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 Schiihsee and involves a rapid increase in Secchi transparency, and a drop in chlorophyll and particulate organic carbon in size fractions ~35 Nm. 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 pm) algae. A distinct period of clear water is typical of the spring algal succession in many mesotrophic and eutrophic lakes of temperate zones (Sommer et al. 1986). An early peak of small, rapidly growing algae (flagellates, small diatoms), which often represents the highest biomass concentration of the annual cycle, is followed by a short period of very clear water and high Secchi transparency. A summer community dominated by large algae and blue-greens develops after the clearwater phase. The period of high Secchi transparency sometimes lasts only 2 weeks or less. The increase in water clarity can be spectacular. In Lake Constance for example transparency may increase from 1 to 10 m in a few days (Lampert

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