Charlène Kermagoret

Joachim Claudet

Valérie Derolez

Maggy M Nugues

Vincent Ouisse

Nolwenn Quillien

Yoann Baulaz

Antoine Carlier

Patrick Le Mao

Pierre Scemama

Diane Vaschalde

Denis Bailly

Rémi Mongruel

How does eutrophication impact bundles of ecosystem services in multiple coastal habitats using state-and-transition models

Keywords:

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The demographic and economic growth of societies is increasingly facing the ecological limits of the planet [START_REF] Meadows | Limits to Growth: The 30-Year Update[END_REF]. This global ecological crisis, as illustrated by major changes in ecosystem states with decreasing availability of natural resources, is accelerated by climate change. The consequences of this crisis are already observable within societies and will most likely spread and generalize in future decades [START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF][START_REF] Isbell | Linking the influence and dependence of people on biodiversity across scales[END_REF]). One of the major scientific challenges for biodiversity conservation is to improve the understanding of the relationship between biodiversity, ecosystem functioning and ecosystem services [START_REF] Harrison | Linkages between biodiversity attributes and ecosystem services: a systematic review[END_REF].

Ecosystem services (ES) seek to account for the dependence of human societies on ecosystems, commonly defined as the contributions of ecosystem structure and function to human well-being [START_REF] Ma | Millenium Ecosystem Assessment)[END_REF]. Originally, the concept of ES and its monetary valuation (see [START_REF] Costanza | The value of the world's ecosystem services and natural capital[END_REF] were primarily intended to alert public opinion and governments about the importance of well-functioning ecosystem for societies and the risks associated with the ecological crisis. Scientific developments of this concept, encouraged by its institutionalization, have gradually clarified its scope and defined multicriteria analysis as the most robust way to evaluate ES, seeking to inform decision-making processes and the establishment of public policy and management policies [START_REF] Keune | Negotiated complexity in ecosystem services science and policy making -Ecosystem Services[END_REF][START_REF] Saarikoski | Multi-criteria decision analysis (MCDA) in ecosystem service valuation[END_REF]. However, 50% of ES studies focus on a single service, or on a limited number of services, without considering interactions and feedback with other services [START_REF] Seppelt | A quantitative review of ecosys-tem service studies: Approaches, shortcomings and the road ahead[END_REF][START_REF] Couvet | Services écosystémiques: des compromis aux synergies[END_REF]). This monofocal vision can lead to an operational ecosystem management based on the maximization of a single ES and potentially to the detriment of the other ones [START_REF] Couvet | Services écosystémiques: des compromis aux synergies[END_REF].

ES are a function of complex interactions among species and their abiotic environment, complex use and utilization patterns and various perceptions by beneficiaries. ES bundles are defined as sets of ES that repeatedly appear together across space or time [START_REF] Raudsepp-Hearne | Ecosystem service bundles for analyzing tradeoffs in diverse landscapes[END_REF]. It is thus a useful tool for improving the management of ecosystems and identifying common ES tradeoffs and synergies: trade-offs arise when the provision of one service is enhanced at the cost of reducing the provision of another service, and synergies arise when multiple services are enhanced simultaneously [START_REF] Raudsepp-Hearne | Ecosystem service bundles for analyzing tradeoffs in diverse landscapes[END_REF]. Bundle analysis seeks to inform management and decision-making for reducing the cost of both tradeoffs and synergies. For example, enhancing some provisioning services might imply the conversion of forest to agricultural land, at the cost of some regulating (e.g. carbon storage) and cultural services (e.g. recreation). More recent scientific developments indicate that separating, a minima, the supply and demand of ES helps to refine and clarify the bundle analysis [START_REF] Villamagna | Capacity, pressure, demand, and flow: a conceptual framework for analyzing ecosystem service provision and delivery[END_REF][START_REF] Burkhard | Ecosystem service potentials, flows and demands-concepts for spatial localisation, indication and quantification[END_REF][START_REF] Levrel | Approches écologiques et économiques de l'offre et la demande de services écosystémiques[END_REF]. The supply represents the ecosystem capacity to provide ES (also called potential), whereas the demand is the amount of services used, consumed but also desired by the society [START_REF] Villamagna | Capacity, pressure, demand, and flow: a conceptual framework for analyzing ecosystem service provision and delivery[END_REF]. Different approaches can be used to analyze tradeoffs and synergies depending on whether the focus is on supply or demand for ES.

Eutrophication occurs when the nutrient enrichment process (especially nitrogen and / or phosphorus compounds) leads to an increase in primary production, growth and biomass of phytoplankton and / or macroalgae, as well as a change in the equilibrium of organisms and a degradation of water quality [START_REF] Cloern | Our evolving conceptual model of the coastal eutrophication problem[END_REF][START_REF] Ferreira | Overview of eutrophication indicators to assess environmental status within the European Marine Strategy Framework Directive[END_REF]. It is a natural phenomenon and ecosystems have a level of resilience that allows them to resist against the high variability of nutrient enrichment. This resilience may be insufficient when excessive nutrient enrichment occurs from human activities. In Europe, the volume of nitrogen transported to the coastal areas is now four times higher than that of natural origin [START_REF] Voss | Nitrogen processes in coastal and marine systems[END_REF]. This eutrophication, with an anthropogenic origin, happens at shorter time scales and is a real issue worldwide because of its important socio-economic consequences: loss of tourist potential and water use for recreational activities, unfit seafood or increased maintenance costs associated with algal removal [START_REF] Lefebvre | Directive Cadre Stratégie pour le Milieu Marin (DCSMM). Définition du Bon État Écologique (BEE) pour le Descripteur 5. Rapport pour le MEDTL -Direction de l'eau et de la biodiversité -Sous-direction du littoral et des milieux marins, juin 2011[END_REF]. From a strictly ecological point of view, the eutrophication manifestations are classically distinguished into two types, namely the development of opportunistic macroalgae and the development of phytoplankton blooms (Menesguen et al., 2001). By modifying environmental conditions, these macroalgal and phytoplankton developments will impact the entire ecosystem.

Coastal habitats are among the habitats the most exposed to current direct and indirect drivers of change (REF). Among them, sandy beaches, coastal lagoons and coral reefs are among the most vulnerable [START_REF] Defeo | Threats to sandy beach ecosystems: A review[END_REF][START_REF] Kennish | Coastal Lagoons. Critical Habitats of Environmental Change[END_REF][START_REF] Pendleton | Multiple Stressors and Ecological Complexity Require a New Approach to Coral Reef Research[END_REF]. Among these multiple stressors, eutrophication is particularly important, prevalent and at the origin of significant ecological changes [START_REF] Smith | Eutrophication of freshwater and coastal marine ecosystems a global problem[END_REF][START_REF] Diaz | Spreading Dead Zones and Consequences for Marine Ecosystems[END_REF][START_REF] Wilkinson | Eutrophication of Freshwater and Coastal Ecosystems[END_REF]. Although the ecological impacts of eutrophication on these ecosystems are well studied today, its effects on ES bundles are little explored. The ES approach can provide an interesting perspective to understand the ecological impacts and associated risks of eutrophication to better inform decision-making processes and management strategies. Here, we assess the effects of eutrophication in sandy beaches, coastal lagoons and coral reefs on ES bundles. Our aim is to identify trade-offs and synergies between ES and the possible societal benefits associated to the recovery of the ecological functions for these ecosystems.

Materials and Methods

State-and-transition model

State-and-transition models are an operational and conceptual framework for organizing and providing information about ecosystem dynamics and management outcomes describing how communities respond to pressures and management [START_REF] Briske | State-and-transition models, thresholds, and rangeland health: a synthesis of ecological concepts and perspectives[END_REF][START_REF] Bestelmeyer | National assessment and critiques of state-and-transition models: The baby with the bathwater[END_REF]. It has been developed by the United States Department of Agriculture -Natural Resources Conservation Service (USDA-NRCS) for rangeland ecological sites in southern Arizona. While its scientific application is widespread for some terrestrial habitats (e.g. Quétier et al., 2007 ;McIntyre and Lavorel, 2007 ;Tarrason et al., 2016), its application in the marine environment remains almost non-existent.

We apply here the two first steps of the operational framework described by [START_REF] Lavorel | Ecological mechanisms underpinning climate adaptation services[END_REF] are applied to three marine ecosystems to explore the evolutions of communities and ES bundles supply along a gradient of eutrophication. Eutrophication and management measures (e.g. nutrient flow, ecological restoration) are seen as drivers determining the ecosystem state, i.e. specific biodiversity and functioning, at a particular time and place. The first step aims to characterize the ecosystem dynamics under eutrophication through state-and-transition models: alternative states may be represented by dominant species and associated biodiversity based on empirical and prospective studies. For each ecosystem state, i.e. eutrophication level, bundles of ecosystem services supplied are identified. The second step aims to describe and, if possible, to quantify ecosystem responses to eutrophication levels. This involves changes affecting ecosystem functions and supply of ES.

