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A Diurnal zooplankton migration study in Lake Mead

of zooplanktdn was first recorded in freshwater lakes by Weismann (1877) in Lake Constance (Bodensee), although Cuvier was credited with observing the migration of Daphniae in 1817 (Cushing, 1955). The migration is best observed in deep oligotrophic lakes and migrations of 50 m per day are described (Worthington, 1931). In addition, two species of chaetognaths are reported to migrate 400 m a day in marine waters off of Lisbon (Waterman and Berry, 1967) while Birge (1895) found little evidence of the phenomenon at all in Lake Mendota, Wisconsin. Most zooplankton that migrate rise at night and sink during the day. Welch (1952) and Pennak (1944) point out that the most important factor involved in diurnal migration is light. It appears that the extent of the diurnal movements varies from lake to lake, among the sexes and among the age of a particular plankter. Crustaceae and some rotifers are the organisms that most often exhibit this phenomenon. Rare cases in which zooplankters show reversed migrations have been recorded (Worthington and Ricardo, 1936; Maloney and Tressler, 1942). Previous studies on Lake Mead have dealt primarily with the physical limnology of Lake Mead (Smith, et al, 1948; National Research Council, 1949) Anderson, 1950; Anderson and Pritchard, 1951; Harbeck, et al, 1958; and Bureau of Reclamation, 1965). Moffett (1943) did a preliminary report on the plankton and fish of the lake from samples collected in November, 1941. A study by Everett (1972) showed that the highest primary productivitiy rates in Lake Mead occurred in the Boulder Basin. The purpose of the present investigation is to find whether nocturnal migration patterns occur in the zooplanktonic fauna of Lake Mead during times of isothermy and to determine the extent of this possible migration. Acknowledgments. The author wishes to acknowledge Dr. Lome G. Everett for his help in collecting the field samples. Special thanks are extended to Dr. W. Thomas Starmer for his statistical aid in the analysis of variance. This work was supported in part by funds provided by the United States Department of the Interior, Bureau of Reclamation, Region III, Boulder City, Nevada and the Office of Water Resources Research, Tucson, Arizona, as authorized under the Water Resources Research Act of 1964.