Date of this Version
Published in Transactions of the Nebraska Academy of Sciences, Volume 3 (1976).
Limnetic microcrustacean communities of 5 flood control reservoirs in the Salt Valley Watershed of Southeastern Nebraska were studied from June 1968 to January 1970. The average community during summer was comprised of 3.2 c1adoceran and 2.4 copepod species. Cladoceran densities usually exceeded those of the copepods. Bosmina longirostris was found in the more productive reservoirs while Bosmina coregoni inhabited the light-limited, nonproductive reservoirs. Congeneneric occurrences of Diaptomus species were common in three of the five reservoirs. Congeneric Daphnia occurrences were found on 61 percent of the sampling dates, excluding one reservoir in which only one species of Daphnia was found. The most abundant species of Daphnia seldom outnumbered the less abundant species by more than 10 times and it was not uncommon to find ratios lower than 5:1.
The species composition of limnetic zooplankton communities is one parameter of limnological importance that has received little study in Midwestern reservoirs. Pearse (1905) and Fordyce (1901) surveyed crustacean fauna in natural lakes of Nebraska but nothing is reported on the structure of zooplankton communities. It has been shown that open water plankton communities are quite simple in terms of the number of species found on any given sampling date (Anderson, 1971; Armitage, 1961; Pennak, 1957; Timms, 1968). This paper considers species composition in some reservoirs in a Midwestern agricultural area where surface runoff provides an abundance of chemical factors necessary to sustain high levels of organic production.
The Study of Reservoirs
Branched Oak, Holmes, Pawnee, Stagecoach and Wagontrain reservoirs, built as flood control and soil conservation projects in the Salt Valley Watershed of eastern Nebraska, were studied from June 1968-December 1970. Drainage areas consist primarily of agricultural and pasture lands. About three-quarters of the 28 inches of annual precipitation occurs in spring and summer. The reservoirs are all shallow with mean depths varying between 2.0 and 4.5 m and ranging in area from 112 to 1800 acres. Quite strong southerly winds that prevail in summer coupled with the shallowness of the reservoirs prevents stable thermal stratification. Holmes and Wagon train are turbid, light-limited reservoirs that have low organic production. Pawnee, Stagecoach and Branched Oak reservoirs are clearer with abundant growths of rooted aquatics and algae in midsummer and showing rapid eutrophication (Hergenrader and Hammer, 1973).