AbstractThe main physical and biological processes that control the seasonal cycle of the plankton dynamics over the Western Black Sea were explored by means of a three-dimensional, 7-compartment, on-line coupled biophysical model that was developed for this study. Adopting high frequency forcing in terms of air-sea interaction and Danube river inputs, we performed a simulation of the coupled model during the 2002–2003 period. A series of 8-day Chl-a SeaWiFS images provided a validation tool that guided us, along with available in situ measurements, to the improvement of model parameterizations and the calibration of the biological parameters. The simulation of the seasonal phytoplankton variability over the entire Western Black Sea, extending from the highly eutrophic river influenced area to the open sea area, was a major challenge that made necessary the representation of both the spatial and time variability of several processes. Despite the model simplicity, the simulated Chl-a patterns presented a good agreement as compared to the SeaWiFS and in situ data. During winter, phytoplankton in coastal areas was shown to be limited by light availability, primarily due to the increased particulate matter concentrations, as a result of resuspension from the sediment and the increased river loads. During summer, the primary production was mostly sustained by riverine nutrients and regeneration processes and thus was strongly linked to the evolution of the Danube plume. The limiting nutrients showed deviations from the observed concentrations, indicating the necessity for a more realistic phytoplankton growth model.