Hydrologie et phytoplancton des lagunes méditerranéennes sous pression du changement climatique (2001-2022) (Projet HYPHEAT’Lag)

The Mediterranean basin has been identified as a climate change (CC) “hotspot” by IPCC experts. Mediterranean lagoons are highly exposed to the impacts of climate change, notably due to their confinement and position at the interface between land and sea. However, at the scale of the French Mediterranean coastline, few studies are available to describe the impacts of CC on the hydrology and phytoplankton of these ecosystems. Against this backdrop, the present study provides : - a review of scientific knowledge on regional CC and the trends observed in the hydrology and phytoplankton of lagoon ecosystems; - an identification of the discriminating factors of lagoons with regard to the pressure of climate change and trophic pressure. Data analyses cover the period 2001-2022 and the regions of Occitanie, Provence Alpes-Côte d'Azur and Corsica, via 3 databases: climate (5 stations), flows (9 stations) and lagoon summer monitoring (22 stations). The results highlight in particular: - atmospheric warming, drying out and lower winds ; - warming, salinization and oxygen depletion in lagoon waters; - oligotrophication trajectories for a majority of lagoons; - the influence of hydromorphological characteristics on the environmental trajectories of lagoons, and particularly on their levels of chlorophyll-a (CHLA) and phycoerythrin picocyanobacteria (CYANO) ; - the marked influence of temperature on CHLA and CYANO variables. The influence of orthophosphates on CHLA. No influence of nutrients on CYANO, but salinity has a greater impact on deep and marinized lagoons. A summary proposes: i) hypotheses on the future evolution of French Mediterranean lagoons in the face of climate projections, and ii) recommendations for further monitoring and studies to better assess the effects of CC in these ecosystems.