The classification of the Common International Classification of Ecosystem Services (CICES) and the list of marine ES defined by [START_REF] Liquete | Current Status and Future Prospects for the Assessment of Marine and Coastal Ecosystem Services: A Systematic Review[END_REF] have served as a support for the definition of the ES constituting the bundles studied (TABLE_1). The main distinction between these classifications concerns supporting services or ecological functions. These latter are the underpinning structures and processes that ultimately give rise to ecosystem services -sometimes defined as 'intermediate services'.

They are not covered in CICES which seeks to only identify the final services that link to the goods and benefits that are valued by people (i.e. demand). Since we focus here on the ES supply, main ecological functions are considered as recommended by [START_REF] Liquete | Current Status and Future Prospects for the Assessment of Marine and Coastal Ecosystem Services: A Systematic Review[END_REF]. were considered : "0: inexistent", "1: very low", "2: low", "3: medium", "4: high", "5: very high".

TABLE_1: Correspondence between CICES and selected ES

Three coastal case studies: sandy beaches, Mediterranean coastal lagoons and coral reefs

The state-and-transition model is applied to sandy beaches, Mediterranean coastal lagoons and coral reefs, three ecosystems with different biophysical characteristics related to contrasted ES supply.

Present at several latitudes, these ecosystems undergo changes proven in various parts of the world, linked to a multitude of pressures among which eutrophication is particularly important. Beyond their ecological functioning, these ecosystems underlie many uses and have important cultural and heritage values. We believe that an increased awareness -by society -of the changes could favor the levers of action to reverse the negative trend they are undergoing (REF).

Sandy beaches are defined as accumulations of non silty fine sediment along coastlines [START_REF] Davis | Beaches in Space and Time: A Global Look at the Beach Environment and How We Use It[END_REF] including the entire foreshore since the level of the Mean High Water Springs until the level of the Mean Low Water Springs. It constitutes a highly dynamic ecotone mainly influenced by its physical environment. Indeed, the composition of species assemblages and the organism abundances are correlated with physical factors such as wind, beach slope, tidal amplitude or sediment granulometry [START_REF] Mclachlan | Global Patterns in Sandy Beach Macrobenthic Communities[END_REF]. Unfairly characterized as lifeless deserts [START_REF] Mcdermott | Food web in the surf zone of an exposed sandy beach along the mid-atlantic coast of the united states[END_REF], sandy beach ecosystems harbor many organisms that are highly specialized and adapted to life in mobile sediments leading to specific ecological functions [START_REF] Mclachlan | The Ecology of sandy shores[END_REF]. Where conditions are favorable to the development of opportunistic macroalgae, eutrophication will generate deposits on the sandy beaches, the anaerobic decomposition will evolve toxic reducing substances, including hydrogen sulfide. Sandy beaches are present worldwide constituting 70% of the ice-free coastline [START_REF] Mclachlan | The Ecology of sandy shores[END_REF]. Eutrophication occurs in many parts of the world [START_REF] Smetacek | Green and golden seaweed tides on the rise[END_REF]. In

France, green tides punctually occur along the Channel-Atlantic coast during spring and summer.

Mediterranean coastal lagoons are semi-enclosed ecosystems spread along the European coasts [START_REF] Barnes | Coastal lagoons[END_REF][START_REF] Kjerfve | Coastal lagoon[END_REF][START_REF] Fiandrino | Spatial patterns in coastal lagoons related to the hydrodynamics of seawater intrusion[END_REF]. They are expanses of shallow coastal water, of varying salinity and water volume, partially separated from the sea by sand banks or shingle, or, less frequently, by rocks [START_REF] Hill | EUNIS habitat classification descriptions[END_REF]). Salinity may vary from oligohaline to hyperhaline ranges depending on rainfall, freshwater inland and underground water supplies, evaporation and through the addition of fresh seawater from storms or tidal exchange. These ecosystems support habitats with or without macroalgae and phanerogams vegetation. Eutrophication particularly occurs in many French mediteranean coastal lagoons with a strong gradient from oligotrophic to hypertrophic states [START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF][START_REF] Souchu | Patterns in nutrient limitation and chlorophyll a along an anthropogenic eutrophication gradient in French Mediterranean coastal lagoons[END_REF][START_REF] Leruste | First steps of ecological restoration in Mediterranean lagoons: shifts in phytoplankton communities[END_REF][START_REF] Fur | Submerged benthic macrophytes in Mediterranean lagoons: distribution patterns in relation to water chemistry and depth[END_REF]. Where conditions are favorable to eutrophication, the development of opportunistic macro-and micro-algae is observed with a marked change of primary producers [START_REF] Schramm | Factors influencing seaweed responses to eutrophication : some results from EUproject EUMAC[END_REF][START_REF] Leruste | First steps of ecological restoration in Mediterranean lagoons: shifts in phytoplankton communities[END_REF][START_REF] Fur | Submerged benthic macrophytes in Mediterranean lagoons: distribution patterns in relation to water chemistry and depth[END_REF].

Coral reefs are developed on the immersed bottoms of volcanic islands in the intertropical zone. The reefs are constructed from a mineral substrate (calcium carbonate) secreted primarily by the polyps of scleractinian corals. This habitat is made of a reef surface and a non-reef surface (lagoon and sedimentary terraces). Despite striving in nutrient-poor waters, coral reefs belong to the most productive ecosystems on Earth due to efficient retention and recycling of carbon and nutrients (famously referred to as the "Darwin's Paradox"). Eutrophication has long-term negative impacts on the structure and functioning of coral reef ecosystems. Increasing nutrient levels can: i) increase the number and prevalence of coral diseases [START_REF] Bruno | Nutrient enrichment can increase the severity of coral diseases[END_REF][START_REF] Voss | Nutrient enrichment enhances black band disease progression in corals[END_REF][START_REF] Vega Thurber | Chronic nutrient enrichment increases prevalence and severity of coral disease and bleaching[END_REF] and the susceptibility of corals to temperature and light-induced bleaching [START_REF] Wiedenmann | Nutrient enrichment can increase the susceptibility of reef corals to bleaching[END_REF], ii) reduce coral reproduction and skeletal growth [START_REF] Tomascik | Effects of eutrophication on reef-building corals[END_REF][START_REF] Koop | ENCORE: the effect of nutrient enrichment on coral reefs. Synthesis of results and conclusions[END_REF], iii) stimulate the growth of algae, heterotrophic sponges and benthic cyanobacterial mats, which in turn can reduce coral recruitment, alter the coral microbiome and drive reef decline further [START_REF] Mumby | Coral reef management and conservation in light of rapidly evolving ecological paradigms[END_REF][START_REF] Brocke | Organic Matter Degradation Drives Benthic Cyanobacterial Mat Abundance on Caribbean Coral Reefs[END_REF][START_REF] Pawlik | A Vicious Circle? Altered Carbon and Nutrient Cycling May Explain the Low Resilience of Caribbean Coral Reefs[END_REF][START_REF] Ford | Evaluation of coral reef management effectiveness using conventional versus resilience-based metrics[END_REF], iv) enhance periodic outbreaks of the corallivorous crown-of-thorns starfish Acanthaster planci [START_REF] Brodie | Are increased nutrient inputs responsible for more outbreaks of crown-of-thorns starfish? An appraisal of the evidence[END_REF], and v) promote higher bioerosion rates by favouring the activity of filter-feeders such as endolithic bivalves and bioeroding sponges [START_REF] Fabricius | Effects of terrestrial runoff on the ecology of corals and coral reefs: review and synthesis[END_REF]. Coral reefs are present worldwide in the intertropical zone. They are one of the most emblematic tropical ecosystems because of its size, geomorphological diversity, biodiversity and high endemicity [START_REF] Gardes | -Récifs coralliens de l'Outre-mer français : suivi et état des lieux[END_REF]. In France, eutrophication occurs in coral reefs located along the most inhabited volcanic islands (e.g. Martinique,

Guadeloupe, La Réunion).

Results

State and transition description

For each ecosystem, ecological functioning is described by dominant species and associated biodiversity for three to four levels of eutrophication (FIGURE_1).

Sandy beaches

First ecological state of sandy beaches is described by groups of species defined as reference species living in a non-eutrophic ecosystem, where no green tides occur. In some French Atlantic sandy beaches, these reference species, for marine benthic macrofauna, are part of Tellinidae, Spionidae, Amphiuridae and Nephtyidae families [START_REF] Quillien | Effects of macroalgal accumulations on the variability in zoobenthos of highenergy macrotidal sandy beaches[END_REF]. The continuous supply of nutrients (exogenous inputs or release from sediments) causes a slight excess and leads to the gradual development of green algae. As a response, dominant species change in this eutrophic ecosystem with an appearance of new dominant species (Donacidae, Oweniidae, Magelonidae) and the decrease, even the disappearance, of some reference species (Tellinidae, Spionidae, Amphiuridae) [START_REF] Quillien | Effects of macroalgal accumulations on the variability in zoobenthos of highenergy macrotidal sandy beaches[END_REF]. Where hydrodynamic conditions are favorable, the massive supply of nutrients leads to the massive and rapid development of green algae forming green tides. Species of reference have disappeared in favor of species (Donacidae, Oweniidae) better adapted to eutrophic conditions.

