Primary Productivity of Southern Indian Lake before, during, and after Impoundment and Churchill River Diversion

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Abstract
The primary productivity of 7 regions of Southern Indian Lake and neighboring Wood Lake was measured during open-water seasons from 1974-1978. The lake had regional differences in chlorophyll concentration and daily rates of integral primary production in 1974 and 1975 prior to impoundment of the lake. Regions receiving Churchill River flow tended to have higher chlorophyll concentrations and production rates than those regions marginal to the flow. Impoundment of the lake resulted in higher efficiencies of primary production in all regions, as indicated by higher light-saturated rates of C uptake per unit chlotophyll and by higher initial slopes of the hyprebolic light response relation to the phytoplankton. Many large basins of the lake had light penetration reduced by high concentrations of suspended sediment from eroding shorelines; other areas had relatively unchanged light penetration. The increased efficiency of C fixation per unit chlorophyll resulted in higher rates of integral production in those regions where light penetration was not greatly affected. Daily rates of integral primary production in lake regions where light penetration had decreased markedly were not significantly different after impoundment because efficiencies of light utilization were higher. Comparison of the mean water column light intensities for those turbid regions with the values of Ik (light intensity at the onset of light saturation) for phytoplankton indicated that these turbid regions are now light deficient on average. P deficiency, as indicated by alkaline phosphatase activity per unit ATP, which was present before impoundment, was eliminated as the mean water column light intensity declined below 5 mEinsteins .cntdot. m-2 .cntdot. min-1. The light environment of a new reservoir can be a significant determinant of integral production, and predicting the consequences of impoundment on phytoplankton production requires accurate prediction of the light enviroment.