The annual course of phytoplankton and bacterial productivity and the carbon requirement of heterotrophic flagellates were studied in the Tvamlinne area, northern Baltic Sea, during 1986. Phytoplankton productivity had a strong spring maximum, which was followed by a bacterial productivity peak formed by cold-adapted bacteria. In summer bacterial productivity was positively correlated with water temperature. Annual bactenal productivity was 15 % of net primary productivity. According to our

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... ulations algal exudation could fulfil 50 to 65 % of the annual bacterial carbon requirement. Bacterial production could satisfy only about half to the flagellate carbon requirement Thls suggests that in order to meet their carbon requirement, heterotrophic flagellates also have to graze on small algae. Of the total net primary production, about 35 % was utilized directly by bacteria or heterotrophic flagellates. This emphasizes the importance of heterotrophic microbes in the pelagic carbon cycle of the northern Baltic Sea.