Étude de l’état de santé des écosystèmes côtiers sud méditerranéens anthropisés à partir du fonctionnement du réseau trophique planctonique : modélisation des indicateurs écologiques en situation de contamination chronique ou pulsée

The Gulf of Gabès, on which 60% of the fishing activity in Tunisia is based, has experienced a serious degradation of its physico-chemical environment and its biocenosis. Within the framework of her Master's degree in 2019, doctoral student Oumayma Chkili provided a first evidence of the combined influence of contamination and hydrodynamics on primary production and its trophic transfer. However, the impact of these forcings on the functional properties of the ecosystem remains to be understood, which will be possible through the modelling of food webs. This will be the first objective of the thesis, based on the complementarity of the two supervisors, Asma Sakka-Hlaili, a specialist in planktonic network analysis, and Nathalie Niquil, a specialist in their modelling. It will thus be possible to characterize the state of health of the Gulf of Gabès by applying the "pelagic habitat" indices used by the OSPAR (North East Atlantic Regional Seas Convention) as well as the indices of the network analysis (of which Nathalie Niquil is a specialist) and to select the most suitable ones in the assessment of the state of health of the Gulf.

The models will then be applied to the data collected during the 2018 oil spill in the Bizerte Lagoon canal. This pole of shellfish farming in Tunisia also supports a strong fishing activity, while experiencing increased anthropization leading to its eutrophication and chemical contamination. Following the crude oil spill, the response of the ecosystem to this contaminating shock is not yet known. We will thus be able to test the health indicators of the planktonic food web in the context of pulsed contamination.

The dataset collected in these two situations of chronic and pulsed pollution by the Phytoplanktonology laboratory headed by Asma Sakka-Hlaili is very valuable for Nathalie Niquil's research objective, which is to characterize the state of ecosystem health using various indicators based on the organization of interaction networks in coastal ecosystems.