Trophic interactions involving protozoa are thought to link the classical (phytoplankton, zooplankton, fish) and the microbial (bacteria and protozoa) components of aquatic food webs. The nature and strength of these interactions were investigated in 1) a meta-analysis study which quantitatively integrated the results of 389 enclosure studies on mesozooplankton effects on planktonic algae, protozoa and bacteria; 2) a series of microcosm experiments designed to determine grazing and growth parameters for mesozooplankton, microzooplankton and phytoplankton under different environmental conditions; and 3) a mesocosm experiment investigating effects of planktivorous fish (golden shiners) on mesozooplankton, ciliates, phytoplankton, and nutrients.

The meta-analysis revealed greater mesozooplankton suppression of protozoa than of algae, and a nonsignificant stimulation of bacteria. Mesozooplankton effects were found to vary with study lake, zooplankton taxonomic group, nutrient availability and lake trophic status. There was evidence for trophic cascades from mesozooplankton via protozoa to bacteria and phytoplankton.

The microcosm experiments combined factorial with dilution experimental designs which made it possible to quantify in situ microzooplankton (protozoa and rotifers) grazing in the presence of mesozooplankton. The results suggest that in mesotrophic lakes, microzooplankton are important grazers on phytoplankton as well as on members of their own community. They may at times be responsible for virtually all grazing on phytoplankton. Microzooplankton were also important prey organisms for mesozooplankton in our experiments. Lake monitoring results supported these experimental findings. Tight coupling between all three plankton components was detected in the epilimnion as well as in the hypolimnion. Hypolimnetic trophic dynamics may greatly influence the cycling of energy, carbon, and nutrients in deep, clear lakes and need further study.

In the mesocosm experiment, golden shiners had a strong negative impact on crustacean zooplankton, a variable impact on rotifers, weaker positive impacts on ciliates and phytoplankton, and no measurable impact on inorganic nutrient concentrations. These results provide evidence for cascading trophic interactions from fish to protozoa in mesotrophic lakes.

In summary, these investigations show close connections between the microbial and the classical components of aquatic food webs and contribute to a more complete understanding of the functioning of aquatic food webs.