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Coastal ecosystems are subjected to an increasing number of anthropogenic drivers, including marine renewable energies and climate change (CC). These drivers can interact in complex ways, which may lead to cumulative effects (CEs) whose potential consequences on the ecosystems need to be addressed. We used a holistic approach—ecological network analysis (ENA)—coupled with a two-dimensional food web model—Ecospace—to conduct an ecosystem study of the CEs of CC plus the operation of an offshore wind farm on ecosystem functioning in the extended Bay of Seine (English Channel). Mapped ENA indices showed that CEs were not restricted to the wind farm area, i.e. where anthropogenic drivers are concomitant. CEs varied both in space and among ecosystem properties, displaying that ENA indices can distinguish between different cumulative pathways that modify ecosystem functioning in multiple ways. Moreover, the effects seemed to be tied to the structuring role of CC, and differed under the 2050 and 2100 conditions. Such changes resulted in stronger loss of ecosystem resilience under the 2100 conditions despite the benefits of the reef and reserve effects of the wind farm.

International audience; Offshore wind farm development has become a key measure of energy transition in recent years. Coastal territories are particularly favorable to the development of offshore wind farms due to their high energy potential. However, these areas are also hotspots of biodiversity, provide attractive landscapes and are under strong anthropogenic pressures. Preserving and sharing the natural and cultural resources of coastal territories while intensifying renewable marine energies, represents one of the most important challenges for future management of coastal environments. Consequently, systemic models that consider all the effects of offshore wind farms on ecosystems and society are essential. Here, we propose a conceptual model for studying these effects, by mobilizing the concept of ecosystem service in a systemic and integrated assessment approach. To that aim, we reviewed the literature and compiled experts' knowledge in order to characterize the effects of offshore wind farms on food webs during the construction and operation phases. Then, we analyzed the contribution of trophic compartments on ecosystem functions, ecosystem services and beneficiaries, and how offshore wind farms modify the relationships between these compartments of the marine coastal social-ecological systems. Our approach helps identify the causal chains that generate the most important modifications in this system. This information could then be used to predict impacts of offshore wind farms on ecosystem services, and to suggest management trade-offs. This study reveals the need for further studies relating marine biodiversity to ecosystem services, and developing systemic approaches at different scales. The analysis of the effects of offshore wind farms on ecosystem services is crucial since it is linked to strong ecological, socioeconomic and political issues.

International audience; We assessed the spatial variability in the size structure of phytoplankton, community composition, primary production and carbon fluxes through the planktonic food web of the Gulf of Gabès (GG; Southeastern Mediterranean Sea) in the fall of 2017 during the MERMEX-MERITE cruise. High concentrations in nutrients, chlorophyll a (~ 2–6 µg L−1) and primary production (1816–3674 mg C m−2 d−1) revealed an eutrophic status of the studied stations in the GG. In accordance with hydrodynamic features, inorganic nutrients showed increases in concentrations from North to South and from coast to offshore, these nutrient gradients impacting the spatial distribution of phytoplankton community. Size-fractioned phytoplankton biomass and production were the lowest in the northernmost zone where they were mainly sustained by pico-sized fraction. Concomitantly, in this area, small aloricate ciliates were dominant leading to a high microbivory. Conversely, higher biomass and production were measured towards the South and offshore with prevalence of larger phytoplankton (nano- and/or micro-sized fractions) supported by diatoms. The herbivorous protozooplankton and metazooplankton were more abundant in these zones, resulting in an increase of the herbivory. The vertical particulate organic carbon flux followed also a north–south and coast-offshore increasing gradient, with a higher contribution of phytoplankton, and zooplankton fecal pellets to the sinking organic matter in the southernmost area. Our results suggest that even in nutrient-rich and highly productive waters, a continuum of trophic pathways, ranging from microbial to multivorous and herbivorous food webs, may exist, which implies different efficiencies in carbon export and carrying capacity within the ecosystem.

