Lise DELATTE

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 most significant threats to estuarine and coastal ecosystems worldwide. This issue represents a global challenge, resulting in extensive research, particularly in the context of widespread warming waters that exacerbate the consequences of eutrophication. This global challenge is driven by local anthropogenic pressures and influences, which depend on the characteristics of local ecosystems (coastline morphology, currents, depth, turbidity, watersheds, etc.), as well as the socio-economic environment of the coastal zone and watersheds. Indeed, at the interface of terrestrial and marine regions, coastal and estuarine ecosystems foster high biological productivity and the associated ecological services. However, due to their interface position, these ecosystems also concentrate pressures from human activities, threatening their functionality and their ability to support the services they provide.

Among the various anthropogenic disturbances affecting coastal ecosystems, agricultural activities and wastewater discharges lead to substantial nutrient inputs, such as nitrogen (N) and phosphorus (P). Excessive nutrient inputs are the root cause of eutrophication, resulting in a significant increase in the biomass of primary producers, such as planktonic microalgae. This is one of the most commonly observed effects, both in the English Channel and in other regions around the world.

This project aims to assess and quantify the impact of dystrophy, employing a multi-scale approach, within the context of significant water warming. It focuses on evaluating how dystrophy affects the structure and composition of phytoplankton communities (specific and functional diversity, size structure, etc.) and its consequences on pelagic primary production and associated carbon fluxes. The project involves a retrospective analysis of in situ observation series, coupled with experimental work conducted in microcosms using natural communities from three sites in the Bay of Seine. Additionally, studies will be carried out on isolated monoclonal cultures in the English Channel, belonging to the main strains and different size classes of the Bay's phytoplankton communities.