Daphnia in Lake Washington1

Abstract

In 1976 Daphnia, a genus that had been inconspicuous in the zooplankton of Lake Washington, suddenly became dominant. The mean summer transparency of the lake doubled. The major change affecting the success of Daphnia evidently was a decrease in the abundance of the predatory Neomysis mercedis in the mid‐1960s. Daphnia probably did not increase at that time because of the continued persistence of significant quantities of Oscillatoria until 1975. Eight species of Daphnia have been seen in the lake, but only three have become abundant. Daphnia pulicaria has been continuously present since its appearance in May 1976, although it becomes very scarce in winter. Daphnia galeata mendotae and Daphnia thorata have had much more limited occurrence. In each year from 1976 to 1980, two species formed separate population maxima, except in fall 1979 when D. thorata, although present, failed to increase. Instead, the plankton became dominated by the rotifer Conochilus hippocrepis which had been increasing for several years. In 1980 D. thorata and D. pulicaria made maxima, but in reverse order from earlier years. Several other species of zooplankton have been less abundant since the resurgence of Daphnia than during the period of eutrophication and recovery.