Protozoan grazing on planktonic bacteria and its impact on bacterial population

Grazing by heterotrophic nanoflagellates and < 100 Fm microzooplankton on planktonic bacteria was followed during a mesocosm experiment in the Baltic Sea between 23 July and 12 August 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 pm (bacterioplankton), < 5 pm (bacteria and heterotrophic nanoflagellates), and < 100 pm (bacteria, heterotrophic nanoflagellates and ciliates). Clearance rate of < 5 pm flagellates was 0.6 to 5.3 nl flag.-' h-' Grazing in the < 5 pm size fraction was 34 to 134 % of grazing in the < 100 pm fraction. The <S pm and < 100 pm 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. When the water temperature was ca 20°C, < 100 pm microzooplankton could consume about 50 to 100 % of flagellate standing stock daily. During the 3 wk mesocosm experiment, protozoan clearance rate fluctuated in the < 5 and < 100 pm filtrates by a factor of 9 and 3, respectively. A fall of water temperature from 18 to 14°C was the main factor affecting the activity of the microbial community in the mesocosm.