Stocking piscivorous salmonids in Lake Michigan produced dramatic alterations in food-web structure, including higher numbers of large-bodied zooplankton (especially Daphnia pulicaria ), lower summer chlorophyll concentrations and increased water transparency. Experimental determinations of epilimnetic phytoplankton growth rates and of zooplankton grazing rates indicate that herbivorous zooplankton controlled algal dynamics during the summer of 1983 because grazers occupied the surface waters throughout the day. In 1985, however, both large- and small-bodied Daphnia made approximately equal contributions to total grazer biomass, and all grazers displayed pronounced diel vertical migrations, visiting epilimnetic waters only at night. This prohibited zooplankton from controlling algal dynamics because grazing losses did not exceed phytoplankton growth rates. The changes in zooplankton community composition and behavior observed in summer 1985 probably resulted from increased predation by visually orienting planktivorous fish, especially bloater chub ( Coregonus hoyi ). Effects of food-web manipulations on phytoplankton dynamics were evident only during July and August. During spring and early summer copepods dominate Lake Michigan's zooplankton community. Owing to their small body size, copepods are less susceptible to fish predation and exhibit much lower filtering rates than Daphnia . Variability in zooplanktivorous fish abundance probably has little effect on phytoplankton dynamics during spring and early summer.
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Dorazio et al. (1987) studied this question.