SUCCESSION AND GROWTH LIMITATION OF PHYTOPLANKTON IN THE GULF OF BOTHNIA (BALTIC SEA)

A one year field study of four stations in the Gulf of Bothnia during 1991 showed that the biomass was ca. two times, and primary productivi ty ca, four times, lower in the north (Bothnian Bay) than in the south (Bothnian Sea) during the summer. Nutrient addition experiments indic ated phosphorus limitation of phytoplankton in the Bothnian Bay and th e coastal areas in the northern Bothnian Sea, but nitrogen limitation in the open Bothnian Sea. A positive correlation between the phosphate concentration and the production/biomass ratio of phytoplankton was d emonstrated, which partly explained the differences in the specific gr owth rate of the phytoplankton during the summer. Differences in photo synthetic active radiation between the stations also showed a covariat ion with the primary productivity. The relative importance of nutrient or light limitation for photosynthetic carbon fixation could not, how ever, be conclusively determined from this study. Marked differences i n phytoplankton species composition from north to south were also obse rved. The number of dominating species was higher in the Bothnian Sea than in the Bothnian Bay. The distribution of some species could be ex plained as due to nutrient availability (e.g. Nodularia spumigena, Aph anizomenon sp.), while salinity probably limits the distribution of so me limnic as well as marine species. The potentially toxic phytoplankt on N. spumigena, Dinophysis acuminata and Chrysochromulina spp. were c ommon in the Bothnian Sea but not in the Bothnian Bay. The pico- and n anoplankton biomass during late summer was higher than previously repo rted due to a revised carbon/volume ratio.