0 WIACKOWSKI, K. BRETT, MT GOLDMAN, CR 1994 LIMNOLOGY AND OCEANOGRAPHY LAWRENCE AMER SOC LIMNOLOGY OCEANOGRAPH 39 3 486-492 MAY 0024-3590 NATURAL ROTIFER DAPHNIA PROTOZOANS PREDATION LAKE An in situ bioassay experiment using Diacyclops bicuspidatus thomasi, Daphnia rosea, Diaptomus novamexicanus, and Holopedium gibberum single-species treatments was conducted to assess the influence of these zooplankters on the ciliate community structure of Castle Lake, California. At peak ambient abundances for adult individuals, these zooplankters all strongly depressed ciliate population growth rates. The ranking of the zooplankton-imposed ciliate death rates, from least to most severe, was Holopedium << Daphnia = Diaptomus <<< Diacyclops. The ranking of ciliate-taxa-response death rates to the zooplankton treatments was Halteria = Mesodinium < Askenasia = Urotricha = (Askenasia <) Strobilidium << Strombidium. As total ciliate abundance was depressed by the zooplankton treatments, the relative frequency of the small ciliates (approximately 5,000 mum3) increased, and the relative frequency of the larger ciliates (approximately 20,000 mum3) decreased. These results suggest that both seasonal and long-term shifts in zooplankton community structure have the potential to dramatically alter the dynamics and structure of ciliate communities.