Zooplankton Standing Crops in the Discharge of Lake Francis Case, 1966-1972

Abstract

An automatic plankton sampler was used for 7 years (1966-1972) to monitor zooplankton standing crops in the discharge of Lake Francis Case, a Missouri River reservoir. Cyclops, the most abundant taxon, had a strongly bimodal annual distribution, with maxima in late spring and early winter. Diaptomus, the second most abundant taxon, exhibited a unimodal annual distribution, with the maximum extending from early spring to midsummer. The annual cycle of Daphnia, the only other frequently encountered taxon, was weakly bimodal, with the primary maximum in late spring and a secondary maximum in early winter. Cyclops standing crops decreased during the 7-year study, while Diaptomus standing crops increased slightly. Daphnia standing crops fluctuated at relatively low levels from year to year, and no long-term trend was discernible. Year-to-year fluctuations in the standing crop of each major taxon were not correlated with young-of-the-year fish abundance or average hydrographie conditions. The long-term changes in the zooplankton standing crops in Lake Francis Case may be related to events in the history of Lake Oahe, a newer and much larger reservoir located about 140 km upstream on the Missouri River.