S�awomira Gromisz and Zbigniew Witek
Sea Fisheries Institute, Kołłątaja 1, 81-332 Gdynia, Poland

Key words: phytoplankton, Baltic Sea, estuary, assemblages, diversity

Abstract.
The comparison of the phytoplankton composition of 156 samples collected during 11 cruises in the Gulf of Gdańsk and the Pomeranian Bay based on clustering analyses led to the identification of ten algae assemblages at a similarity lower than 50%. The most distinct phytoplankton assemblages were found in samples collected at stations located directly near the Vistula River mouth and a local, single-species assemblage formed by dinoflagellate Peridiniella catenata in the Gulf of Gdańsk. Other assemblages were relevant to subsequent seasons of the vegetative cycle: winter-spring, summer and autumn.
The relationship of the phytoplankton assemblage with environmental conditions (low salinity, high nutrient concentrations) was the most evident near the Vistula River mouth. In other areas, phytoplankton composition changed seasonally and was mainly correlated with temperature, and/or day length. The differences in phytoplankton composition between the Gulf of Gdańsk and the Pomeranian Bay were relatively small at a similarity higher than 50%.