On the following subject:
"Study of the impact of climate change and terrigenous inputs of nutrients on the biogeochemical functioning of the Mediterranean Sea using coupled physical/biogeochemical modelling".
Supervisor: Melika Baklouti, Professor AMU
Co-director: Thierry Moutin, Professor AMU
Summary:
The Mediterranean Sea (MED) is now clearly identified as an area that is particularly sensitive to climate change. In addition, it is a semi-enclosed sea, strongly connected to the continental part, in particular via nutrient inputs from rivers. Predicting and understanding its evolution between now and the end of the century requires the implementation of integrated studies, taking into account the evolution of dynamic and energetic forcings at the various interfaces in order to understand the complexity of this system.
It is against this backdrop that the LaSeR-Med project (Towards an integrated prediction of Land and Sea Responses to global change in the Mediterranean Basin) was set up. The aim of this project is to study the effects of climate change alone, or combined with economic scenarios, on the key ecosystem services provided by the Mediterranean basin.
This thesis focused on the marine component of the project. The fundamental question that guided all of this work can be summarised as follows:
How is the biogeochemistry of the MED likely to react to changes in river inputs and climate change?
The first stage of this work was to evaluate the coupled NEMO-MED12/Eco3M-MED model in the light of available observations. At the end of this preliminary work, we were interested in the effects, on the biogeochemistry of the MED, of the variations observed since the 1980s in terrigenous inputs (rivers and run-off) of nutrients. This thesis has shown that these variations have had a significant influence on the biogeochemistry of the MED, particularly in the eastern basin, causing a deepening of the phosphacline and a reduction in the availability of PO4 in surface waters. These initial results have highlighted the need to include relevant scenarios in climate change scenarios to represent possible changes in riverine nutrient inputs.
The aim of the second part was to study the effects on the biogeochemistry of the MED and on the functioning of its planktonic food web of variations in hydrodynamic forcing alone induced by climate change. To do this, the IPCC RCP 8.5 scenario was used to force the coupled atmosphere/ocean/biogeochemistry model ALADIN/NEMO-MED8/Eco3M-MED. This scenario predicts an increase in MED temperature and salinity, as well as a decrease in the maximum mean depth of the mixed layer. The simulations (scenario + control) carried out as part of this thesis showed a significant effect of these physical changes on the biogeochemistry of the MED, such as a fairly significant decrease in integrated primary production (-10 %) and carbon export at 1000 m (-15 to -20 %). The scenario also predicts a change in the planktonic food web to the detriment of large phytoplankton (-15 %) due to the reduction in nutrients available in the surface layers (up to -45 % for PO4). Finally, for unchanged river inputs, the regions where organisms will be limited or co-limited by nitrogen should be more numerous in the Mediterranean by 2100. In view of this ever-increasing oligotrophication of the MED, it is likely that nutrient inputs from rivers will play an increasingly important role in the biogeochemistry of the MED and in the functioning of its planktonic food web.