Abstract
We investigated the role of a macrograzer (the filter feeding mussel Dreissena polymorpha ) in mediating effects of high summer temperatures on the dominant components of natural river plankton (i.e., bacteria, algae, and heterotrophic flagellates) in flow channel experiments. Effects of adaptation (by comparing mussels from a southern and a northern population) and thermal acclimation of the mussels were considered. Both heterotrophic flagellates and algae are released from grazing pressure and increase in abundance at temperatures above 20°C. Bacterial abundance, however, decreased with increasing temperature, suggesting a trophic cascade (mussel‐flagellates‐bacteria) that is altered by the temperature response of the mussel ingestion rate. Warm acclimation of the mussels did not change the outcome of the experiments. The dreissenids from the southern population showed a significantly higher ingestion rate than those from the northern population only in July. The general pattern (i.e., decreasing ingestion rates at high temperatures) was found in both populations. Microbial communities controlled by macrofauna can experience substantial changes in warm summers because of differential development of direct and indirect grazing effects with increasing temperature.