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 Bornholm Sea, increasingly giving rise to a dinoflagellate bloom. The new production of the spring phytoplankton 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.