Abundance and biomass are higher in this new eutrophic ecosystem, but the species richness is lower [START_REF] Quillien | Effects of macroalgal accumulations on the variability in zoobenthos of highenergy macrotidal sandy beaches[END_REF].

Mediterranean coastal lagoons

First ecological state of primary production of Mediterranean coastal lagoon is characterized by a dominance of reference species that are typical of a lagoon environment in oligotrophic conditions (State I). For French Mediterranean coastal lagoons, the reference genus are the marine phanerogams Zostera and Ruppia which form seagrass beds, and perennial benthic macroalgae (eg. Cystoseira sp., Acetabularia sp.). The continuous supply of nutrients causes a slight excess and leads to the gradual disappearance of the reference species and the slow and sustainable development of algae [START_REF] Schramm | Factors influencing seaweed responses to eutrophication : some results from EUproject EUMAC[END_REF]. State II is dominated by a dominance of opportunistic and epiphytic macroalgae. Most are red or brown algae, which can form drifting populations or seasonaly bloom on substrates or other macrophytes. The massive supply of nutrients leads to the massive and rapid dominance of freefloating blooming opportunistic algae (State III). These algae have a shorter lifetime and a higher growth rate than state II algae. Their ability to absorb nutrients is higher, making them more competitive than other species in highly eutrophic environments. In case of proliferation, they can cover and eliminate seagrass beds. In case of massive development of these species, they fill the whole column of water and reach the surface, forming green tides. In the most eutrophicated systems, phytoplankton community dominates the water column (State IV). The proliferation of macroalgae and phytoplankton can contribute to triggering the phenomenon of anoxic crisis.

Coral reefs

Combining anthropogenic and natural stressors lead to changes in the ecological functioning of coral reefs [START_REF] Hughes | Catastrophes, phase shifts, and large-scale degradation of a Caribbean coral reef[END_REF][START_REF] Jackson | Status and trends of Caribbean coral reefs: 1970-2012[END_REF][START_REF] De Bakker | Long-term shifts in coral communities on shallow to deep reef slopes of Curac¸ao and Bonaire: are there any winners?[END_REF]. First ecological state of coral reefs is characterized by the dominance of hard corals. In the Caribbean reefs of Curaçao and Bonaire, hard coral represented two thirds of the benthic community cover in this initial state. Other benthic communities present were algal turfs, crustose coralline algae, sponges and benthic cyanobacterial mats [START_REF] De Bakker | 40 Years of benthic community change on the Caribbean reefs of Curacao and Bonaire: the rise of slimy cyanobacterial mats[END_REF]. The increasing pressure (transition 1) leads to the development of algal turfs and fleshy macroalgae which are fast-growing organisms and a gradual decline of coral cover (e.g.

Hughes 1994), in particular from competitive losses against algae under conditions of reduced herbivory [START_REF] Vermeij | The effects of nutrient enrichment and herbivore abundance on the ability of turf algae to overgrow coral in the Caribbean[END_REF]) (State II). Algal turfs are multispecies assemblages of diminutive, mostly filamentous algae, including cyanobacteria. Due to their opportunistic life-history characteristics, they are able to rapidly occupy newly available substratum. Besides, they inhibit coral recruitment. Fleshy macroalgae are commonly defined as more upright and anatomically complex algae with frond extension > 1 cm (e.g., Dictyota spp. and Lobophora spp.). They are frequently superior competitors against corals, inhibiting coral growth, reproduction, and recruitment [START_REF] Nugues | Differential competitive abilities between Caribbean coral species and a brown alga: a year of experiments and a long-term perspective[END_REF]. With a continuous and increasing pressure (transition 2), benthic cyanobacterial mats increase and become dominant at the expense of algal turfs and macroalgae while sponges showed a more limited but significant increase. Benthic cyanobacteria mats benefit from increased levels of nutrient [START_REF] Brocke | Organic Matter Degradation Drives Benthic Cyanobacterial Mat Abundance on Caribbean Coral Reefs[END_REF] but also from high grazing pressure and elevated water temperature [START_REF] Kuffner | Effects of nitrate, phosphate and iron on the growth of macroalgae and benthic cyanobacteria from Cocos Lagoon, Guam[END_REF][START_REF] Bender | Warming and acidification promote cyanobacterial dominance in turf algal assemblages[END_REF]

ES bundle description

ES bundles are described by relative levels of functions and services supplied by each ecosystem for three to four levels of eutrophication (FIGURE_1). As supply is difficult to quantify or even to characterize, it is sometimes described qualitatively by the state of the ES and the conditions necessary for its effectiveness.

Sandy beaches

Coastal protection is provided by both the physical structure of the beach and specific fauna and flora.

As ES only considers the roles played by biodiversity, coastal protection (R1) focuses on the latter, ables to reduce the hydrodynamics or to stabilize the substrate. Indeed, bioturbating organisms contribute to the stabilization of the substrate and the tide mark also limits the erosion phenomenon by trapping the sand. But the presence of Ulva mats (States II and III) impacts the hydrodynamics (Hull, 1987), thus affecting the sediment transport and ultimately the ES. Nutrient regulation (R2) decreases along the gradient of eutrophication: beach ecosystems are important in processing large quantities of organic material and recycling nutrients back to coastal waters (Schlacher et al., 2008) but the release of excess nutrients and the presence of green macroalgae mats probably saturate the filtering function and the ES as well. The capacity of beach ecosystems to provide a service of pathogen regulation (R3)

is not well-documented but in the same way as for nutrient regulation (R2) the alteration of the filtering function can affect this ES. Climate regulation (R4) is constant between state I to III as sequestration through the phytoplanktonic, microphytobenthic and green algae activity is a short term function.

Human food (P1) is highly decreasing along the eutrophication gradient because of the changes in species assemblages affecting the shellfish fishing activities. Indeed, sandy beaches support professionnal fisheries of the bivalve Donax trunculus, which is of commercial importance (Augris et al., 2005 ;[START_REF] Mclachlan | The Ecology of sandy shores[END_REF] but at eutrophication states (States II and III), a decrease in its density has been shown [START_REF] Quillien | Effects of macroalgal accumulations on the variability in zoobenthos of highenergy macrotidal sandy beaches[END_REF]. Materials (P2) is slightly provided by driftwood and seashell which can be collected but highly increase in state III because of the capacity of green algae to be collected and used in industry (pet food, cosmetics...). Molecules (P3) is potentially provided in States I and II as the diversity that is harboured by sandy beaches is high, specialized and unique and effectively provided through Arenicola marina, which is collected and bred to get hemoglobin for medical uses. In state III, molecules are effectively extracted from Ulva but the eutrophication impacts the other organisms, and more specifically affects the overall diversity thus decreasing the potential pool of molecules.

Cultural ES (C1 to C4) decrease along the eutrophication gradient as the landscape, the leisure activities, the territorial identity and the emblematic biodiversity are affected by green tides [START_REF] Mclachlan | The Ecology of sandy shores[END_REF]Schlacher, 2008 ;Levain, 2014).

The habitat function (F1) is altered as green tides affect nurseries of various species (Quiniou, 1986 ;[START_REF] Mclachlan | The Ecology of sandy shores[END_REF]Quillien et al., 2017 ;Le Luherne, 2017). Trophic networks (F2) in state I is high because the food web is complex, showing several potential carbon pathways and diverse trophic niches while in eutrophication states, the trophic network is homogenized/simplified and shows less niche differentiation [START_REF] Quillien | Dynamic ecosystems under anthropogenic stress : how does macrotidal sandy beach fauna respond to green tides ?[END_REF]. Recruitment (F3) is high in State I for many species and increases between states I and II as the presence of Ulva mats influence local hydrodynamics, which in turn influence the recruitment of some species. For example, in Brittany, the presence of heterogenous cover of Ulva enhances the recruitment of the bivalve Donax vittattus [START_REF] Quillien | Effects of macroalgal accumulations on the variability in zoobenthos of highenergy macrotidal sandy beaches[END_REF]. However when the Ulva biomass is high, macroalgae affect the recruitment, community structure and production of benthic fauna, including meiofauna, macrofauna and flatfish [START_REF] Quillien | Dynamic ecosystems under anthropogenic stress : how does macrotidal sandy beach fauna respond to green tides ?[END_REF].

