Zooplankton of the Big Eau Pleine Reservoir
University of Wisconsin-Stevens Point, College of Natural Resources
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Composition, abundance, and distribution of zooplankton were determined during September, 1973 to September, 1974 in the Big Eau Pleine Reservoir, a eutrophic, 2832 ha (10.93 m^2 ) fluctuating reservoir in central Wisconsin. Yearly water level fluctuations of up to 9.6m (31.5 ft.) occur commonly, and maximum drawdown occurs during winter. The yearly drawdown appeared to have little affect on the zooplankton populations because: a) zooplankton species composition in the Big Eau Pleine Reservoir was typical for lakes of the Great Lakes Region, b) the mean annual total number of 298 zooplankton per liter and biomass of 362 mg/m^3 (13.8 kg/ha) for the ice-free period of September 20 through December 1, 1973 and May 13 through September 5, 1974, were high in comparison with other bodies of water, c) the 30-year mean annual flushing rate of 158 days is considerably greater than that thought to be limiting to zooplankton, and d) a winter drawdown, as in the Big Eau Pleine Reservoir, occurs when zooplankton numbers are low and has less affect than if it occurred during summer, when zooplankton numbers are high. The reservoir remained thermally unstratified except for a brief period during late June and early July. Low water transparency could probably be attributed to algae blooms and suspended sediments during summer and suspended sediments during winter. Dissolved oxygen was minimal in winter, reaching concentrations as low as 0.1 ppm in January and February. Of 25 zooplankton taxa in the reservoir, at least 17 were found at each of 6 sampling sites. Copepod nauplii, Rotifera, and Chydorus sphaericus comprised 66 to 75% of zooplankton numbers. As is typical in other bodies of water, maximum biomass and number per liter occurred during summer and minima during winter. The reservoir, probably due to a lack of horizontal and vertical thermal stratification, had little horizontal or vertical differences in zooplankton number per liter, and no significant horizontal differences in zooplankton biomass.