Contribution of different zooplankton groups in grazing on phytoplankton in shallow eutrophic Lake Vortsjarv (Estonia)

The grazing impact of different sized zooplankton on 'edible' and total phytoplankton biomass and primary production was measured in L. Vortsjarv during a seasonal study in 1998 and 2000. The organisms of 48-100 mu m size class, composed of ciliates and rotifers, contributed significantly to the total grazing of zooplankton community throughout the study period (average 68%). The average daily filtering and grazing rate of the whole zooplankton community (micro- and macro-zooplankton) remained low, corresponding to a filtration of 44% of the water volume, 4% of the total phytoplankton biomass and 29% of primary production. However, a strong grazing pressure on small-sized phytoplankton (< 30 mu m) was estimated in most of the study period (average 44% d(-1)). Among size classes of 'edible' phytoplankton, the size range 5-15 mu m was the most important algal food for the dominant zooplankton grazers (herbivorous ciliates, Polyarthra spp., Chydorus sphaericus and Daphnia cucullata) in L. Vortsjarv.