Can current velocity mediate trophic cascades in a mountain stream?

Abstract

Summary Trophic cascades can be context‐dependent in that they occur under some conditions but not others. An important context for stream benthic communities is near‐bed current velocity, which varies at the spatial scale at which benthic species interactions occur. To investigate how centimetre‐scale variation in near‐bed current velocity influenced the occurrence of benthic cascades, I studied a stonefly/mayfly/algal system in a high‐gradient Rocky Mountain stream and examined the effect of 10 and 25 cm s−1 current (i.e. ‘slow’ and ‘fast’, respectively) on stonefly predation and its top‐down effects. There were two experiments using stream‐side mesocosms and a third using in‐stream troughs. The two mesocosm experiments had known numbers of stoneflies and mayflies and examined how near‐bed current influenced (i) top‐down cascades driven by non‐trophic, behaviourally mediated predator effects (stonefly mouthparts were glued shut) and (ii) direct predation. The in‐stream experiment allowed mayfly grazers to arrive and depart freely and examined how near‐bed current and stoneflies influenced algal accumulation and community structure. Results from these experiments showed that top‐down cascades occurred in slow, but not fast current, and that these cascades are driven largely by the non‐trophic effects of predation. The mesocosm experiment showed that mayflies removed less algae in slow current when stoneflies with glued mouthparts were present. There was 33% greater algal biomass in slow compared with fast current, and algal biomass in the slow current did not differ from that in the grazer‐free controls. The predation experiment showed that stoneflies captured approximately twice as many mayflies in slow as compared to fast current. The in‐stream experiment found that algal biomass increased when stoneflies were present, but grazer densities did not differ from stonefly‐free controls, supporting the hypothesis that effects mediated by behaviour drove the cascade. This study suggests that near‐bed current can turn stonefly/mayfly/algae cascades ‘on’ and ‘off’ at small spatial scales and provides a new perspective for understanding algal heterogeneity on natural streambeds.