The nature and cause of the midsummer decline of littoral zooplankton in Lake Itasca, Minnesota

Inshore zooplankton in Lake Itasca, Minnesota experienced dramatic midsummer population declines when environmental conditions, such as temperature and food, should have been favorable for high reproductive rates. Birth rates remained high during the decline, which suggested that predation was the cause of the decline. No size selection was evident across the size classes, as determined by two-way analysis of variance. Shoreward migrations of young-of-the-year yellow perch coincided temporally with the zooplankton decline.

Chambers for experimental manipulations were set in the lake prior to the perch migration. Zooplankton populations protected from young-of-the-year perch predation maintained high numbers, while populations exposed to young-of-the-year perch predation experienced reductions in population numbers and size class abundance similar to those in the lake.