EFFECT OF LARGE-BODIED AND OF SMALL-BODIED ZOOPLANKTON ON PHYTOPLANKTON IN A EUTROPHIC OXBOW

Macrozooplankton and microzooplankton effects on the phytoplankton wer e measured in situ in a eutrophic lake. Indigenous phytoplankton were incubated for 5 days in 30 1 mesocosms with either the macro- and micr ozooplankton (complete), microzooplankton only (micro) or no zooplankt on (none). Changes in phytoplankton biovolume were investigated. Rotif er densities became significantly higher in the 'micro' treatment than in the 'complete' and 'none' treatments. Total algal biovolume change d little in the 'complete' and 'none' treatments, but increased signif icantly in the 'micro' treatment. The results suggest that macrozoopla nkton (Daphnia magna) suppressed it and microzooplankton (Keratella co chlearis) enhanced it. They had opposite net effects on the phytoplank ton. Suppression of microzooplankton by Daphnia probably had an indire ct negative effect on the phytoplankton.