The Rise and Fall of a Dominant Planktivore: Direct and Indirect Effects on Zooplankton

Abstract

We analyzed a 14—yr time series (1976—1989) of planktivorous fish and zooplankton from Lake Mendota, Wisconsin. Planktivory rates changed by an order of magnitude during this time period, primarily due to the rise and fall of the 1977 year class of cisco (Coregonus artedi) that dominated planktivory rates for a period of 10 yr. Planktivory increased between 1977 and 1978 due to an increase in biomass of that year class and decreased in August 1987 after a summer kill of cisco in the lake. Yellow perch (Perca flavescens) and other cisco year classes contributed °25% of the total planktivory during 1978 to 1987. Time series analysis revealed that this 10—yr pulse in planktivory rates was associated with changes in the Daphnia species and biomass. In years with low planktivory rates, higher biomass of daphnids dominated by Daphnia pulicaria developed earlier in the spring and lasted longer into the summer. This was also associated with an earlier and longer spring clear—water phase. In years with high planktivory rates, daphnid biomass was generally lower and dominated by the smaller Daphnia galeata mendotae. However, there was no significant effect of planktivory on the early summer peak in Daphnia biomass that is associated with a clear—water phase. The dynamics of this Daphnia peak are apparently regulated by Daphnia—algae interactions and not by planktivory rates. The seasonal and interyear changes in Daphnia species composition can be explained by the combined effects of pl…