Functional Interaction of Phytoplankton and Zooplankton along the Trophic Gradient in Green Bay, Lake Michigan

Abstract

The functional interaction of phytoplankton and zooplankton, expressed in terms of the numerical difference between phytoplankton growth rates per day (in situ, 14 C method) and zooplankton grazing rates per day (in situ feeding experiments), was studied along the trophic gradient in Green Bay, Lake Michigan. Growth–grazing differences increased with trophic conditions, averaging 0.08 for the water column in the meso-oligotrophic northern bay and 0.56 in the eutrophic southern bay for the summers of 1986, 1987, and 1988. Eutrophic conditions produced dominance of growth by large-size cyanobacteria and low grazing rates by microcrustaceans Small and occasionally negative growth–grazing differences in the meso-oligotrophic region were associated with dominance of larger cladocerans and calanoid copepods and small algal species Phytoplankton growth rates in the northern bay averaged about 28% those of the eutrophic region. A unimodal phytoplankton growth response to increased grazing was observed in the northern bay, suggesting variation in positive (growth stimulating) and negative (grazing losses) effects of zooplankton on the phytoplankton.