The Occurrence and Cause of Midsummer Zooplankton Declines: Cyanobacteria and the Implications for Freshwater Ecology

The current paradigm of lacustrine ecology is that zooplankton communities are structured by the nature am intensity of predation upon them. One corollary to this paradigm is that the observed phenomenon of midsummer zooplankton decline is caused by predation. A second is the hypothesis of "biomanipulation." One aspect of "biomanipulation" proposes that in certain circumstances the removal of predators of zooplankton might result in reduced algal biomass. The assumption of both these corollaries is that aquatic food webs are manipulable from the top. 'The data supporting predation-caused midsummer declines and biomanipulation are examined analytically and theoretically by considering biomass, foraging cost and evolution. It is concluded that the present data suggest it is more likely that algal biomass and composition determine zooplankton community structure at a given time than does predation.