A coupling between a 3-D mesoscale hydrodynamic model and a biogeochemical model was performed in the north-western Medi ter ranean to reproduce the major processes of carbon and nitrogen cycles involved in the new production of phytoplankton biomass, The study was focused on simulating the real conditions of the EURHOGLI campaign, performed in summer in the Gulf of Lions (June-July 1983). Results of the coupled model showed the high variability of the Rhone river plume position and the associated phytoplankton biomass, The validation ofthe model was in good accordance with field measurements and CZCS and AVHRR remote sensing pictures available at the period considered. In addition, results showed that the phytoplankton biomass induced by freshwater nutr ients was strongly controlled by the variability of the local wind conditions, The model, performed with representative forcing conditions, allowed to better understand the representative phytoplankton f eatur es that prevails in summer in the Gulf of Lions, where both strong winds and freshwater runoffs interact.