A zero dimensional ecosystem model is applied to the Black Sea to simulate the behaviour of its regional ecosystems. The modelling is guided by the analyses of the seasonal changes of mixed layer depth, nutrients and chlorophyll-a in the model regions, based on the available data in these areas of the Black Sea. Nutrients are continuously supplied to the adjacent coastal area by rivers, and in the other regions, mixed layer nutrient concentrations generally increase by mixing in winter. Despite the scarcity of data, it appears that the maximum chlorophyll-a concentration occurs in February-March in the central Black Sea, and in April-May in the peripheral regions, where the level is also an order of magnitude higher, as a result of riverine and coastal sources. The model successfully reproduces basic features of seasonal plankton and nutrient changes, and helps interpret the available observations and identify the factors creating the observed regional differences in productivity. The computed seasonal cycle of the chlorophyll-a compares well with the chlorophyll measurements in the central Black Sea. On the other hand, advection of nutrients is found to be important along the western and southern Black Sea coastal areas downstream of the river sources. Near the Bosphorus, reasonable agreement of model results and observations could only be ensured when the seasonal pattern of advection of river nutrients were taken into account. Better representation of the seasonal cycles including spring and autumn blooms are obtained with a nine compartment size fractionated model. Preliminary results are presented for a case including jelly organisms such as the ctenophore Mnemiopsis leidyi.