Phytoplankton in large river plumes in the Baltic Sea

Coastal regions of the south-eastern Baltic Sea subjected to large river in flow (Oder, Vistula, Klaipeda Strait, Daugava) were investigated from 1993 to 1997. The gradual increase in salinity due to mixing of river plumes wit h sea water was associated with a decrease in nutrient concentration, Chi a , and phytoplankton biomass. Dilution of the high biomass of riverine phyto plankton, however, does not result in a steady decrease of all components, but in the successive disappearance of different freshwater species. When s alinity increased to about 7, most limnetic (oligohalobic) species (chlorop hyceae, most of the cyanobacteria, some diatoms) disappeared almost complet ely. Therefore, this concentration was used to define the border of the plu me. Sometimes, different water bodies can be distinguished more readily by their species composition than by physico-chemical parameters. From an ecos ystem perspective, the coastal waters must be distinguished from the open s ea because of their special characteristics: e.g. higher N/P ratios indicat ing potential P-limitation and higher phytoplankton biomass (>2 g m(-3)). I n contrast to the open Baltic Proper, the river plumes were characterized b y spring blooms of diatoms instead of dinoflagellates. (C) 1999 Internation al Council for the Exploration of the Sea.