Abstract
The influence of discharges from an upstream reservoir, Lake Francis Case, on the standing crops in Lewis and Clark Lake, 71 km down the Missouri River, was assessed by automatic plankton samplers installed in the powerhouses. Composite samples were collected weekly from January 1966 through December 1967. Densities of total Copepoda and Cladocera were significantly greater in the upstream reservoir; populations downstream were dependent on upstream discharges. Discharge and water exchange rates in Lewis and Clark Lake were too rapid for significant development of Copepoda and Cladocera; only three species showed population increases between powerhouses. In contrast, phytoplankton and rotifers were more abundant downstream, especially in summer when introductions from the Niobrara River and production within Lewis and Clark Lake produced marked differences between reservoirs.Phytoplankton and zooplankton populations in both reservoirs were exceptionally low during summer. Low concentrations of nutrients and low photosynthetic rates in the upstream reservoir appear to be the limiting factors.
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