LONG-TERM CHANGES IN ZOOPLANKTIVOROUS FISH COMMUNITY COMPOSITION - IMPLICATIONS FOR FOOD WEBS

The zooplanktivorous fish community of Lake Mendota has been dominated by cisco (Coregonus artedi), yellow perch (Perca flavescens), and whi te bass (Morone chrysops) for over 100 years. Shifts in their abundanc es have led to changes in zooplankton community composition. When tota l zooplanktivory was less than 0.5 g Daphnia . m(-2). day(-1), large-b odied Daphnia pulicaria dominated. At predation rates above 0.9 g Daph nia . m(-2). day(-1), D. pulicaria were rare. Large differences exist in the ability of each planktivore to regulate zooplankton community s tructure. In its lifetime, one cisco will consume as much Daphnia spp. as 14 yellow perch or 8 white bass. For equal biomass, cisco consume twice as much Daphnia spp. as do yellow perch or white bass. Almost 80 % of the zooplanktivory by cisco is attributable to adults, while juve nile yellow perch and white bass account for over 50% of their species ' predation. Cisco predation is highest during spring and fall and con trols D. pulicaria population dynamics. Yellow perch and white bass pr edation is most intense during summer. When cisco biomass was less tha n 80 kg . ha(-1), D. pulicaria was the dominant zooplankter, regardles s of yellow perch or white bass biomass. Continued improvements in wat er quality of Lake Mendota through biomanipulation require sustained l ow cisco biomass, enabling D, pulicaria populations to flourish.