Macrozooplankton and microzooplankton effects on the phytoplankton were measured in situ in a eutrophic lake. Indigenous phytoplankton were incubated for 5 days in 301 mesocosms with either the macro- and microzooplankton (complete), microzooplankton only (micro) or no zooplankton
(none). Changes in phytoplankton biovolume were investigated. Rotifer densities became significantly
higher in the 'micro' treatment than in the 'complete' and 'none' treatments. Total algal biovolume changed little in the 'complete' and 'none' treatments, but increased significantly in the 'micro' treatment. The results suggest that macrozooplankton (Daphnia magna) suppressed it and
microzooplankton (Keratella cochlearis) enhanced it. They had opposite net effects on the phytoplankton.
Suppression of microzooplankton by Daphnia probably had an indirect negative effect on the phytoplankton
