Phytoplankton in the sea area around the Loviisa nuclear power station, south coast of Finland

The effects of thermal discharges on the phytoplankton were studied with the methods of Lorenzen and Utermöhl in the Loviisa archipelago in 1971-1982. Throughout the study period, the species composition was typical of oligotrophic areas off the southern Finnish coast. Due to the general increase of salinity and nutrients in the Gulf of Finland, the chlorophyll a concentration and biomass increased, and the species composition became more marine in character in the late seventies. After the first unit of the power station came into use in 1977, the increase of the average phytoplankton quantities was greater in the area in which the cooling water was discharged than in the intake area. The changes in the species composition were similar in the two areas. The warmed water did not cause significant alterations in the winter plankton, but seemed slightly to advance the start of the spring phytoplankton phase in the discharge area. The temperature rise had a stimulative effect on the phytoplankton during the growing period. Nutrients, however, were a more important regulating factor than temperature for both the chlorophyll a concentration and phytoplankton biomass.