Growth of an entire planktonic protozoan community in Lake Michigan (nanoflagellates microflagellates, and ciliates) was measured on 10 dates in 1988-1989 in fractionated lake water (<3, < 8, < 30, and < 153 mum) incubated in bottles (in vitro), while in June and July 1989, in vitro growth rate estimates were compared with in situ growth rates derived from the mitotic index. Comparisons of in situ vs. in vitro growth rates for the taxa assayed were similar, indicating that in vitro experiments provide reasonable estimates of protozoan growth, and these rates were similar to those measured from other oligotrophic/mesotrophic habitats. However, containment artifacts in some bottle experiments influenced community growth as some taxa experienced mortality in bottles, thus underscoring the importance of censusing the entire community. Protozoa in Lake Michigan contribute significantly to both heterotrophic and phototrophic carbon pools; phototrophic flagellates represented 24% of primary production, while heterotrophic nanoflagellates and ciliates collectively constituted 40% of bacterial production. The fate of high bacterial production can be accounted for through grazing by both heterotrophic nanoflagellates and ciliates, indicating the trophic importance of protozoa in Lake Michigan.