Mediterranean coastal lagoons

On Mediterranean coastal lagoon ecosystems, coastal protection (R1) is estimated to decrease with the alteration and decline of seagrass meadows which have the capacity to attenuate waves and to slow down currents [START_REF] Paquier | Sedimentation and erosion patterns in a low shoot-density Zostera noltii meadow in the fetch-limited Berre lagoon, Mediterranean France[END_REF]. In a logical way, nutrient regulation (R2) decreases along the gradient of eutrophication. Seagrass beds play an important role in regulating benthic nutrient fluxes in lagoons as they increase the ability to store nutrients sustainably. The flow of nutrients from the sediment to the water column and, at the same time, eutrophication levels are thus greater in lagoons without seagrass [START_REF] Viaroli | Community shifts, alternative stable states, biogeochemical controls and feedbacks in eutrophic coastal lagoons : a brief overview[END_REF][START_REF] Ouisse | Restauration des écosystèmes lagunaires : évaluation du rôle du sédiment et des herbiers à phanérogames[END_REF]. Pathogen regulation (R3) is more provided in states I and II than in states III and IV because of the algicidal effects of Zostera marina L. and Zostera noltei Hornem. on Alexandrium catenella [START_REF] Laabir | Algicidal effects of Zostera marina L. and Zostera noltii Hornem. extracts on the neuro-toxic bloom-forming dinoflagellate Alexandrium catenella[END_REF]. More generally, seagrass ecosystems reduce exposure to bacterial pathogens of humans, fishes, and invertebrates [START_REF] Lamb | Seagrass ecosystems reduce exposure to bacterial pathogens of humans, fishes and invertebrates[END_REF]. However, emergence of toxic dinoflagellate is observed in oligotrophic conditions [START_REF] Collos | Oligotrophication and emergence of picocyanobacteria and a toxic dinoflagellate in Thau lagoon , southern France Regional index term : Mediterranean[END_REF] which leads to weighting the pathogen regulation service in state I. Climate regulation (R4) is particularly high in state I and II because of the potential long-term capacity to sequestrate greenhouse gases in the sediment through perennial macrophytes.

Human food (P1) is the only provisioning service for Mediterranean coastal lagoons [START_REF] Brito | Assessing, quantifying and valuing the ecosystem services of coastal lagoons[END_REF] and is mainly based on shellfish farming. The quantity or the state of the suitable areas for shellfish farming indicate the state of this ES. As shellfish farming needs a high rate of primary productivity to feed shellfish, the state II meets the most optimal conditions. On the one hand, oligotrophic conditions can lead to an under-capacity of production and on the other hand, a massive supply of nutrients can lead to anoxic crisis and the death of shellfish stocks [START_REF] Cloern | Our evolving conceptual model of the coastal eutrophication problem[END_REF].

Emblematic biodiversity (C4) is varying along the eutrophication gradient. In states I and II, protected and rare species like Zostera sp., Hippocampus sp., avifauna (e.g. Anas penelope, Cygnus olor, Egretta garzetta, Ardea cinerea) are able to contribute to human well-being because of their mere existence, but also because of their role in supporting of some recreational and leisure activities (C4) like scubadiving, snorkeling and nature watching. In states III and IV, the presence of these protected and rare species decreases for the benefit of a more restrictive number of other protected species like flamingos (de [START_REF] Wit | Restoration ecology of coastal lagoons: new methods for the prediction of ecological trajectories and economic valuation[END_REF]. The contribution of Mediterranean coastal lagoon ecosystems to a pleasant landscape (C2) is mainly based on avifauna, and in a lesser extent, on other living components including underwater seascape (Chazée et al., 2016). Avifauna is present from state I to IV even if the assemblage of species varies between them. However, the capacity of Mediterranean coastal lagoons to provide pleasant underwater seascape can be considered altered with the degradation and decline of Zostera meadows. Biodiversity of coastal lagoons also contributes to culture and territorial identity (C3) as they are a socialization area, sometimes assimilated to a urban park [START_REF] Chazée | Les services culturels récréatifs et éducatifs des zones humides en Méditerranée -Des services sous-estimés malgré les avantages qu'ils procurent: résultats d'études en Méditerranée[END_REF].

The habitat function (F1) is altered with degradation and decline of Zostera meadows since they are a habitat for many species. Indeed, the leaf canopy and the network of rhizomes and roots create hiding places to avoid predation. Mediterranean coastal lagoons also provide higher temperature during growth and food to some fish species like Sparus aurata, which allow good lipid reserves, and large sizes of juveniles, which may be very important to their survival over winter [START_REF] Tournois | Temporal stability of otolith elemental fingerprints dis-criminates among lagoon nursery habitats[END_REF][START_REF] Isnard | Getting a Good Start in Life? A Comparative Analysis of the Quality of Lagoons as Juvenile Habitats for the Gilthead Seabream Sparus aurata in the Gulf of Lions[END_REF]. Trophic networks (F2) in state I is high because of its high complexity, which then decreases along the eutrophication gradient until low complexity in degraded states [START_REF] Pearson | Macrobenthic succession in relation to organic enrichment and pollution of the marine environment[END_REF]. As a consequence, if enrichment of food webs in lagoons is altered, the consequences for fishery resources could be important with an impact on recruitment (F3). However, the carrying capacity for juvenile oysters of oligotrophic lagoons is questioned [START_REF] Lagarde | Recruitment of the Pacific oyster Crassostrea gigas in a shellfish-exploited Mediterranean lagoon: Discovery, driving factors and a favorable environmental window[END_REF] and leads to weighting the recruitment function in state I.

Coral reefs

Coastal protection (R1) is an important service provided by coral reefs as they can dissipate 97% of the wave energy that would otherwise impact shorelines (Ferrario et al., 2013). As eutrophication leads to the loss of corals, water depth between the reef crest and the surface increases and should result in a less effective, or even nonexistent, ES. Nutrient regulation (R2) decreases along the gradient of eutrophication. This ES is intensively performed by zooxanthellae in living coral reefs (state I). The risk of hyper-eutrophication is thus greater in altered or dead coral reefs. Moreover, cyanobacterial mats and sponges who are also able to produce the ES in state III can be easily washed away by storms.

Pathogen regulation (R3) also decreases along from state I to III. Turf algae and macroalgae (state II)

alter the coral microbiome and elevate putative pathogen loads (Vega-Thurber et al., 2012 ;Zanefeld et al., 2016 ;Pratte et al., 2018). Living coral reefs (state I) also widely contribute to climate regulation (R4), stocking greenhouse gases through the production of carbonates (Roberts et al., 2017).

Human food (P1) is mainly based on reef fisheries. Coral-dominated reefs (state I) are the most productive [START_REF] Hughes | Coral reefs in the Anthropocene[END_REF]. Depending on structural complexity, multi-species fisheries can shift towards herbivorous fish species (state II). However, overfishing of herbivorous fishes prevents the return to state I [START_REF] Hicks | Social drivers forewarn of marine regime shifts[END_REF]. Bottoms lacking structural complexity (state III) become very poor in target species.

Coral ecosystems are particularly rich in molecules (P3) because of the wide competition between species that leads to a diversity of chemical defense by organisms (Banaigs et al. 2016). State I supports a high diversity of organisms and thus potential biomolecules. Cyanobacteria and sponges (state III) are also chemically rich. However, chemical defense could lessen in absence of consumers. For example, sponge communities have become dominated by fast-growing species that lack chemical defenses on reefs where sponge-eating angelfishes and parrotfishes have been removed by overfishing [START_REF] Loh | Chemical defenses and resource trade-offs structure sponge communities on Caribbean coral reefs[END_REF].

In many tropical societies, relations to nature are often very different from those related to the Western lifestyle and the distinction between culture and nature is sometimes blurred. In these tropical contexts, the difficult resilience and adaptive capacity to abrupt changes in coral reefs (eutrophication and other pressures) can alter cultural ESs (C1 ; C2 ; C3) from state I to state III [START_REF] Sterling | Culturally Grounded Indicators of Resilience in Social-Ecological Systems[END_REF].

Emblematic biodiversity (C4) is particularly rich in coral reefs and decreases along the eutrophication gradient. Coral-dominated reefs (state I) are richer in habitat (F1), trophic networks (F2) and recruitment (F3) than algal dominated reefs and cyanobacterial mats. Corals provide shelter and food for a large diversity of benthic organisms and allow the creation of complex trophic networks. Algaldominated state can benefit some herbivorous fishes, but large fleshy macroalgae and cyanobacterial mats are often unpalatable to fishes. Mesopredators can switch prey, shortening food chains, in response to coral reef degradation [START_REF] Hempson | Coral reef mesopredators switch prey, shortening food chains, in response to habitat degradation[END_REF]. The three dimensional structure of corals are important to fish recruitment, which can, in turn, increase herbivory and favor coral dominance via positive feedback mechanisms [START_REF] Mumby | Coral reef management and conservation in light of rapidly evolving ecological paradigms[END_REF].