Integrative and spatialized tools for studying the effects of a wide variety of ecosystem drivers are needed to implement ecosystem-based management and marine spatial planning. We developed a tool for analyzing the direct and indirect effects of anthropic activities on the structure and functioning of coastal and marine ecosystems. Using innovative modelling techniques, we ran a spatially explicit model to carry out an ecological network analysis (ENA) of the effects of climate change (CC), of an offshore wind farm (OWF) and of multiple fishing scenarios on the Bay of Seine (eastern part of the English Channel) ecosystem. ENA indices described the effects of those different drivers in a holistic and spatial way. The spatial analysis of ecosystem properties revealed local and global patterns of modifications attributed to CC, while the OWF resulted in localized changes in the ecosystem. This ability of ENA indicators to detect human-induced changes in ecosystem functioning at various spatial scales allows for a more integrative view of the effects of human activities on ecosystems. ENA indices could be used to link both local and global ecosystem changes, for a more cross-scale approach to ecosystem management.

As the human population grows, the number and the intensity of anthropogenic activities increases oncoastal systems. These activities can have significant effects on the environment and on ecosystems.Furthermore, the wide range of coastal activities implies that coastal ecosystems are almost always subject tomultiple ecological drivers induced by humans. These drivers can interact with each other, leading to cumulativeeffects, which can have significant, and difficult to predict, consequences on ecosystems. In the context of thedevelopment of marine renewable energy in France, this thesis implements innovative methods to quantify theseparate and combined effects of multiple ecological drivers, including: the future Courseulles-sur-Mer offshorewind farm, climate change, and the effect of Brexit on fishing regimes. We studied the combined effects ofthese drivers on the functioning and resilience of the Seine Bay ecosystem using holistic ecosystem approaches.Our results indicate that interactions between drivers are numerous and complex, yet the potentialstructuring effects of drivers on ecosystem functioning play a determining role in the formation of cumulativeeffects. Cumulative effects appear to be heterogeneous between the functional properties of ecosystems andspecific to the studied ecosystems. It is therefore important to carry out local functional studies of thecombination of multiple drivers, to determine the effects of humans on ecosystems, as well as to evaluate theeffects of combined drivers on the provisioning of ecosystem services.; Avec l’accroissement de la population humaine sur Terre, le nombre et l’intensité des activités humaines sur les littoraux ne cessent d’augmenter. Ces activités peuvent avoir des effets importants sur l’environnement et sur les écosystèmes. De plus, la grande diversité de ces activités sur les côtes fait que les écosystèmes côtiers sont presque toujours soumis à de multiples facteurs de changements d’origine humaine. Ces facteurs peuvent interagir entre eux, provoquant des effets dit cumulés, qui peuvent avoir des conséquences importantes, et difficilement prévisibles, sur les écosystèmes. C’est dans le cadre du développement des énergies marines renouvelables en France, que cette thèse met en place des méthodes innovantes pour quantifier les effets séparés et combinés de multiples facteurs de changements, comprenant : le futur parc éolien de Courseulles-sur-Mer, le changement climatique, et l’effet du Brexit sur les régimes de pêche. Les effets combinés de ces facteurs sur le fonctionnement et la résilience de l’écosystème de la baie de Seine ont été quantifiés avec l’aide d’approches écosystémiques holistiques.Nos résultats indiquent que les mécanismes d’interactions des facteurs sont complexes et multiples,néanmoins les potentiels effets structurants des facteurs sur le fonctionnement des écosystèmes jouent un rôle déterminant sur la genèse des effets cumulés. Les effets cumulés apparaissent hétérogènes entre les propriétés fonctionnelles des écosystèmes et spécifiques aux écosystèmes étudiés. Il est donc important de réaliser des études fonctionnelles du cumul de multiples facteurs à des échelles locales, pour déterminer les effets de l’homme sur les écosystèmes, et vice versa, en prenant en compte les multiples services rendus par les écosystèmes.