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 st ructure. However, most ultimately piscivorous fish pass through a peri od of planktivory before becoming piscivorous. In 1993 and 1994, two l arge cohorts of young-of-the-year largemouth bass (Micropterus salmoid es) were recorded in Paul Lake, Michigan. The 1993 and 1994 cohorts we re the largest recorded in data extending back to 1984 and occurred du ring a period in which adult and juvenile bass biomass was low. In 199 3, consumption of large-bodied zooplankton by the young-of-the-year co hort eliminated large-bodied cladocerans by the middle of August. As l arge cladocerans declined, small cladocerans, especially Bosmina longi rostris, increased. By early September the biomass of B. longirostris was similar to that of the entire cladoceran community in previous yea rs. Coincident with the shifts in the cladoceran community were increa ses in epilimnetic Chl a concentrations <35 mu m. However, total epili mnetic chl a did not increase. In contrast, the 1994 cohort had no dis cernible effect on pelagic food web structure because production of la rge-bodied zooplankton rapidly exceeded predation. Our results show th e potential for large cohorts of piscivorous fish to affect food web s tructure, at a seasonal scale, through intense planktivory.