Nutrient limitationand grazing control of the planktonic community were studied in the northern Baltic Sea off the SW coast of Finland during the phytoplankton growth season of 1985. In situ experiments based on a 23 factorial design were performed in mesocosm enclosures on 10 occasions. The manipulations used included phosphorus (PO,? ) and nitrogen (NH,+ ) additions and the removal of metazooplankton by 1 OO+m prefiltration.In each experiment, the responses of phytoplankton, bacterioplankton, heterotrophic nanoflagellates, and protozooplankton were followed for 2 d. Orthogonal multiple regression analysis was used to reveal which manipulations had statistically significant effects. Nitrogen was found to be the basic limiting nutrient for phytoplankton throughout the productive season. During early summer, only the combined addition of P and N evoked a clear increase in the growth of phytoplankton.In general, bacterial productivity was not highly affected by the manipulations. In summer the removal of metazooplankton caused a rapid increase in the amount of protozooplankton in the units with loo-pm prefiltration or prefiltration combined with N addition. In the absence of metazooplankton, the nutrient-induced increase in primary productivity was channeled to protozooplankton, whose growth in the units where metazooplankton was present was severely limited by food competition or by direct metazooplankton grazing.

Ohgotrich ciliates are a major microzooplankton component in the northern Baltic Sea during summer The use of wheat starch particles as a tracer-level food source enabled us to simultaneously evaluate in situ clearance rates and food particle size selection of natural ciliate assemblages. Average clearance rates of 9 ciliates of the genera Lohmanniella, Strobilidium, Strombidium, Tintinnidium and Tintinnopsis on their most favored particle sizes ranged from 1.9 to 11.4 p1 cell-' h-'. Particle size selectivity was studied by the use of x2-based selectivity index. Each species showed a specific particle size preference pattern. The optimal particle size varied from 1.4 pm (Strombidium sp. 20 pm in size) to 9.8 pm (Strobilidium sp. 40 pm). Most species were able to ingest effectively nanoflagellate-size food, but only 2 of the species showed effective grazing on the smallest particles, suggesting a possible ability to utilize bacteria-size prey. The overall ciliate food size spectrum covered the size range of the most abundant food items in the Baltic summer plankton. Among the ciliates investigated, there appears to be 2 different feeding strategies: specialistic and generalistic, where the ciliates either concentrate on feeding on a narrow size range of food organisms, or use food particles of a wide size range, with little or no preferences within this range.

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ABSTRACT. 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 calculations 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.

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