A biogeochemical model, BIOMELL (BIOgeochemical Model of the Euphotic Layer of Lodyc), has been developed to simulate the temporal evolution of the main nitrogen stocks and fluxes at the DyFAMed station (Dynamique des Flux Atmospheriques en Mediterranée), located in the Northwestern Mediterranean Sea. This mainly oligotrophic region is characterized by a strong seasonal cycle, and a significant export of dissolved organic matter compared to the particulate export measured by sediment traps. Validation of the model is made using temperature, nitrate and chlorophyll profiles acquired at DyFAMed approximately every month in 1991. Extended datasets from specific years are also used to validate seasonal variations of other variables, for which the coverage in 1991 was rather poor (new and total production, particulate export, dissolved organic matter export, bacteria, zooplankton). Sensitivity studies on selected parameters are carried out in order to give an idea of the margin of error in the model predictions. The model is then used to analyze the behavior of the biogeochemical system during various production regimes (winter, spring bloom, oligotrophy, autumn bloom). It is shown that several processes, which are often neglected in biogeochemical models, must be taken into account: phytoplankton growth limitation by deep mixing, C:Chl ratio dependence on light, nitrification, and semi-refractory dissolved organic matter accumulation in the surface layer.