Large heterotrophic flagellates form a significant part of protozooplankton biomass in lakes and rivers

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Abstract
Stimulated by studies from marine waters there is now increasing evidence that the components of the microbial food web contribute to the pelagic flux of matter as significandy in freshwater environments as they do in the ocean. Up to now generally only heterotrophic nanoflagellates and cili-ates have been quantitatively considered among limnetic protozooplankton. Our investigations using a special live-counting technique showed that large heterotrophic flagellates (LHF, ≥ 15 μm; mainly chrysomonads and dinoflagellates) have been overlooked in quantitative estimations of limnetic protozooplankton biomass. We present estimations from 19 German lakes of different trophy during different seasons and from 4 rivers as well as detailed seasonal studies of a eutrophic lake and a brook indicating that LHF can form up to 50% (mean about one third) of protozooplankton biomass. LHF reached highest importance in early spring when LHF were one of the most significant herbivore groups. Large heterotrophic flagellates should be considered as significant components of limnetic food webs. This is in agreement with the present knowledge of marine pelagic ecosystems where LHF (mainly dinoflagellates) have already been recognized as an important part of the pelagic microbial web.