Protozoan grazing on planktonic bacteria and its impact on bacterial population

Abstract

Grazing by heterotrophic nanoflagellates and < 100 mu m microzooplankton on planktonic bacteria was followed during a mesocosm experiment in the Baltic Sea between 23 Jul and 12 Aug 1988 on the SW coast of Finland. During the succession of the planktonic community in one mesocosm, 4 grazing experiments were run with a size-fractionation technique. The size fractions used were: < 1 mu m (bacterioplankton), < 5 mu m (bacteria and heterotrophic nanoflagellates), and < 100 mu m (bacteria heterotrophic nanoflagellates and ciliates). Clearance rate of < 5 mu m flagellates was 0.6 to 5.3 nl flag/h. Grazing in the < 5 mu m size fraction was 34 to 134% of grazing in the < 100 mu m fraction. The < 5 mu m and < 100 mu m protozoa harvested hourly on average 75 and 90% of bacterial production, respectively. Nonetheless, heterotrophic nanoflagellates could not satisfy their carbon demand from bacteria. Grazing by protozoa altered bacterial size distribution, and reduced bacterial cell number and production.