Biological and chemical processes occurring in seawater from the Gulf of Gdansk, 'enriched' with Vistula river water, were studied in a series of microcosm experiments covering different seasons of the year. Concentrations of inorganic forms of nitrogen were, with one exception, 25 - 800 times higher in the Vistula water than those in the sea; phosphates were from 3.5 to 70 times higher. Two phases were observed during the incubations: during the first there were marked increases in primary production, chlorophyll concentration and bacterial production, and a de-crease in nutrient concentrations. The second phase was characterised by a lack of phosphates, a gradual decrease in chlorophyll concentrations, and an increase in zooplankton abundance. During the growing season the first phase lasted only a few days, during which nitrates and silicates were taken up at a rate of 2-4 mmol?dm-3?d-1 and phosphates and ammonia at a rate of 0.05-0.3 mmol?dm-3?d-1. Further phytoplankton growth and further nutrient uptake were most frequently limited by phosphorus depletion. Phosphorus was probably also a factor limiting bacte-rioplankton growth, which appeared to be correlated with primary production. In spring and autumn phytoplankton growth was additionally limited due to grazing by protozooplankton and rotifers, in summer by nitrogen exhaustion.