The effects of a biomanipulation with adult carp (Cyprinus carpio) removals on water quality and the abundance, composition, and body size of zooplankton were investigated in Bowman-Haley Reservoir, North Dakota in a two-year study. In both years, decreases in zooplankton abundance, Daphnia body size, and Daphnia clutch size in carpuneradicated areas coincided with the appearance and subsequent domination of the blue-green algae Aphanizomenon. Concurrently, in areas where carp were eradicated, water clarity increased, total suspended solids decreased, and prevalence of blue-green algae blooms decreased. As a result of these improved feeding conditions, zooplankton abundance, Daphnia size, and Daphnia clutch size increased. Because adult carp, which fed to a very limited extent on zooplankton in spring, represented over 95% of the fish removed, changes in the zooplankton community were not attributable to decreased predation by fish. Furthermore, it is unlikely that predation by fish significantly affected the zooplankton community in the extremely turbid water observed prior to fish removals. In this study, depletions of the benthivorous carp in areas of a shallow, turbid reservoir resulted in changes to zooplankton communities similar to those observed with removal of planktivorous fishes in other water bodies. These results indicate that bioturbation by carp can affect zooplankton in ways similar to predation by planktivorous fishes.
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