Unraveling the implementation of episodic hypoxia events in a coastal ecosystem under anthropogenic nutrient loads.

In the context of increasingly frequent extreme summer events, it has become essential to develop new hypoxia alert tools in order to improve the management of shellfish farming areas and fish nursery habitats. Indicators need to be developed not only in areas where persistent hypoxia are observed for long periods but also in areas subjected to intermittent (a few hours) or episodic (a few days to a few weeks) hypoxia. Repeated short-term hypoxic events can be more detrimental to benthic communities than a single extended period of anoxia. Our study explores the emergence of episodic hypoxic events in a shallow coastal ecosystem directly influenced by nutrient-rich river plumes. This ecosystem is located in the Bay of Vilaine (French Atlantic coast), one of France's main shellfish-growing regions, which has experienced recurrent hypoxic events since the 1980s. Using high-frequency measurements of temperature, salinity, dissolved oxygen and chlorophyll-a concentrations, we examine the interactions between surface phytoplankton blooms, stratification and bottom oxygen depletion events. We also highlight the environmental conditions that promote the formation of stratification and thus hypoxia. Finally, we use a three-dimensional model simulating extreme event scenarios to assess the increased risk of hypoxia in different parts of the bay.