Bacteria as a food source for microzooplankton in the Schlei Fjord and Baltic Sea with special references to ciliates

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Abstract
In situ investigations revealed that number and biomass of microzooplankton increased with eutrophication along the length of the Schlei Fjord. 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 biomass of microzooplankton in both bodies of water can mostly be attributed to protozoans of the 3-30 .mu.m fraction. Determined with the aid of radioactively labeled 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 5-58% of the water per day. In the central Baltic Sea in Aug. 1982 the rate was 70% day-1 during the last stage of a decaying blue-green algae bloom. Laboratory experiments with Uronema marinum 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-1 is attained does a proliferation of ciliates commence. Bacteria can represent an important food source of microzooplankton, especially in biotopes with a high bacterial number and biomass.

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