Discussion

Evolution of bundles of ecosystem services

ES bundles are very important in state I for each ecosystem, determined in part by their ability to provide the 3 ecological functions (habitat, recruitment, food networks) that support the ESs set. The bundle is thus particularly strong for coral reefs but also very important for sandy beaches and coastal lagoons. For all ecosystems, the general trend is a degradation of the ES bundles along the eutrophication gradient, particularly for nutrients and pathogen regulation/sequestration or the support of recreational and leisure activities. However, some exceptions exist. For instance, the capacity of coral reefs to provide molecules for medicine decreases between the two first states but increases again in state III, dominated by chemically rich cyanobacteria and sponges . The capacity of sandy beaches to support recruitment function increases with the proliferation of green algae as the latter can reduce hydrodynamics intensity allowing the fixation of larvae. When green tides occur, sandy beaches also increase their capacity to provide material as algae might be collected and used in industry for pet food and cosmetics. Finally, the capacity of Mediterranean coastal lagoon to provide food through shellfish farming increases between states I and II as nutrient flows increase shellfish productivity. However, excessive nutrient flows occurring in states III to IV can lead to shellfish mortality thus decreasing ES supply. Trade-offs exist between these ES which are enhanced by eutrophication and other ES of the bundles which are altered along the same gradient.

Potential impacts on ES demand

ES demand is not explored here, but we can easily imagine that a deterioration of almost all ES in increasing eutrophic would no longer meet the societal demand for use and consumption of these ESs, thus turning into an expression of a societal demand for the conservation of these ecosystems. For example, in Brittany (France), local environmental protection associations have been created and mobilized against green algae after the the deaths of a horse, dogs and wild boar linked to gas emissions generated by green tides was broadcasted into local and national medias [START_REF] Levain | Faire face aux « marées vertes », penser les crises du vivant. ethnographiques.org[END_REF].

Another example is the French NGO Coral Guardian, created in 2012 in response to the multiple pressures these ecosystems are undergoing and their impacts on local communities.

In the few cases where increased eutrophication can favor a given ES, some stakeholders can express a new demand, positioning themselves in favor of a slight or a significant eutrophication because an economic activity could result of it. This could be the case for shellfish farmers in Mediterranean lagoons, for companies cleaning and transforming green algae along the French Atlantic coast, or for pharmaceutical industries towards molecules of cyanobacterial mats and sponges in tropical environments. Even if the cleaning process of macroalgae is still costly (Morand and Merceron, 2005), industrial utilization of this macroalgal biomass is in full growth worldwide (Abd El-Baky et al., 2008 ;[START_REF] Khan | Biodiesel production from green seaweed Ulva fasciata catalyzed by novel waste catalysts from Pakistan Steel Industry[END_REF][START_REF] Qiu | Potential of Ulva lactuca for municipal wastewater bioremediation and fly food[END_REF] and could bear these costs for economic purposes which would not solve the problem of eutrophication at source. Hence the interest of communicating at the scale of the ES bundles so that society and decision-makers can balance these few potential gains obtained from eutrophication against all the ecological functions and ES which have been altered. Indeed, in the case of the Water Framework Directive, cost-benefit analysis of achieving the good environmental status can lead to the argument that costs are disproportionate regarding the benefits. This is due to methodological and conceptual issues regarding the assessment of benefits [START_REF] Feuillette | The use of cost-benefit analysis in environmental policies: Some issues raised by the Water Framework Directive implementation in France[END_REF]. In this way, decision-makers could really use state-and-transition models to better understand the consequences of eutrophication on the impacted users.

How to reverse the situation?

The transition from eutrophic states to initial states depends on the ecosystem resilience. While Mediterranean coastal lagoons have already proved a resilience after years of efforts to reduce nutrient inputs [START_REF] Leruste | First steps of ecological restoration in Mediterranean lagoons: shifts in phytoplankton communities[END_REF], reef recovery to a coral-dominated state is rare (but see Adjeroud et al.

2018

). The transition from coral-dominated to macro-algal dominated reefs can be a regime shift, making it difficult to reverse to the previous state because of strong feedback processes [START_REF] Mumby | Coral reef management and conservation in light of rapidly evolving ecological paradigms[END_REF]. In addition, the eutrophication management must be carried out taking into account also the multiple stressors context (global change, overfishing, urbanization, etc.).

Specifically, the eutrophication management can be carried out at two levels: upstream by focusing on the causes (control of pollution flows from domestic, agricultural or industrial sources); or downstream by tackling these symptoms (dilution action via mixing water, water reoxygenation, green algae cleaning…) (Charlier et al., 2016). The latter does not orient the ecosystem towards an initial state so we believe that upstream strategies for the restoration of eutrophic ecosystems are essential, before any other remedial measures. In 2016, the eutrophication management costed € 272 million in metropolitan

France distributed as follows: € 6.6 million of monitoring and information costs; € 262 million of avoidance costs targeting both reduction of agricultural inputs (41%) and domestic inputs (59%); € 2.5 million of restoration and mitigation costs [START_REF] Henry | Coûts associés à l'eutrophisation[END_REF]. The management of coastal ecosystems needs to be more strongly integrated with the management of terrestrial ecosystems.

Ecological restoration is the process accompanying the recovery of an ecosystem that has been degraded to its historical trajectory. However, depending on the source type of pollution it may be challenging to control the pollution flows. (Souchu et al., 1998 ;[START_REF] Derolez | Restoration trends of the Thau lagoon's water ecological status and phytoplankton communities in response to changes in anthropogenic nutrient inputs . COAST Bordeaux[END_REF]. On the other hand, even if agricultural pollution has been identified as the main source of eutrophication since several decades, green tides are still occurring in Brittany since effective tools are more difficult to implement and should be more based on a redesigned agricultural practices [START_REF] Molénat | Mechanisms of nitrate transfer from soil to stream in an agricultural watershed of French Brittany[END_REF][START_REF] Roberts | Effect on nitrate concentration in stream water of agricultural practices in small catchments in Brittany: I. Annual nitrogen budgets[END_REF]. Thus a strong political will is needed to effectively regulate the sources of pollution.

An alternative would be the establishment of hybrid markets to regulate eutrophication such as water quality trading in which participants can voluntarily exchange their water pollution rights taking into account the respect of certain biophysical criteria related to the water quality. Even if they are still facing obstacles to be effective (Heberling, 2010), these markets are expected to act as win-win solutions as they reduce the costly regulatory burden on the State while mobilizing new sources of private funding to address water quality problems. They can also provide new sources of revenue to farmers through direct payments for nutrient credit offsets and offer greater flexibility to the farmers in how to achieve environmental goals (Abdalla et al. 2007;Ribaudo et Gottlieb, 2011;Corrales et al., 2013).

In addition to these management strategies, supra-local and supra-national actions are needed. For

Concluding remarks: challenges and opportunities regarding state-and-transition model

The use of state-and-transition models to explore the dynamics of ecosystems and ESs is slightly growing [START_REF] Briske | State-and-transition models, thresholds, and rangeland health: a synthesis of ecological concepts and perspectives[END_REF][START_REF] Bestelmeyer | National assessment and critiques of state-and-transition models: The baby with the bathwater[END_REF], but its application to marine and coastal ecosystems remained almost non-existent. The lessons learned from this exploratory application are multiple. The lack of data and the knowledge gaps between ecosystems, ecosystem services, states and transitions are the main challenges to consider. For example, there are more quantitative data on coral reefs than on sandy beaches as the former are much more explored. To our knowledge, there is thus a lack of studies exploring the climate regulation function of sandy beaches, as well as their role in pathogen sequestration, or in harboring species showing interesting molecules for medicine. Experts then recognized that there are more data concerning the supply of provisioning ES than cultural ES since the latter is more studied in the light of the societal demand, based on socio-economic indicators. Finally a last example is that the transition from coral-dominated to algae-dominated reefs and its associated states are much more documented than the state and transition to cyanobacterial mats. To fulfill these gaps it might be very useful to conduct studies using a gradient approach studying response variables at each state. It can also be difficult to quantify each ES based on qualitative and heterogeneous quantitative data. To create a consensus regarding the level of supply granted to each ES, the analytical framework could be complemented by the use of focus groups or by expanding the size of the panel of experts complemented by the use of the Delphi method for example [START_REF] Filyushkina | Applying the Delphi method to assess impacts of forest management on biodiversity and habitat preservation[END_REF].

State-and-transition model can be a very powerful framework to work in an interdisciplinary perspective taking into account all functions and services delivered by the ecosystem and avoiding the conclusions focused on single service. It is also relevant to take into account the temporal dynamics of ES which are too often ignored. We conclude by saying that the effect of eutrophication on ecosystems is not linear and that thresholds and tipping points have to be further considered as well as the fact that eutrophication does not act alone and that the multi-stress context has to be taken into account.

