Summary 1. Over the course of this 17‐month study, we assessed the potential loss of plankton (bacteria, algae, heterotrophic flagellates) to consumers (ciliates and rotifers) within mature biofilms established on natural substrata exposed to the main current of the River Rhine (Germany). Once a month, in flow cells in a bypass system to the River Rhine, we measured the clearance rates of the biofilm‐associated consumers on the different groups within the natural plankton. 2. Ciliates were the most dominant consumers, among which planktivorous groups, particularly peritrichs and (in spring and summer) heterotrichs dominated. Consumer biomass varied with season, with the highest density occurring directly after the appearance of the phytoplankton spring peak. 3. Clearance rates on plankton ranged from 96 to 565 L m−2 d−1 for bacteria and 66–749 L m−2 d−1 for algae, with a preference for algae in summer and for bacteria in winter. This pattern coincided with seasonal changes in the structures of the grazer communities. The consumers (both ciliates and rotifers with total standing stocks ranging between 19 and 572 mg C m−2) imported a substantial amount of organic matter (between 15 and 137 mg C m−2 d−1) into the biofilm. 4. These results highlight the potential importance of consumers in the biofilm as a trophic link between the plankton and the benthos, a function that has hitherto mostly been attributed to filter‐feeding bivalves. In contrast to bivalves, the biofilm‐dwelling consumers show a more dynamic response towards the plankton density and composition. Such dynamic components need to be considered when estimating total plankton consumption by the benthos.

Biofilms, the complex microbial communities associated with surfaces, can play an important role in the flux of matter in running waters. In addition to bacteria and algae, heterotrophic protists and small metazoans can also occur in high abundances in biofilms. By grazing on planktonic prey, biofilm-dwelling grazers can act as a trophic link between planktonic and benthic food webs, parallel to filter-feeding benthic macroinvertebrates. Here we introduce a flow cell method for the quantification of grazing rates of semi-natural biofilms on suspended bacteria, parallel to microscopic biofilm community analysis. This technique was used to measure the temperature response of the grazing rates of complex biofilm communities (established in a bypass system to the River Rhine, Germany) on planktonic bacteria. Results showed that 14 d old biofilms (dominated by ciliates) clear planktonic bacteria at rates between 29 (in winter) and 99 l m -2 biofilm d -1 (in summer) at ambient temperature. These rates increased with test temperature, even at temperatures (> 30°C) at which the grazing rates of filter-feeding macrofauna in moderate climates are negatively influenced. The acclimation temperatures under which the biofilm developed can have a significant effect on community clearance rates, largely due to a temperature effect on the abundance and structure of the grazer community, whereas no indication of an acclimation effect on individual grazing rates could be found. In the function of bentho-pelagic couplers, biofilm-dwelling consumer communities can react to temperature changes substantially faster than macrofaunal filter feeders.

Climate models predict an increasing frequency of extremely hot summer events in the northern hemisphere for the near future. We hypothesised that microbial grazing by the metazoan macrofauna is an interaction that becomes unbalanced at high temperatures due to the different development of the grazing rates of the metazoans and the growth rates of the microbial community with increasing temperature. In order to test this hypothesis, we performed grazing experiments in which we measured the impact of increasing temperatures on the development of the grazing rates of riverine mussels in relation to the growth rates of a unicellular prey community (a natural heterotrophic flagellate community from a large river). In a first experimental series using Corbicula fluminea as a grazer and under the addition of a carbon source (yeast extract), the increase of the prey's growth rates was considerably stronger than that of the predator's grazing rates when temperatures were increased from 19 to over 25 degrees C. This was also the outcome when the mussels had been acclimatized to warm temperatures. Hereafter, specific experiments with natural river water at temperatures of 25 and 30 degrees C were performed. Again, a strong decrease of the mussels' grazing rates in relation to the flagellate growth rates with increasing temperature occurred for two mussel species (C. fluminea and Dreissena polymorpha). When performing the same experiment using a benthic microbial predator community (biofilms dominated by ciliates) instead of the benthic mussels, an increase of the grazing rates relative to the growth rates with temperature could be observed. Our data suggest that predator-prey interactions (between metazoans and microbes) that are balanced at moderate temperatures could become unbalanced at high temperatures. This could have significant effects on the structure and function of microbial communities in light of the predicted increasing frequency of summer heat waves.