A six-compartment ecosystem model defined by a simple nitrogen cycle is coupled with a general circulation model in the Black Sea so as to examine the seasonal variability of the ecohydrodynamics. Model results show that the annual cycle of the biological productivity of the whole basin is characterized by the presence of a winter--early spring bloom. In all the regions this bloom precedes the onset of the seasonal thermocline and occurs as soon as the vertical winter mixing decreases. Phytoplankton development starts in winter in the central basin, while in coastal areas (except in the river discharge area) it begins in early spring. In the Danube's discharge area and along the western coast, where surface waters are almost continuously enriched in nutrient by river inputs, the phytoplankton development is sustained during the whole year at the surface. The seasonal variability of the northwestern shelf circulation induced by the seasonal variations in the Danube discharge and the wind stress intensity has been found to have a major impact on the primary production repartition of the area. In the central basin the primary production in the surface layer relies essentially on nutrients being entrained in the upper layer from below. Simulated phytoplankton concentrations are compared with satellite and field data. It has been found that the model is able to reproduce the main characteristics of the space-time evolution of the Black Sea's biological productivity but underestimates the phytoplankton biomass especially in regions extremely rich in nutrients such as the Danube discharge area.