The importance of zooplankton‐protozoan trophic couplings in Lake Michigan

The importance of the zooplankton-protozoan trophic coupling was determined experimentally by measured changes in protozoan growth rates with increasing zooplankton biomass. In five of six experiments conducted in Lake Michigan, a significant inverse relationship between protozoan growth and zooplankton biomass was observed (avg r2 = 70%), Zooplankton clearance rates on protozoan assemblages (range, 1.0-6.2 ml (pg dry wt)-I d ‘1 were comparable to those previously measured for phytoplankton which suggested that protozoa are important prey for zooplankton. Clearance rates on individual protozoan taxa [O-15.6 ml (Fg dry wt)-I d-l] were size-dependent. Rates were greatest for taxa ~20 grn in size (mainly nanoflagellates and small ciliates). In contrast to findings for phytoplankton, no evidence emerged for grazer resistance nor growth enhancement by planktonic protozoa in response to grazing. The high flux rates for macrozooplankton on heterotrophic nanoflagellates observed in all experiments (0.2-6.0 pg C liter-’ d-l) provided evidence that a large fraction of picoplankton C may be directly transferred to higher trophic levels via a picoplankton-flagellate-zooplankton coupling. Debate continues as to whether macrozooplankton (rotifers, crustacean zooplankton) can effectively graze on components of the microbial food web (picoplankton, protozoa), and thus, whether a significant amount of C is transferred from picoplankton to higher trophic levels (e.g. Sherr et al. 1987). Recent studies suggest that picoplankton-sized cells can be ingested by macrozooplankton; however, picoplankton production is not efficiently grazed or grazing may be intermittent (Pace et al. 1990). ’ Present address for correspondence: Department of Fisheries and Aquaculture, University of Florida, 7922 NW 71st St., Gainesville 32606. 2 Present address: Department of Biology, University of Alabama, Tuscaloosa 35487. Acknowledgments We thank the crew of the RV Shenehon: B. Burns, J. Grimes, and D. Morse. Also, J. Cavaletto, M.J.P.G. R. McCormick, and T. Patton provided assistance in the field. M. Brenner, W. S. Gardner, C. L. Schelske, and H. A. Vanderploeg gave criticisms on the manuscript. Discussions with J. T. Lehman concerning expcrimental design proved helpful. Contribution 736, Great Lakes Environmental Research Laboratory. This paper is dedicated to the memory of Hunter Joseph Carrick, Sr. Moreover, the harvesting efficiency of macrozooplankton on picoplankton cells varies among age classes and species of zooplankton consumers (Pace et al. 1983). Because most grazing on picoplankton is by protozoa (Sanders et al. 1989; Fahnenstiel et al. 199 1), without information on the fate of protozoa (Weisse et al. 1990) it remains difficult to assess whether picoplankton production is an important trophic link to macrozooplankton. Information is limited on macrozooplankton predation on protozoa (see Stoecker and Capuzzo 1990). Of the studies that do exist, most are laboratory investigations which have demonstrated that single copepod species can actively graze protozoa and that a diet composed entirely of flagellated or ciliated protozoa can support growth and reproduction of the macrozooplankton species tested (e.g. Stoecker and Egloff 1987; Sanders and Porter 1990). There is also some indication that copepods prefer protozoa over algae and that macrozooplankton taxa fed seston ingest particles in the nanoto microplankton size range (e.g. Nival and Nival 1976). Little is known, however, about predation by intact zoo-