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-the-year cohort eliminated large-bodied cladocerans by the middle of August. As large cladocerans declined, small cladocerans, especially Bosmina longirostris, increased. By early September the biomass of B. longirostris 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 mu m. However, total epilimnetic 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.