Phytoplankton control by grazing zooplankton: A study on the spring clear‐water phase1

We tested the hypothesis that a clear‐water period, regularly observed in many meso‐ and eutrophic lakes, is caused by grazing herbivorous zooplankton. Such a clear‐water phase occurs during mid‐May in the moderately eutrophic Schöhsee and involves a rapid increase in Secchi transparency, and a drop in chlorophyll and particulate organic carbon in size fractions <35 µ m. Maxima of zooplankton biomass and community grazing rates (170% of volume cleared per day) coincided with the greatest transparency. The algal decline was not related to nutrient depletion or climatic events. Before the clear‐water phase small phytoplankton contributed up to 88% of the primary production, but the contribution of large particles was more important after the zooplankton maximum. The effects of herbivory by zooplankton were examined in a series of time‐overlapping enclosure experiments. Concentrations of small (<35 µ m) particles were always higher in the bags lacking zooplankton than in the controls. A mass development of small algae occurred in the zooplankton‐free bags initiated during the clear‐water phase, although the presence of zooplankton stimulated the growth of large (>35 µ m) algae.