The response of phytoplankton to increased temperature in the Loviisa archipelago, Gulf of Finland

Phytoplankton was studied in the Loviisa archipelago (south coast of Finland) in 1971– 1994. Since 1977, thermal effluents from the Loviisa nuclear power plant have been dis­ charged into the sea there. A general increase of nutrients in the Gulf of Finland contributed to an increase in phytoplankton biomass and primary production in the Loviisa archipelago during the late 1970s and 1980s, though biomass seemed to decrease again in the 1990s. The rise in temperature was, however, a more important factor than nutrients stimulat­ ing the production and biomass in the area close to the cooling water outlet. The thermal discharges increased especially the biomass of Aphanizomenon spp., which is the most abundant cyanoprocaryote (blue-green alga) and one of the most common phytoplankton taxa in the study area. At the intake area, total amounts of phosphorus best explained the changes in total biomass. The results indicate that increased temperature can lengthen the growing season, advance eutrophication and somewhat change species dominances in the circumstances prevailing in the northern Baltic Sea.