PICOPLANKTONIC ALGAE IN THE NORTHERN BALTIC SEA - SEASONAL DYNAMICS AND FLAGELLATE GRAZING

The seasonal succession of picoplanktonic algae was followed from March to December 1986 at a station off Tvarminne, northern Baltic Sea. This area is characterized by strong seasonal changes in water temperature and stratification. Picoeukaryotes were abundant during the whole year (10(3) to 10(4) ml-1), but picocyanobacteria thrived only during summer (10(5) to 10(6) ml-1). The contrasting seasonal dynamics of these 2 groups led to the conclusion that growth of picocyanobacteria is more temperature dependent than that of the almost equally sized picoeukaryotes. However, the cyanobacterial dynamics could not be explained by temperature alone and light climatic changes may have contributed to the drastic decline of cyanobacterial numbers in the warm deep-mixed water column in the autumn. Estimates of flagellate grazing on picoplanktonic cyanobacteria were obtained by differential filtration experiments. These showed that flagellates could graze vigorously on picoalgae. Total flagellate grazing on picoalgae amounted to 15 % of annual primary production, and was estimated to be 32 and 42 % during summer and autumn, respectively. The microbial loop seems to be of major quantitative importance in the boreal northern Baltic Sea.