The pelagic planktonic ecosystem dynamics of the north-western Mediterranean Sea has been studied by a realistic modelling approach. The dynamics of coastal ecosystems sustained by the Rhone River nutrient loads, and offshore ecosystems are considered separately. Through an high resolution modelling of the Rhone River plume, a specific calibration dedicated to river plume ecosystems has been proposed and validated on in situ data. Our results suggest a positive influence of river loads on the zooplankton biomass, which in turn limits the organic carbon deposition on the Gulf of Lion shelf through grazing on organic detritus. Low salinity water lenses detached from the Rhone River plume, owing to their positive influence on zooplankton development, could favour this process. Offshore, an extended simulation of 30 years shows that the interannual variability of planktonic ecosystems is controlled by the vertical mixing, in relation with atmospheric forcing. The winter deep convection determines the structure of the phytoplanktonic ecosystem by favouring microphytoplancton, and seems unfavourable to zooplankton and bacterial communities. The vertical mixing also controls the interannual variability of particulate and dissolved organic carbon export. The net metabolism in the MEDOC area tends to be weakly heterotroph during a weak convection period, reducing the sink pattern of the MEDOC area. However, there is an increased accumulation of carbon within planktonic ecosystems and organic detritus.
