Phytoplankton spring blooms in the southern Baltic Sea—spatio-temporal development and long-term trends

The seasonal and long-term development of the phytoplankton spring bloom in different regions of the southern Baltic Sea was investigated on the basis of monitoring data. The development of a spring bloom starts when the upper mixed layer becomes shallower than the euphotic zone, as proved also by a mesocosm experiment. This already happens in March in Mecklenburg Bight and the western part of the Arkona Sea, leading to a diatom bloom, but only in April in the Bomholm Sea, increasingly giving rise to a dinoflagellate bloom. The new production of the spring phyto- plankton may be calculated from the decrease in nutrients during 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 and the southern Gotland Seas is related to eutrophication, whereas the decrease in diatoms in favour of the dinoflagellates is related to mild winters. The lack of deep-reaching circulation after mild winters may be one reason for the suppression of the non- motile diatoms.