A 13-compartment model of primary production and degradation of dissolved organic matter has been coupled with a general circulation model in an open coastal environment (Gulf of Lions, Mediterranean) so as to quantify exchanges with the open sea. The biogeochemical model had been previously calibrated with a one-dimensional vertical version on a 1-year data set, and this simulation provides boundary conditions for the three-dimensional model. After a 1-year spin-up simulation, quasi-equilibrium is obtained for all the compartments, and the results of an additional annual simulation are compared with coastal zone color scanner images and other data collected in the area: the simulation of chlorophyll a, nutrient concentrations, and primary production is quite satisfactory, although the spring bloom starts slightly earlier than usual. Total gross community production is estimated at 76 g C m-2 y-1, and New Gross Community Production at 45 g Carbon m-2 y-1 (f=0.37). The model was used to understand the nitrate annual cycle in the coastal zone of the Gulf of Lions. Nitrate input from the Rho⁁ne river, from the sediment, and from marine advection are compared in terms of potential fertilization. Model results indicate that, regarding the open sea, the margin acts most of the time as a sink for nitrate. However, during winter, when phytoplankton growth is reduced and cascading of dense waters is active along the shelf, the margin is shown to export nitrate toward the open sea. These results may qualitatively apply to other open Mediterranean margins, although the intensity of the fluxes would be modulated by local features. ¿ 1998 American Geophysical Union