In situ investigations revealed that number and biomass of microzooplankton increased with eutrophication along the length of the Schlei Fjord (FRG). The same observation was made for total bacterial numbers and biomass. Microzooplankton of the Schlei and total bacterial numbers showed a minimum in winter and major periods of development in late summer/autumn and spring. Usually the microzooplankton biomass in the Schlei was greater than the bacterial biomass. In contrast. the bacterial biomass for 5 of 6 stations in the Baltic Sea surpassed that of the microzooplankton during summer. Number and biornass of microzooplankton in both bodies of water can mostly be attributed to protozoans of the 3 to 30 pm fraction. Determined with the aid of radioactively labelled bacterial cultures, the filtration rate of 'natural' microzooplankton populations exhibited a distinct dependency on microzooplanktonic biomass and water temperature. In March 1982 microzooplankton populations in the eutrophic Schlei Fjord filtered 5 to 58 % of the water per day. In the central Baltic Sea in August 1982 the rate was 70 % d-' during the late stage of a decaying blue-green algae bloom. Laboratory experiments with Uronema marinurn clearly showed that bacteria concentrations exert a considerable influence on the development of ciliates. Only when a limiting concentration of about 1 X 106 bacteria ml-' is attained does a proliferation of ciliates commence. Hence, bacteria can represent an important food source for microzooplankton, especially in biotopes with a high bacterial number and biomass.

Total bacterial numbers and bacterial biomass were determined, and the size-class spectrum analysed, of the bacterial population in the Gotland Basin (Central Baltic Sea) during pronounced cyanobacterial blooms. Numbers of aerobic and anaerobic saprophytic bacteria and of sulphate-reducers are also reported. Below the oxic-anoxic interface a morphological change in the microfloral populations was observed. The size-class spectrum in anoxic bottom waters shifted towards larger cell forms. Average cell volume increased about 3-fold over that found in oxic waters. Anaerobic saprophytes and sulphate-reducers became more important in the H2S-rich bottom waters. Anaerobic micro-environments in 02-saturated water layers are discussed as important microbiotopes for anaerobic bacteria (e.g. sulphate-reducers).

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