Abstract
Coastal environments, including coastal lagoons, at the interface between land and sea, often display a rich biological productivity. This unique position creates significant variability of the physical, chemical (e.g., temperature, nutrient concentration) and biological variables, thus influencing how these ecosystems function. In the context of global climate change, widely accepted by the scientific community, this variability could be amplified particularly by extreme intense events such as storm and flood events. It is therefore essential to develop multidisciplinary scientific studies on these coastal marine environments in order to better understand the natural variabilities of this system. Additionally, it is needed to quantify the trend and magnitude of the variability associated with medium and long-term environmental changes.In this thesis, the impact of storm and flood episodes on Mediterranean coastal marine ecosystems was studied on a particular coastal zone: the Thau lagoon (Sète, France) and its nearshore. In addition, the impact of a potential modification of these intense hydro-meteorological events, in relation with climate change, on these same ecosystems was also studied. To answer the main objective, a 3D coupled hydrodynamic-biogeochemistry modeling approach (SYMPHONY - ECO3M-S) was used, describing the evolution of physical and chemical variables (currents, pressure, water level, temperature, salinity, etc.), and biogeochemical variables (biomass of marine micro-organisms, particulate and dissolved matter, sediment).The ECO3M-S model was first adapted to the Thau lagoon, a privileged location for oyster farming activities, by integrating the forcing by oyster on the lower trophic compartments (zooplankton, phytoplankton and bacteria), showing a negative impact on phytoplankton (filtration) and promoting bacteria. Then, a first 3D modeling experiment was performed on the “cevenol” episode of November 2008, validating the approach followed and giving a first analysis of the short-term impact of this type of event on the studied ecosystem. Despite significant river inputs into Thau Lagoon, phytoplankton biomass did not increase during this event, due to possible light and water temperature limitations. The results highlights contradictory effects of these events on the ecosystem functioning. A longer period (2015 - 2016) was then considered, including several intense events, to assess a medium- and long-term impact. The modelling results was validated using in situ observations and statistical tests were performed. Finally, the impact of different scenarios of a modification of extreme events on the biogeochemical balance of the Thau Lagoon and its nearshore was quantified. In the short term, the biogeochemical balances and biomass of micro-organisms were significantly affected by intense events. In the long term, the system showed a good resilience to the modification of environmental forcings, even if differences persist one year after extreme events. This was linked to different inertia of the biogeochemical processes within the water column and the sedimentary compartment.This thesis provides a better understanding of the short, medium and long-term impact of storm and flood events on Mediterranean coastal marine ecosystems. In addition, this thesis shows the possible impact of a potential future modification of intense events on the biogeochemical evolution of the North-Western Mediterranean.