FIGURE_1:

  FIGURE_1: Main characteristics of the ecosystem states and transitions (sources:[START_REF] Quillien | Effects of macroalgal accumulations on the variability in zoobenthos of highenergy macrotidal sandy beaches[END_REF] for sandy beaches;[START_REF] Schramm | Factors influencing seaweed responses to eutrophication : some results from EUproject EUMAC[END_REF] for Mediterranean coastal lagoons;[START_REF] De Bakker | 40 Years of benthic community change on the Caribbean reefs of Curacao and Bonaire: the rise of slimy cyanobacterial mats[END_REF] for coral reefs) and relative levels of ES supplied by the ecosystem in each state of eutrophication (0 = inexistent ; 1 = very low ; 2 = low ; 3 = medium ; 4 = high ; 5 = very high ; R1 = Coastal protection (vegetal or animal reef supplying a protection against erosion and submersion) ; R2 = Nutrient regulation/sequestration (ecosystem capacity to supply a "good quality water", limiting the risk of eutrophication, encouraging shell fish farming…) ; R3 = Pathogen regulation/sequestration (ability of

  example, land use and climate change in northern Africa can influence the seasonal deposition of dust in the Caribbean, adding other types of nutrients (e.g. iron) and potential coral pathogens, with negative effects on Caribbean corals. Thus, local mitigation efforts need to be coupled with actions from nations far from the coastal areas where reefs are found. This eutrophication management can also mitigate the effects of global warming and climate change by decreasing coral susceptibility to disease and bleaching.

CICES Liquete et al. 2013 Selected ES for the study Section Division

		Mediation of waste, toxics and other nuisances		-
		Mediation of flows		R1. Coastal protection
	Regulation & Maintenance	Maintenance of physical, chemical, biological conditions	Water purification Air quality regulation Coastal protection Climate regulation Weather regulation	R2. Nutrient regulation/sequestration R3. Pathogen regulation/sequestration R4. Climate regulation
			Biological regulation
				C1. Support of recreational and leisure
	Cultural	Physical and intellectual interactions with biota, ecosystems, and land-/seascapes [environmental settings]	Recreation and tourism Cognitive effects	activities C2. Contribution to a pleasant landscape C3. Contribution to culture and territorial identity
		Spiritual, symbolic and other interactions with biota, ecosystems, and land-/seascapes [environmental settings]	Symbolic and aesthetic values	C4. Emblematic biodiversity
	-	-	Ocean nourrishment Life cycle maintenance	F1. Habitat F2. Trophic networks F3. Recruitment
	Each step involves a literature review regarding ecosystem responses to eutrophication that is
	supplemented with expert-knowledge. The literature review encompasses knowledge obtained and
	disseminated on a global scale, while expert knowledge focuses on data observed on a more local scale,
	based on French case studies. However, experts have a good understanding of these ecosystems which
	allow them to pronounce in a qualitative way where data gaps are identified. All information is
	compiled within a matrix (SUPPLEMENTARY_FILE). Information was then coded and analyzed to
	produce spider plots summarizing the variation of ES supply between states. Five levels of ES supplied
		Nutrition	Food provision	P1. Food through fisheries
	Provisioning	Materials	Biotic materials and biofuels	P2. Material P3. Molecules
		Energy		-

  . In the Caribbean reefs of Curaçao and Bonaire, hard coral and algal turfs represented both around 10% of the benthic community cover in this State III while benthic cyanobacterial mats represented more than 20% (de Bakker et al. 2017).

  Since they are easily identifiable and generally few in the territory, point sources of pollution (e.g. domestic pollution) are easier to control, to monitor and to effectively regulate than non-point source or diffuse pollution (e.g. agricultural pollution) characterized by random and intermittent occurrence, and influenced by different drivers (e.g. land use, soil type, management practices)(Hu and Huang, 2014 ; Duncan, 2017). For example, the considerable efforts made to water depuration systems on the watershed have induced a significant decrease of nutrient pollution since the 1970s and the late 2000s and have gradually led to a good environmental status of the Thau lagoon according to the Water Framework Directive (WFD)


David Dupenne

Marc Lubineau

Antoine Lonjon
email: antoine.lonjon@univtlse3.fr

Colette G>

Lacabanne

Eric Dantras

Thierry Pierré

Carbon fiber reinforced polymer metallization via a conductive silver nanowires polyurethane coating for electromagnetic shielding

INTRODUCTION

Abstract

For electromagnetic shielding in space environment, the metallization of car bon fiber reinforced polymer (CFRP) is required. A specific attention is paid due to the thermal expansion coefficient difference between substrate and metal coating. A surface metallization of a CFRP has been elaborated by elec trodeposition. This study presents an original process based on an electrically conductive polymer coating elaborated from a polyurethane matrix filled with a low content of silver nanowires ( <10%vol). A continuous and adherent deposit is obtained by an optirnization of the electroplating parameters. A min imal volume fraction was determined at 3%vol associated with an applied cur rent density estirnated near 0.1 Ndm 2

• The growth speed is 7 µm/h at 0.1 Ndm 2

• The adhesion was checked in severe environmental conditions (-196 to 165 °C). The effectiveness shielding obtained with this solution reaches an attenuation value higher than 90 dB between 1 and 26 GHz neces sary for spaoe communication applications.

KEYWOR OS

ooating;, composites, electromagnetic shielding. nanocrystals, nanoparticles, nanowires

The weight reduction in spaoe transportation applications is a key point to limit the energy consumption. A large weight increase of the embedded electronic devices is due to the protection against the electromagnetic interfer ences (EMI). The EMI could affect the signal information generated or received by electronic equipment. The con ventional solution consists in a shielding of the equip ment by an irnpervious metallic box containing the electronic devices. In the case of satellites telecommuni cations systems, the weight of this metallic structure could me more important than the electronic equipment itself. The EMI encountered in satellites could emanate from a wide variety of sources (equipment and devioes) 1 and affect the electronic and electrical equipment perfor mances. This kind of application must be protected with high shielding effectiveness in the gigahertz frequency range. 2 -s The best results for shielding are obtained with metals 9 due to their high electrical conductivity. The metal structure was realized in aluminum due to his low density: then, the surface is electroplated by a high elec trical conductive metal like gold or silver.

The weight reduction of these metallic shielding structure can be achieved by replacing the metal compo nents structure by carbon fiber reinforoed polymer (CFRP), according to mechanical and mass properties requirements. The weight saving relative to the mechanical performances is estimated near 30%. 10 In all cases the CFRP must be metallized to reach the shielding requirement. The CFRP surface conductivity is not enough to allow an electrodeposition classically used on aluminum. The application of a conductive underlayer is necessary to reach a highly conductive surface. This underlayer can be realized by different processes as electroless deposition, 11 chemical vapor deposition, 12 physical vapor deposition, 13 and dynamic chemical plating. 14 These processes create a metallic layer directly on the CFRP surface. But, this metal layer exhibits a weak surface adhesion. A dramatic detachment of this metal layer was observed for large temperature variation due to the difference of thermal expansion coefficient (CTE) with the CFRP substrate. This parameter is crucial for space applications due to the environmental temperature variation.

The conductive layer between the CFRP and the electroplated metal surface is the key point to permit an effective shielding on a wide temperature range. This layer must be sufficiently conductive to allow the electroplating, mechanically optimized to withstand the difference of thermal expansion coefficient and promoted a significant final grip of the metal surface layer. These specifications orientate our choice towards organic conductive layers. Intrinsic conductive polymers 15-18 could be used as conductive layer but their electrical conductivity are very dependent from temperature. Furthermore, their sensitivity to oxidation prevent them from an industrial application. A conductive composite coating polymer-based was preferred. Their extrinsic conductivity is time stable and along the electroplating process. The choice of the metallic filler to realize the conductive coating was essentially led by their electrical conductivity level and their resistance to oxidation. Metallic particles provide the highest electrical conductivity such as silver, 19,20 gold, 21,22 nickel, 23 and copper. 20,24 The conductive composites filled with silver particles are preferred to prevent the oxidation phenomenon. The percolation threshold is established around 15%vol 25 for spherical silver particles and the conductivity level is reached around 10 2 S/m. 25,26 Conventional conductive coatings are filled with 20%vol of silver particles. This filler content increases significantly the composite weight and deteriorates its mechanical properties. Lower percolation threshold could be obtained by increasing the particles aspect ratio ξ (ratio length to diameter). 25,27 Previous works of conductive composites filled with carbon fibers and Carbon NanoTubes (CNT) exhibit a low percolation threshold near 0.1-1%wt and an electrical conductivity around 10 -2 -10 -1 S/m above the percolation threshold. [28][29][30] Silver nanowires (AgNWs) with high aspect ratio allow us to provide a higher electrical conductivity for a small amount of nanowires (below 5%vol). For these low rates, 31 the mechanical properties were preserved.

