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The water quality of two prairie Jakes was evaluated from the followmg measurements taken weekly to biweekly during the open water season for six years in Lake Hendricks, S. D. and three years in Oak Lake, S. D.: 1. water transparency, 2. nutrient levels, 3. algal densities, and 4. zooplankton densities. Two of the years sampled in each lake followed partial winterkill of fish. Data were separated into spring, early summer, and late summer seasons and tested for significant differences between years by Duncan multiple range test. Lake Hendricks showed significantly improved water quality !n terms of lower algal densities and higher water transparency in 1978, following the most severe winterkill. Ammonia-nitrogen and phosphate-phosphorus levels were significantly elevated in 1978, and zooplankton densities were significantly increased in the early summer season. The same significant changes were not noted after years of less extensive winterkill (Lake Hendricks 1975, Oak Lake 1978 and 1979). Impact of fish population density on nutrient levels and plankton densities was further investigated through in situ container experiments with and without varying densities of fish tested for significance by Duncan multiple range test and linear 'correlation analysis. Both tests showed significantly lower algal populations and higher zooplankton populations in containers without fish (Duncan) or with lower fish biomass (correlation), the same results as observed after severe winterkill. Nutrient levels in containers showed opposite trends from nutrient levels in the lake after severe winterkill, however, with significantly lower nutrient levels in containers without fish. Presence of dead and decomposing fish in the lake, but not in the containers without living fish, is assumed to be the reason for the discrepancy. Since nutrient levels were unusually high after winterkill, the reason for decreased algal bloom density and improved water transparency was probably due to increased zooplankton grazing. Other measures which increase the zoop!ankton effectiveness, such as management for larger numbers of piscivorous fishes which prey upon zooplanktivorous fishes might also be expected to improve water quality.

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Proceeding of the South Dakota Academy of Science



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