Description du sujet de thèse

Functioning of plankton ecosystem and carbon and oxygen cycles in the coastal region of the Gulf of Lion (northwestern Mediterranean): climate variability and influence of extreme events

This project studies the impact of climate change and anthropogenic pressures on the functioning of marine coastal ecosystems and biogeochemical cycles on the Gulf of Lion shelf in the northwestern Mediterranean. The Gulf of Lion is an oceanic margin characterized by high primary productivity in an oligotrophic sea (Bosc et al., 2004). This high productivity is linked to nutrient inputs from rivers, as well as the strong winter mixing of surface waters with underlying waters (Many et al., 2021) and wind-induced upwelling processes (Fraysse et al., 2014), which cause nutrients to rise into the light layer.
The Mediterranean Sea is one of the regions most vulnerable to climate change and is subject to strong anthropogenic pressures (Durrieu de Madron et al., 2011). Several studies based on in situ observations in the offshore region of the Gulf of Lion, or the coastal region of the Ligurian Sea adjacent to the Gulf of Lion, have documented trends of decreasing oxygen in deep waters, decreasing pH and increasing dissolved inorganic carbon, alkalinity and pCO2 (Touratier and Goyet, 2011; Kapsenberg et al., 2017; Coppola et al., 2020; Fourrier et al., 2021; Wimart-Rousseau et al., 2023). Salgado-Hernanz et al. (2019) also show, from satellite images, a decrease in surface chlorophyll in the coastal region over the period 1998-2014.

Given the limited spatio-temporal coverage of in situ observations, coupled physical-biogeochemical numerical models are very useful tools for complementing observations and better understanding the seasonal, interannual and climatic variability of biogeochemical cycles and the ecosystem, as well as estimating the contribution of different processes and forecasting their future evolution. Modeling studies over 20-year periods (Cossarini et al., 2021) have been carried out for the entire Mediterranean. They show trends similar to observations, with an increase in atmospheric CO2 uptake over the whole of the north-western Mediterranean over the periods 1999-2019, mainly explained by an increase in heat content and atmospheric CO2 content. However, the authors highlight the uncertainties of their results in the coastal region, linked to uncertainties over the fate of continental inputs and the dynamics of biogeochemical processes in the sediment.
The aim of the Riomar Priority Research Project is to gain a better understanding of the evolution of coastal ecosystems influenced by rivers, using an approach that combines high-resolution numerical modelling with in situ and satellite observations. In this context, the aim of this thesis is to (1) characterize the variability of the carbon and oxygen cycles, the carbonate system and the functioning of marine ecosystems, (2) better understand the factors controlling them, (3) characterize extreme events and their impacts on ecosystems and biogeochemical cycles. Several controlling factors will be studied, including atmospheric regimes, river inputs and ocean conditions.
The study will be based on high-resolution coupled physical/biogeochemical modeling. Simulations will be compared with observations for validation and improvement purposes.

Contexte de travail

The proposed thesis will be carried out within the framework of the PPR Riomar and within the LEGOS ECOLA team, which develops the hydrodynamic and biogeochemical models used, and has solid expertise in modeling the Gulf of Lion.
Weekly meetings with the scientific managers will be organized to discuss the progress of the thesis. The PhD student will also interact with researchers and engineers from the ECOLA team, other institutions involved in the PPR Riomar and the Simed and Med-CORDEX communities. She/he will travel in France and abroad to attend meetings and present her/his results at conferences.