Cladoceran and rotifer grazing on bacteria and phytoplankton in two shallow eutrophic lakes:: in situ measurement with fluorescent microspheres

Metazooplankton grazing on bacteria and on the phytoplankton of various sizes was estimated in shallow eutrophic lakes Kaiavere and Vortsjarv (Estonia) by in situ feeding experiments with fluorescent microspheres (diameters 0.5 mu m for bacteria and 3, 6 and 24 mu m for phytoplankton). Zooplankton community composition, abundance and food density were important factors determining grazing rates in these lakes. Cladocerans and rotifers filtering rates (FR) and ingestion rates (IR) on bacteria and phytoplankton were several times higher in Lake Kaiavere where bacterivorous rotifers and Daphnia contributed more to zooplankton assemblage. While cladocerans were generally the main phytoplankton consumers, both lakes differed with respect to the groups of bacterivores. Based on consumption of fluorescent microspheres, the metazooplankton grazing rates were relatively low and had low impact on production and standing stock of bacteria and ingestible phytoplankton (< 30 mu m). On average, 0.5 and 0.1% of standing stock of bacteria and 2.6 and 1.0% of standing stock of ingestible phytoplankton was grazed daily by metazooplankton in lakes Kaiavere and Vortsjarv, respectively. That corresponded to daily grazing of 4.1% of the bacterial production and 0.43% of the total primary production (PP) by metazooplankton in Lake Kaiavere compared with 4.3 and 0.06% in Lake Vortsjarv, respectively. The results suggest that the majority of consumption of the bacterial and phytoplankton PP is most likely channelled through the microbial loop.