SUMMARY.1. Effects of two size‐classes of micrometazoa (assemblages of rotifers and of rotifers/cladocerans/copepods) on the abundance and composition of a co‐occurring, substrate‐associated protistan assemblage were assessed by selectively transferring these groups from a small pond to laboratory microcosms.2. Addition of micrometazoa to the microcosms resulted in significant changes in the dominance of different protistan groups within 7 days, including a substantial decrease in the abundance of larger omnivorous and bactivorous Protozoa, mostly ciliates, and dominant algal taxa, and an increase in the abundance of heterotrophic microflagellates.3. Mechanisms whereby micrometazoa reduce the abundance of larger heterotrophic protists may include competition for overlapping resources, but probably involves interference and predation as well. Positive effects of Metazoa on the abundance of heterotrophic microflagellates may result from the elimination of ciliates that prey on and compete with smaller Protozoa.4. Interactions indicated in this study may alter both quantitative and qualitative aspects of energy flow and mineral cycling in benthic food‐webs and be part of a larger trophic cascade involving other Metazoa such
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