perennial rivers and streams[END_REF], as well as many other arid landscapes, such as those found in Central and South America, Africa and Asia. The majority of studies about river systems impacted by damming were from China, with some studies in Brazil, French Guiana and the USA. Moreover, most of the studies done on IRES systems were conducted at the reach scale, with few studies at the global or network scale. These scales are important for understanding the ramifications of the highly variable spatial patterns in IRES. Similarly, very few studies examined the impacts of damming on GHG fluxes in rivers at a network scale. The magnitudes and drivers of fluxes may vary across the longitudinal gradient from headwaters to the mainstem. Furthermore, studies that measured GHG fluxes from dry and rewetting IRES most commonly measured CO2, with few studies measuring CH4 and N2O. A similar tendency was observed in studies of river networks fragmented by damming, where fewer studies included measurements of N2O when compared to CH4 and CO2.

2. Since our literature review, several notable studies examining GHG fluxes from dry inland waters have been published (e.g. [START_REF] Arce | Role of dry watercourses of an arid watershed in carbon and nitrogen processing along an agricultural impact gradient[END_REF][START_REF] Bretz | Integrating Ecosystem Patch Contributions to Stream Corridor Carbon Dioxide and Methane Fluxes[END_REF][START_REF] Delvecchia | Biogeochemical characteristics and hydroperiod affect carbon dioxide flux rates from exposed high-elevation pond sediments[END_REF][START_REF] Koschorreck | Temporal patterns and potential drivers of CO2 emission from dry sediments of a large river[END_REF][START_REF] Paranaíba | Crosscontinental importance of CH4 emissions from dry inland-waters[END_REF][START_REF] Pinto | Effects of dry-wet cycles on nitrous oxide emissions in freshwater sediments: A synthesis[END_REF][START_REF] Schreckinger | When water returns: Drying history shapes respiration and nutrients release of intermittent river sediment[END_REF]. However, there is still a marked shortage of studies on GHG fluxes from isolated pools in IRES, with most studies focused on dry sediments [START_REF] Bonada | Conservation and Management of Isolated Pools in Temporary Rivers[END_REF]. For example, we did not find a single study about N2O fluxes from isolated pools. Although isolated pools may occur at a lower frequency and over a smaller area than dry states, they may have a high GHG production potential [START_REF] Bonada | Conservation and Management of Isolated Pools in Temporary Rivers[END_REF]. Isolated pools have lentic waters, warm water temperatures, and high organic matter accumulation, which may result in anoxic conditions which can promote methanogenesis and denitrification [START_REF] Bonada | Conservation and Management of Isolated Pools in Temporary Rivers[END_REF][START_REF] Day | Water quality in non-perennial rivers[END_REF]. Thus, isolated pools represent a potential source for CH4 and N2O, particularly under future predictions of increasing freshwater temperatures [START_REF] Van Vliet | Global river discharge and water temperature under climate change[END_REF] and augmented terrestrial subsidies to inland waters (Regnier et al., 2022).

3. We found that most studies focused on large reservoirs, while small reservoirs (e.g. hydropower plants with < 10 megawatts) were studied to a lesser extent, even though there are significantly more of them [START_REF] Chanudet | Emission of greenhouse gases from French temperate hydropower reservoirs[END_REF][START_REF] Couto | Global proliferation of small hydropower plants -science and policy[END_REF].

There is currently a boom in small hydropower plants as the hydropower potential of larger rivers has already been exploited, public support for new large reservoirs has diminished, and small reservoirs are perceived to have less of an ecological impact [START_REF] Couto | Global proliferation of small hydropower plants -science and policy[END_REF][START_REF] Lange | Basin-scale effects of small hydropower on biodiversity dynamics[END_REF]. Although their surface area and therefore total local GHG flux potential may be smaller, small reservoirs may have important cumulative fragmentation effects on GHG fluxes at the network scale which warrant further investigation. 4. Fragmentation by drying and damming do not occur independently. Therefore, it is important to understand how drying and damming interact to influence the dynamics of GHG fluxes. We found six studies that covered this topic in our review of the literature, predominantly conducted in the Mediterranean, with mixed results.

Reservoirs may support downstream flow during drought periods, or may exacerbate drying downstream. Given the increasing occurrences of drying and damming (Döll & on GHG fluxes in river networks, thereby better integrating global GHG budgets over space and time. In summary, there has been a recent increase in studies examining GHG fluxes in river networks fragmented by drying and damming. We propose that the location and type of fragmentation within a river network are important factors influencing GHG dynamics. There are still many unexplored gaps in the research, which, if addressed, can improve our understanding of these complex and globally important systems and help to inform local management of fragmented river networks as well as climate change mitigation strategies. S3 through S6

FiguresFigure

  Figures

  Figure 1. The global distribution of publications about greenhouse gas fluxes in intermittent rivers and ephemeral streams (IRES) and river networks fragmented by damming (Dam), identified by our literature search (1900-2021). Excludes four global studies in IRES and seven global studies about fragmentation by damming

Figure 2 .Figure 3 .

 23 Figure 2. Number of publications investigating A. each greenhouse gas (CO2, CH4, and N2O) per fragmentation type: fragmentation by damming (Dam) and fragmentation by drying in intermittent rivers and ephemeral streams (IRES); and B. study scale per fragmentation type

Figure 4 .

 4 Figure 4. The principle factors influencing greenhouse gas (GHG; CO2, CH4, N2O) fluxes from all scales in river networks impacted by damming (top) and intermittent rivers and ephemeral streams (IRES; including all phases; bottom) as identified in our review of the literature. The number in the circle, and circle size denotes the number of studies identifying the factor. Factors identified by at least two studies were included for IRES, and at least three studies for damming. Insufficient data for N2O (n = 1) in IRES

Acknowledgments

We are grateful to whose comments onymous reviewers and the an Gener -Lluís Gómez helped us improve the quality of the manuscript. This work was supported by European Union's Horizon 2020 research and innovation programme through the DRYvER project (Securing Biodiversity, Functional Integrity, and Ecosystem Services in Drying River Networks, award number: 869226). Additional funding to the first author was provided by INRAE (l'Institut national de recherche pour l'agriculture, l'alimentation et l'environnement) and EDF (Électricité de France) through the HYNES collaborative framework.

Data Availability Statement

The datasets collected in our literature review as well as the code used to analyze the data and create the figures for this study are available in the open access Zenodo repository: https://doi.org/10.5281/zenodo.7431750

Author Contribution Statement

Conceptualisation, data interpretation, and writing: TS, TD, VC, AF, and NLR. Data collection and analysis: TS and NLR. Preparation of figures and tables: TS. All authors read and approved the final manuscript. All authors agree to be accountable for the accuracy and integrity of the work.

Conflicts of interest/Competing interests

The authors have no conflicts of interest to declare that are relevant to the content of this article. reaches, and dashed lines indicate intermittent reaches. We expect higher dry sediment GHG fluxes in the headwaters where there are generally greater per area inputs of organic matter (A). We expect reservoirs in the mainstem to have higher per area GHG fluxes due to the warmer water temperatures compared to the shaded upstream reaches (B). The additive interactive effect of drying and damming could result in more organic matter process in dry reaches and reservoirs, thereby resulting in more GHGs emitted to the atmosphere (C)