Eutrophication and dystrophy in the Bay of Seine : Impact on the structure of phytoplankton communities and their productivity

Coastal eutrophication caused by anthropogenic nutrient inputs is one of the greatest threats to the health of estuarine and coastal ecosystems worldwide. This issue reflects a global challenge that is generating a great deal of research in the context of widespread water warming, which intensifies the consequences of eutrophication. This global issue is linked to local anthropogenic forcings and pressures that depend both on the characteristics of local ecosystems (coastline morphology, currents, depth, turbidity, watershed, etc.) and on the socio-economic environment of the coastal zone and watersheds. Indeed, coastal and estuarine ecosystems, forming the interface between terrestrial and marine environments, promote high biological productivity and the associated ecological services. Due to their position, these ecosystems also concentrate the pressures associated with human activities, which endanger their functioning and their ability to sustain the services they provide.

Among the various anthropogenic disturbances affecting coastal ecosystems, agricultural activities and wastewater discharges result in massive inputs of nutrients such as nitrogen (N) and phosphorus (P). Excessive inputs of these nutrients cause eutrophication and result in a significant increase in the biomass of primary producers, such as planktonic microalgae, which is one of the most commonly observed effects in the English Channel and other regions of the globe.

This project aims to qualify and quantify the impact of dystrophy, using a multi-scale approach, in a context of significant water warming, on the structure and composition of phytoplankton communities (specific and functional diversity, size structure, etc.) and the consequences on pelagic primary production and associated carbon fluxes. A retrospective analysis of in situ observation series will be carried out in conjunction with an experimental approach that will combine microcosm work on natural communities from three sites in the Bay of Seine with studies on monoclonal cultures isolated in the English Channel belonging to the main lineages and different size classes of the Bay’s communities.