Effects of temperature increase on the grazing of planktonic bacteria by biofilm-dwelling consumers

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 1 m(-2) biofilm d(-1) (in summer) at ambient temperature, These rates increased with test temperature, even at temperatures (> 30 degrees 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.