The aim of this study is to metallize CFRP substrate via a conductive underlayer composed of polyurethane (PU) matrix and AgNWs. We use a waterborne twocomponent PU to limit solvent emissions (volatile organic compounds) studied in a previous work. 32 The electrical conductivity behavior of the PU/AgNWs coating is presented. Above the percolation threshold, the conductivity level allows the metallic electroplating. The minimal volume fraction of AgNWs to obtain a continuous metallic deposit is studied. The adhesion of this deposit according to the applied current density is discussed. The conductive coating adhesion is analyzed as function of temperature. Finally, the shielding effectiveness for different configurations is measured and shows the interest to metallize CFRP.

| EXPERIMENTAL SECTION

| Nanowires processing

AgNWs were synthesized by reducing AgNO 3 with ethylene glycol in presence of poly(vinyl pyrrolidone) (M w = 55,000 g/mol). The reaction was carried out at 160 C in a round-bottom balloon with magnetic stirring bar for 1 h. This technique and the solution concentrations were described by Sun et al. 33,34 AgNWs were elaborated in this work with only few variations. 19 This process permits to obtain nanowires with an aspect ratio around 222. 19 After the process, AgNWs were cleaned by successive washing of water and ethanol. Clean AgNWs were stored in ethanol and dispersed using an ultrasonic bath for short time.

| Electrodeposition processing

Silver electroplating of CFRP process was achieved in two steps: First, the coating application of PU/AgNWs on CFRP to obtain a conductive surface layer followed by the silver electroplating. For this last step, an original silver electroplating cyanide free bath based was used. a.

Step 1: PU/AgNWs coating on CFRP: The PU coating is a two components system: Poly(2-hydroxypropylmethacrylate) (PHPMA), MACRYNAL® VSM 6299w/42WA (Allnex, Belgium), in 40%wt water; and Easaqua™ X D 401 (Vencorex, France) composed of hexamethylene diisocyanate, isocyanurate (45%wt), and 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (30%wt) in 15%wt n-butyl acetate. The two parts are mixed at room temperature (ratio 3:1) with the appropriate volume fraction of AgNWs. The mixture is sprayed with a high volume low pressure (HVLP) spray gun. The curing process is 30 min at 80 C. b.

Step 2: Silver electroplating of PU/AgNWs: The electrolysis was performed at 50 C with silver as anode and the PU/AgNWs coating on CFRP as cathode. The composition of silver electroplating bath is gathered in Table 1. The metallic coating adhesion depends on the current density. The current density value will be discussed in this work.

| Scanning electron microscopy

The coating surface was observed using a JEOL JSM 6700F. Images were collected under 10 keV accelerating voltage and were obtained using the back-scattered electron detection mode.

| Electrical conductivity

The electrical conductivity of the PU/AgNWs composites was measured by a four-point probe technique using a Keithley 2420 for sample impedance less than 10 Ω. The electrical conductivity measurements were carried out by recording the complex conductivity σ * (ω) using a Novocontrol broadband spectrometer (BDS) for impedance above 10 Ω. Measurements were done in the frequency range from 10 -2 to 10 6 Hz at room temperature. The real part, σ 0 (ω) of the complex conductivity σ * (ω) was investigated. The value of σ 0 (ω) at 10 -2 Hz was taken as DC conductivity σ dc .

The bulk electrical conductivity is determined from the measured resistance of the sample between two electrodes. Samples were disks with a diameter of 20 mm and thickness of 500 μm. Samples were placed between 20 mm gold plated electrodes.

The surface electrical conductivity σ surf is related to the measured resistance R surf according to ASTM D257 between two concentric electrodes separated by an insulator at the surface of the sample:

σ surf = ln R 2 R 1 2πR surf ð1Þ
With R 1 and R 2 the radius of the inner and outer electrode, respectively. Samples were coating (thickness $ 50 μm) on isolated substrates of 12 × 12 cm 2 . The conductivity is normalized to surface unit (S.Sqr).

| Adhesion test

The adhesion strength of the plated silver deposit was estimated by a cross-cut tape test (ISO 2409). This test consists in applying and removing a pressure-sensitive adhesive tape over 25 cross-hatched squares of 2 × 2 mm 2 . The cut area of the test coating was examined and classified from 0 (excellent adhesion) to 5 (bad adhesion) by a visual comparison with illustrations in the standard, depending on the amount of flaked coating.

| Thermal expansion and complex mechanical compliance

The thermal expansion coefficient α and the complex mechanical compliance J Ã ω T ð Þ was determined by dynamic mechanical analysis using an ARES G1 strain controlled rheometer (Rheometrics Scientific). Experiments were carried out in rectangular torsion geometry over the linear range (10 -2 % strain at angular frequency of 1 rad/s) from -145 to 150 C with an heating rate of 3 C/min. PU and composites samples were sprayed on silicone mold with 45 mm in length and 10 mm in width. After the curing process, the sample thickness is approximately 500 μm.

| Electromagnetic shielding

The electromagnetic shielding was carried out with a mode-stirred reverberation chamber SMART 700 Chamber (ETS-Lindgren). The frequency range was from 1 to 26 GHz. The samples dimensions were 12 × 12 cm 2 screwed on an aluminum casing with a silicone seal filled with silvered copper particles. The treated surface (conductive coating or metallic deposit) has been oriented inwards to maximize electrical continuity. The effectiveness criterion over the frequency range tested is chosen at 90 dB (dependent on application). Before each test, the shielding effectiveness of a 50 Ω standard is measured to check the coaxial cable state linked to aluminum casing. The effectiveness measured is the maximal effectiveness accessible by the mode-stirred reverberation chamber.

| RESULTS AND DISCUSSIONS

| Conductive PU/AgNWs coating

The bulk and surface conductivity of the PU/AgNWs coating were measured as function of AgNWs volume fraction and given in Table 2. At low volume fraction, the conductivity is close to the neat PU value, that is, 10 -14 S/m for bulk and 10 -13 S.Sqr for surface. The percolation threshold has been determined near 0.6%vol for bulk and 0.8%vol for surface. Above the percolation threshold, electrical conductivity values, 100 S/m for bulk and 10 -2 S.Sqr for surface, are enough for electrodeposition process.

3.2 | Electrolytic deposit

| Electrodeposition lower limit

At low volume fraction of AgNWs in PU matrix, the electrodeposition process is possible. A continuous metallic deposit is obtained since a regular surface conductive network is formed at the microscopic scale.

The gap between nanowires is an important parameter directly linked to the quantity of nanowires dispersed in the matrix. The electrolytic deposits have been realized on coatings as function of AgNWs volume fraction with a current density of 0.1 A/dm 2 during 2 h. The scanning electron microscopy (SEM) images are shown in Figure 1. No silver deposit was observed for a volume fraction of 0.7%vol close to the percolation threshold. Some metallic silver germs began to grow at 1%vol of AgNWs. The germs density increased with the volume fraction of nanowires and coalesced to form little clusters from 2.5%vol. The surface was totally covered by the silver deposit above 3%vol. This value is the lower limit to obtain a continuous silver layer on the conductive coating surface. The same result was confirmed for higher content of AgNWs. The optimal rate of nanowires was chosen at 4%vol to insure a good reproducibility of electrolytic deposit.

For the same rate of AgNWs, the current density influences the metallic deposit speed but also its microstructure. The germination phenomenon was improved by high current density inducing a silver layer growth in all directions. This uncontrolled growth provides a spongy silver structure. For low density, the germination becomes less important promoting the crystal growth as large crystal structures (Figure 2).

| Apparent current density

The metallic deposit adhesion has been checked as function of the current density. The CFRP substrates have been coated with a layer of PU/AgNWs filled with 4% vol. The electrodeposition times have been adjusted to obtain metallic deposits with a quasiconstant thickness (between 10 and 20 μm). The adhesion quality, according to ISO 2409 standard, is represented versus the current density on Figure 2. For current densities lower that 0.2 A/dm 2 , the adhesion class is 0 corresponding to a maximum adhesion. For higher current densities, the deposit adhesion is altered. This phenomenon is accentuated by the current density increase. In addition, we have observed a polymer surface degradation certainly due to a local increase of the temperature at the vicinity of 1 × 10 -13 ± 5 × 10 -14 0.2 8 × 10 -14 ± 1 × 10 -14 0.5 1 × 10 -13 ± 3 × 10 -14

5 × 10 -13 ± 2 × 10 -13 0.7 1 × 10 -2 ± 3 × 10 -3 1 2 × 10 -1 ± 1 × 10 -2 3 × 10 -5 ± 1 × 10 -6 1.5 5.2 ± 4 × 10 -1 2 1 0 ± 2 2 × 10 -3 ± 3 × 10 -4 3 33 ± 11 1 × 10 -2 ± 4 × 10 -3 4 68 ± 23 3 × 10 -2 ± 7 × 10 -3 5 125 ± 44 7 × 10 -2 ± 6 × 10 -3 6 120 ± 32 9 × 10 -2 ± 3 × 10 -3
Abbreviations: AgNWs, silver nanowires; PU, polyurethane.

metallic germs for high current density. This thermal degradation implies a decrease of the adhesion. We observe a lack of adhesion of metallic deposit for high current density. In order to understand, the current density really seen by the AgNWs was studied. The current density J is dependent from the applied current A and the conductive surface S. The current lines are focused on the conductive fillers; that is, the conductive coating is composed of an insulated polymer matrix and conductive nanowires. The electrolytic deposit is only initiated on the site occupied by AgNWs at the surface of the coating (S Ag ).

