Phytoplankton control by grazing zooplankton: A study on the spring clear-water phase l

We tested the hypothesis that a clear-water period, regularly observed in many mesoand 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) 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 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 1978a; Schober 1980; Tilzer 1983). The timing of the event can be very predictable and relatively independent of weather conditions (Geller 1980; Sommer et al. 1986). The clear-water phase in Schohsee, occurring in mid-May, clearly demonstrates the phenomenon (Fig. 1). The Coulter Counter analysis of biovolume and particle distribution shows a sharp drop of the biovolume of all particles present after the l This research was supported by the Deutsche Forschungsgemeinschaft through a grant to W.L. 2 Present address: Savannah River Ecology Laboratory, Drawer E, Aiken, South Carolina 2980 1. “spring bloom.” Subsequent increases in biovolume occur first in the larger size classes, while small particles remain low in abundance. Although it is a widespread phenomenon (for a collection of examples see Lampert 1985; Sommer et al. 1986), a clear-water phase is only apparent when sampling is frequent (once or more a week) (H. BernerFankhauser pers. comm.). A number of seasonal records of phytoplankton biomass show clear spring depressions (e.g. Blaauboer 1982; Bailey-Watts 1982; Riemann 1983; Barko et al. 1984; Meffert and Overbeck 1985) but do not mention this explicitly. The clear-water phase usually coincides with a spring peak of the filter-feeding zooplankton and the increase in water clarity has sometimes been attributed to grazing activity (Lampert and Schober 1978). Alternatively nutrient depletion, climatic events, or parasitism have been suggested as causes for the algal crash (cf. Reynolds 1984). Although there are several descriptions of the time-course of phytoand zooplankton abundance during this period of the year (e.g. Krambeck et al. 1978; Oskam 1978; Petersen 1983; Kuparinen et al. 1984), the causes of the clear-water phase have not been analyzed directly. We here test the hypothesis that zooplankton grazing causes the