Abstract
ABSTRACT Despite diversion of sewage effluent and septic leachate from Delavan Lake in 1981, dense blue-green algal blooms caused numerous complaints in the summer of 1983. The U.S. Geological Survey started a comprehensive two-year water quality investigation in 1983. All major external inputs and outflows of phosphorus were measured to assess the importance of each source. Internal loads were calculated as a residual of a mass-balance budget. This paper reports the 1984 water year. Although external loads of total phosphorus were sufficient to cause eutrophic conditions, internal loading was 2.6 times the external supply, mostly from summertime anoxic release of phosphorus from lake sediments. Continuous streamflow and water quality monitoring in the subbasins of the Delavan Lake watershed showed a wide range of annual phosphorus yields from 154 kg/km2 in a subbasin of Jackson Creek draining the village of Elkhorn to 50.4 kg/km2 at Jackson Creek's headwaters where the largest concentration of livestock is found. The other major tributary (unnamed) to Delavan Lake had an extremely low phosphorus yield of 10.3 kg/km2 because of significantly reduced runoff from storage and evapotranspiration in a large wetland surrounding a large pond in the stream's basin.
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