ANNUAL CYCLE OF PROTOZOOPLANKTON (CILIATES, FLAGELLATES AND SARCODINES) IN RELATION TO PHYTOZOOPLANKTON AND METAZOOPLANKTON IN LAKE NEUMUHLER-SEE (MECKLENBURG, GERMANY)

The seasonal changes of the abundance and biomass of protozooplankton were investigated by a live-counting technique in the mesoeutrophic Lake Neumuhler See. In addition, the biomass of metazoans and phytoplankton was studied. During the annual cycle, the percentage contribution of the different protozoan groups changed significantly. Ciliates dominated the biomass in late spring, summer and autumn, heterotrophic nanoflagellates (<15 mu m, HNF) were most important in late summer and in winter, whereas large heterotrophic flagellates (LHF) were dominant in early spring. Ciliates (mainly oligotrichs, less important were peritrichs, prostomatids and heterotrichs) formed 50 % of protozoan biomass on annual average, HNF (mainly chrysomonads, less important were kathablepharids and choanoflagellates) 29 %, LHF (mainly dinoflagellates, less important were chrysomonads) 19 % and sarcodines (mainly heliozoans and testacean, less important were naked amoebae) 2 %, respectively. Protozooplankton percentage contribution to total zooplankton biovolume ranged between 3 % and 78 % and was about 21 % of annual average total biovolume. Maximum percentage contributions of protozoans were registered in early spring and during summer. Since the metabolic rates of protozoans are several times higher than those of metazooplankton protozooplankton should be considered as important within the pelagic material flux of Lake Neumuhler See as the metazooplankton. The seasonal dynamics of the protozoan community may well be explained by changes in the impact of bottom-up and top-down effects. Small centric diatoms may have supported the growth of protozoans in early spring; copepods and daphnids probably exerted a strong grazing pressure during late spring. High temperatures and low predation pressure may have supported a protozoan peak during summer which was then probably reduced by metazoan predation (rotifers). Small cryptomonades should have supported a short-term maximum of protozoans in autumn. Protozoan herbivory probably had an important impact on phytoplankton mortality during the early spring bloom.