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The objectives of this study were to describe and compare inflow rates, nutrient concentrations and phytoplankton taxa in two South Dakota prairie lakes of different trophy: Eutrophic Lake Cochrane (Deuel County) and hypertrophic Oak Lake (Brookings County). Stream flows, total phosphorus (TP), total Kjeldahl nitrogen (TKN), nitritenitrate- N, iron, silica, manganese, calcium, sodium, conductivity and algal species composition and density were measured from Lake Cochrane on three dates in 1997. Sampling sites included a natural inflow below a sediment control dam, an artificial diversion pipe from Lake Oliver to Lake Cochrane, an artificial outflow from Lake Cochrane and a mid-basin site. Lake Oak was sampled on two dates in 1997 from major inflows from the west and north, spring seepage flowing into the lake on the west shore, and a mid-basin site. Mean concentrations of TP in Lake Cochrane were highest in the inflow from the sediment control dam, intermediate in the inflow from Lake Oliver and lowest midbasin. TN concentrations were highest midbasin and in the inflow from Lake Oliver, and lowest in the inflow from the sediment control dam. Average P loading rates were 116.3 g/da from Lake Oliver and 8.2 g/da from the sediment dam. Outlet removal was estimated at 24.8 g/da. TN loading rates were 43.7 g/da from the sediment control dam and 2005.6 g/day from Lake Oliver. Outlet removal was estimated at 919.2 g/day. Populations of green algae and cryptophyte flagellates were highest in the in the inflow from the sediment control dam. Dinoflagellates and Botryococcus were highest midbasin. Chrysophyte flagellates and coccoid bluegreen algae were most abundant in the outlet. In Oak Lake, mean concentrations of both TP and TKN were highest midbasin and lowest in the inflows. Average P loading rates were 25.5 g/da from the west inflow, 2.6 g/da from the west seepage area and 68.3 g/da from the north inflow. Average N loading rates were 94.0 g/da from the west inflow, 67.7 g/day from the west seepage and 715.3 g/day from the north inflow. Populations of most taxonomic groups of algae were greatest at the midbasin site. Loading of P into Lake Cochrane may threaten the high water quality currently present in that lake. The Lake Oliver inflow is of particular concern. Oak lake may be generating much of its P internally. N-fixing bluegreen algae are stimulated by high P levels and contribute to high TKN levels measured midlake. Restoration of higher water quality to Oak Lake may require sediment removal.

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Proceedings of the South Dakota Academy of Science



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