Seasonal effects of variable recruitment of a dominant piscivore on pelagic food web structure

Piscivorous fish play an important role in regulating lake food web structure. However, most ultimately piscivorous fish pass through a period of planktivory before becoming piscivorous. In 1993 and 1994, two large cohorts of young-of-the-year largemouth bass (Micropterus salmoides) were recorded in Paul Lake, Michigan. The 1993 and 1994 cohorts were the largest recorded in data extending back to 1984 and occurred during a period in which adult and juvenile bass biomass was low. In 1993, consumption of large-bodied zooplankton by the young-of-theyear cohort eliminated large-bodied cladocerans by the middle of August. As large cladocerans declined, small cladocerans, especially Bosminu Zongirostris, increased. By early Septcmbcr the biomass of B. Zongirostris was similar to that of the entire cladoceran community in previous years. Coincident with the shifts in the cladoceran community were increases in epilimnetic Chl a concentrations <35 pm. However, total cpilimnetic Chl a did not increase. In contrast, the 1994 cohort had no discernible effect on pelagic food web structure because production of large-bodied zooplankton rapidly exceeded predation. Our results show the potential for large cohorts of piscivorous fish to affect food web structure, at a seasonal scale, through intense planktivory. The effects of piscivorous fish on food web structure are central to biomanipulation efforts to improve water quality (Shapiro et al. 1975; Benndorf 1990; Jeppesen et al. 1990; Kitchell 1992). Piscivorous fish are important to lake food webs because they may depress planktivorous fish populations, thereby allowing large-bodied zooplankton to flourish, which results in relatively low phytoplankton biomass. Yet, the strength of this cascading interaction in unmanipulatcd systems remains a topic of continued debate (Crowder et al. 1988; McQueen et al. 1989; DeMelo et al. 1992). The effect of fish populations on food web dynamics is mediated by recruitment (Carpenter 1988; Rudstam et al. 1993), an inherently variable process (LeCren 1962; Fogarty et al. 1991). Previous studies have demonstrated seasonal food web effects of a large year class of young-of-the-year (YOY) planktivorous fish (Mills et al. 1987; Carpenter and Leavitt 1991; Helminen 1994) and long-term food web efI Present address: Section of Ecology and Systematics, Corson Hall, Cornell University, Ithaca, New York 14853. 2 Present address: National Center for Ecological Analysis and Synthesis, 735 State St., Suite 300, Santa Barbara, California 93101. J Present address: Department of Zoology, University of Washington, Seattle, Washington 98 195.