Abstract: International audience; The end-to-end OSMOSE-GoL model parameterized, calibrated and evaluated for the Gulf of Lions ecosystem (Northwestern Mediterranean Sea) has been used to investigate the effects of introducing two-ways coupling between the dynamics of Low and High Trophic Level groups. The use of a fully dynamic two-ways coupling between the models of Low and High Trophic Levels organisms provided some insights in the functioning of the food web in the Gulf of Lions. On the whole microphytoplank-ton and mesozooplankton were found to be preyed upon by High Trophic Levels planktivorous groups at rates lower than 10% and 25% of their respective natural mortality rates, but these relatively low rates involved some important alterations in the infra-seasonal and annual cycles of both High and Low Trophic Levels groups. They induced significant changes in biomass, fisheries landings and food web interactions by cascading effects. Spatial differential impacts of High Trophic Levels predation on plankton are less clear except in areas in which primary productivity is high. Higher predation rates on plankton groups were encountered within the area of the Rhone river's influence and in areas associated to the presence of mesoscale eddies in the Northwestern part of the Gulf of Lions, especially. Generally, the pressure of the High Trophic Levels predation was the highest in areas of highest biomass whatever the plankton group considered. The two-ways coupling between Low and High Trophic Levels models revealed both bottom-up and top-down controls in the ecosystem with effects on planktivorous species similar to those observed in the field. The use of the end-to-end model enabled to propose a set of potential mechanisms that may explain the observed decrease in small pelagic catches by the French Mediterranean artisanal fisheries over the last decade.
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