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2740070 
Technical Report 
Lake restoration by biomanipulation: Round Lake, Minnesota (USA), the first 2 years 
Shapiro, J; Wright, DI 
1984 
HEEP/85/01660 
14 
4 
371-384 
English 
HEEP COPYRIGHT: BIOL ABS. Rotenone was applied to (eutrophic) Round Lake in the autumn of 1980 to eliminate predominantly planktivorous and benthivorous fish. The lake was subsequently restocked with a higher population density of piscivores. The effect of this biomanipulation on the phytoplankton and zooplankton communities and on total nutrient concentrations was monitored at fortnightly intervals during the summers, from May 1980-Sept. 1982. The abundance of phytoplankton was much lower after biomanipulation and was consistent with observed changes in Secchi disc transparency, total attenuation coefficient and chlorophyll a concentration. Zooplankters were also less abundant in 1981 and 1982 but the decrease in numbers was more than offset by the large increase in the mean sizes of the zooplankters present, so that the estimated grazing pressures in 1981 and 1982 were at least double the 1980 value. Daphnia, rare in 1980, became the dominant genus in 1981 and 1982, and a shift to progressively larger-bodied Daphnia species was observed. Although total N and total P levels were generally lower after biomanipulation, their decline could not explain the reduction in phytoplankton abundance which was attributed to the increased grazing pressure. Possible causes of the observed declines in nutrient concentrations are discussed.