PHYTOPLANKTON SPRING BLOOMS IN THE SOUTHERN BALTIC SEA - SPATIOTEMPORAL DEVELOPMENT AND LONG-TERM TRENDS

The seasonal and long-term development of the phytoplankton spring blo om in different regions of the southern Baltic Sea was investigated on the basis of monitoring data. The development of a spring bloom start s when the upper mixed layer becomes shallower than the euphotic zone, as proved also by a mesocosm experiment. This already happens in Marc h in Mecklenburg Eight and the western part of the Arkona Sea, leading to a diatom bloom, but only in April in the Bornholm Sea, increasingl y giving rise to a dinoflagellate bloom. The new production of the spr ing phytoplankton may be calculated from the decrease in nutrients dur ing spring. In comparison with the Redfield ratio, phosphorus is taken up in excess (N:P = 9.2-10.2). The consumption of silicate in spring has been reduced in the southern Baltic proper since 1989, pointing to a decline in diatoms. The increase in chlorophyll a in the Bornholm a nd the southern Gotland Seas is related to eutrophication, whereas the decrease in diatoms in favour of the dinoflagellates is related to mi ld winters. The lack of deep-reaching circulation after mild winters m ay be one reason for the suppression of the nonmotile diatoms.