Phytoplankton in large river plumes in the Baltic Sea

Abstract

Abstract Coastal regions of the south-eastern Baltic Sea subjected to large river inflow (Oder, Vistula, Klaipeda Strait, Daugava) were investigated from 1993 to 1997. The gradual increase in salinity due to mixing of river plumes with sea water was associated with a decrease in nutrient concentration, Chl a, and phytoplankton biomass. Dilution of the high biomass of riverine phytoplankton, however, does not result in a steady decrease of all components, but in the successive disappearance of different freshwater species. When salinity increased to about 7, most limnetic (oligohalobic) species (chlorophyceae, most of the cyanobacteria, some diatoms) disappeared almost completely. Therefore, this concentration was used to define the border of the plume. Sometimes, different water bodies can be distinguished more readily by their species composition than by physico-chemical parameters. From an ecosystem perspective, the coastal waters must be distinguished from the open sea because of their special characteristics: e.g. higher N/P ratios indicating potential P-limitation and higher phytoplankton biomass (>2 g m−3). In contrast to the open Baltic Proper, the river plumes were characterized by spring blooms of diatoms instead of dinoflagellates.