The involved surface S Ag has been determined by coupling the SEM images analysis of the PU/AgNWs coating surface with the nanowires volume fraction. This surface S Ag corresponds to AgNWs area in direct contact with Figure 4 shows the apparent current density versus the AgNWs volume fraction for different applied current densities. The apparent current density corresponds to the current density normalized by S Ag , determined by SEM observations of coating surface.

The applied current densities in electroplating on metal, as aluminum, are 0.5-1 A/dm 2 . For a 4%vol coating filled, we determined an apparent current density near 10 A/dm 2 . This high value induces an increase of local temperature at the AgNWs/PU matrix interface leading to polymer degradation. After the adhesion test, nanowires present on the surface are removed from the PU matrix and stay with the electrolytic deposit. The polymer degradation induced by high current density probably damage also the interactions between nanowires and polymer matrix.

If we applied a maximum value of current densities close to 0.2 A/dm 2 for a coating filled with 4%vol of AgNWs, the apparent current densities have similar values (or lower) than current densities values used classically for bulk metal surface. The interface between nanowires and polymer was preserved and a good value for adhesion test is obtained. In this work, we have chosen to process electrolytic deposits with a current density of 0.1 A/dm 2 . This last value allows us to obtain good reproducibility in term of the electrolytic deposit adhesion (Figure 5).

| Metallic deposit thickness

Parameters leading the deposit thickness are the current density value and the time. The deposit thickness has been studied for AgNWs content above the electrodeposition limit (from 3 to 6%vol) with a current density of 0.1 A/dm 2 during 2 h (Figure 5(a)). The thickness looks to be independent from the volume fraction. The homogeneous conductive network coated on the surface allows us to cover it with a continuous metal layer.

The Figure 5(b) describes the growth kinetics of the electrolytic deposit on the PU/AgNWs coating filled with 4%vol with a current density of 0.1 A/dm 2 . The growth is linear with a deposit speed around 7 μm/h. Literature reports an optimized electromagnetic shielding value for an electroplating thickness between 5 and 10 μm. [START_REF] Ghanem | Elaboration et caractérisation de films de cuivre par la méthode de dépôt chimique dynamique[END_REF] In this case, it is possible to obtain a shielding higher than 90 dB for the high frequencies domain close to 1 GHz. The Figure 5(b) allows us to extrapolate the corresponding electrolytic treatment time near 45 min to obtain the expected thickness.

| Adaptation of thermal expansion

Due to the space environment, the coatings adhesion is required on a wide temperature range (between -180 and 165 C). The thermal expansion coefficient (CTE) of CFRP is very different from the metallic layer. The CTE difference would create mechanical stresses at the interface. The metallic layer adhesion could be altered during temperature cycling. So, weakly filled polymer coating between metallic layer and CFRP substrate could absorb the expansion difference.

In order to check these assumptions, different cases of stress versus CTE for a three layers assembly are shown in Figure 6. The first layer can be assimilated to the metallic deposit, the second layer to PU/AgNWs coating and the third layer to CFRP substrate. In the case 1 and 4, when the expansion coefficient of the conductive coating is higher or lower than two others layers (CFRP and metal), tensile and compressive stresses are opposite to interfaces, creating shear stresses. The PU/AgNWs coating presents a good F I G U R E 6 Schematic representation of stresses induced by the different coatings thermal expansion coefficients compatibility with the CFRP substrate, which allows to maintain adhesion with the presence of these stresses at interface. In the case 2 and 3, the expansion coefficient is intermediate which generates supplementary shear stresses in the PU/AgNWs coating. The conductive coating should have an expansion coefficient higher or lower than two others layers to limit shear stresses.

Strain versus temperature is shown for the CFRP substrate, the PU matrix, the PU/AgNWs coating filled with 2%-6%vol and bulk silver on the Figure 7(a). The thermal expansion coefficients are given in Table 3.

For silver bulk and CFRP substrate the strain behavior as function of temperature is quasi linear. For PU matrix and PU/AgNWs composites, the thermal expansion coefficient is dependent on T g . Two coefficients α are observed according to vitreous and rubbery states. α value is not dependent on nanowires ratio in the vitreous state; it is due to the weak mobility below T g . In contrast, α values increases above T g and decrease with nanowires content. Nanowires play the role of additional entanglements for macromolecules on the rubbery plateau. Despite these modification, PU/AgNWs composites point out an α value higher than silver bulk and CFRP substrate. This configuration is consistent with the case 1 of Figure 6.

The adaptation of thermal expansion phenomenon could be described by the thermal expansion coefficient and mechanical compliance. The PU/AgNWs coating should have a high mechanical compliance to adapt the shear stresses at the interfaces. The storage compliance J' versus temperature is shown in Figure 7(b), calculated from storage G' and dissipative G" shear modulus as following:

J 0 = G 0 G 02 + G 00 2 ð2Þ
We note a storage compliance of the PU matrix higher than the silver and the substrate mechanical compliance over all the temperature range. At low temperature, the value is around 10 -9 Pa -1 and reaches 10 -6 Pa -1 for high temperature. The mechanical compliance decreases by insertion of AgNWs. For low volume fraction of AgNWs, the PU/AgNWs coating absorbs stresses by adapting the induced strain with the thermal expansion of different materials (case 1, Figure 6).

The thermal shocks cycling process was chosen to simulate the worst case for expansion adaptation. The estimated time between two extreme values of temperature is about 1 min. The maximal temperatures chosen are similar with the thermal cycling temperature (-180 to 165 C) in space environment. The system suffered two successive thermal shocks (cooling and heating) with temperature variation of 360 C. Figure 8 reports the schematic representation of the thermal cycling.

After two thermal shocks, adhesion test was performed to confirm the expansion adaptation of the CFRP metallized sample with the PU filled with 4%vol of AgNWs. An excellent adhesion value was obtained before the thermal treatment for a metallic deposit with a current density of 0.1 A/dm 2 . After these two accelerated aging, a class 1 adhesion result was measured corresponding to the industry requirements. The PU matrix adapts the difference of thermal expansion coefficient of the CFRP substrate and the silver deposit absorbs the induced mechanical stresses. The higher compliance with a maintain of ductility is obtained with a volume fraction of AgNWs lower than 5%vol 31 and provides the expected adhesion level.

F I G U R E 7 Strain and storage compliance of polyurethane (PU)/silver nanowires (AgNWs) coatings

| Electromagnetic shielding effectiveness

The shielding effectiveness tests are presented on the frequency range from 1 to 26 GHz. The shielding effectiveness and the 90 dB criterion expected by spatial industry are reported on Figure 9.

The CFRP substrate has the lowest attenuation and its shielding effectiveness decreases linearly with frequency range. This frequency behavior is well known in the literature for metallic grids. [START_REF] Evans | Design guidelines for shielding effectiveness, cur rent carrying capability, and the enhancement of conductivity of composite materials[END_REF] The studied CFRP is constituted of twill weave carbon fiber; its shielding effectiveness was compared to a grid model illuminated by a plane wave:

SE = 20 log λ 2g ð3Þ
With g the grid mesh and λ the wavelength.

For the grid model with for g = 20 μm, results are reported in dashed line on Figure 9. The CFRP frequency behavior could be compared to a 20 μm grid. The equivalent spacing is the same order of magnitude than carbon fibers diameter near 7 μm.

The CFRP substrate coated with PU/AgNWs filled with 4%vol presents a shielding effectiveness higher than 90 dB up to 20 GHz. For specific frequency, as 2 GHz for example, we observe shielding effectiveness loss peak (for

T A B L E 2

 2 Bulk and surface conductivities of PU/AgNWsVolume fraction

F

  I G U R E 1 Electrolytic deposits realized on coatings as function of silver nanowires volume fraction observed by scanning electron microscopy the electroplating bath and involve in the electrodeposition process. Data are reported in Figure3. The behavior law is linear for coatings filled up to 6%vol. The electroactive AgNWs occupied surface is less than 20% for low volume fractions.

F

  I G U R E 2 Adhesion of electrolytic coating according to current density values F I G U R E 3 Surface occupied by silver nanowires (AgNWs) versus the nanowires volume fraction F I G U R E 4 Apparent current density of the polyurethane/ silver nanowires (AgNWs) for different current density

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  I G U R E 5 Electrolytic deposition thickness versus volume fraction of silver nanowires (AgNWs) (a) and time (b)

T A B L E 3

 3 Abbreviations: AgNWs, silver nanowires; CFRP, carbon fiber reinforced polymer; PU, polyurethane.