Todd Wellnitz1 and William Hintz2 Do small-scale trophic cascades contribute to benthic heterogeneity in streams? Todd Wellnitz1 and William Hintz2 1 Department of Biology, University of Wisconsin- Eau Claire, Eau Claire, WI; 2 Fisheries and Illinois Aquaculture Center, Southern Illinois University, Carbondale, IL The Context Findings Trophic cascades are known to be mediated by environmental conditions, but have generally been examined at large scales (> 10 m). Where fine-grained environmental heterogeneity exists, however, small-scale cascades may create mosaics of high and low primary producer biomass. “Patchy” cascades may be particularly important for stream benthic habitats where heterogeneous near-bed current may influence species interactions at local scales. Conclusions Experiment 1. Near-bed current velocity affected algal accrual and interacted with Megarcys to influence Baetis grazing. As compared to “fast” current (25 cm s-1), cobbles in “slow” current (12 cm s-1) had 53% more algae after 3 d; however, when Megarcys was present Baetis removed less algae (ANOVA, p < 0.001). The presence of Megarcys affected algal removal through an interaction with near-bed current such that a top-down cascade occurred in slow, but not fast current. This cascade appears to have behavioral as well as functional components, since Megarcys could not feed in the first experiment. These results suggest that spatial variation in near-bed current may contribute to heterogeneity in algal biomass through small-scale trophic cascades. It also suggests that the detection of trophic cascades in natural systems may be scale-dependent. Questions We investigated a stonefly-mayfly-algae cascade in streamside mesocosms to replicate conditions in Copper Creek, a high-gradient mountain stream near the Rocky Mountain Biological Laboratory in Gothic, CO. We addressed two questions: Can near-bed current velocity mediate trophic cacades on the streambed ? Does near-bed current influence prey capture rates by stonefly predators? Relevance Experiment 2. Megarcys was a more effective predator in slow current. After 12 h, these stoneflies consumed an average of 4 more Baetis in slow than in fast current (t-test, p < 0.01). To understand how stream discharge influences stream function it is critical to understand the ways in which current velocity affects species interactions. Flow and current define stream ecosystems and if the discharge regimes are altered, the distribution of near-bed current across the streambed will change. 40 30 20 10 50 cm Near-bed current velocity (cm s-1) The figure at left shows near-bed current measured across the same section of streambed at 4.8 and 1.3 m3 s-1 discharge Note that at high discharge near-bed current is not just slower, but more homogeneous. Experiments Algae was colonized on stream cobbles placed in 24 circular channels in which average near-bed current velocity was set to either 11 or 25 cm s-1 (“slow” and “fast”, respectively). Baetis mayflies were stocked to match mean streambed densities in Copper Creek. Stonefly treatments had two Megarcys/channel. Experiment 1 employed a factorial design with slow and fast velocities and 3 trophic treatments: algae only, algae + Baetis, and algae + Baetis + Megarcys. Megarcys could not feed because their mouthparts were glued shut. Experiment 2 examined Baetis prey capture by Megarcys at the two velocities. Experiment 3 examined trophic-level interactions across a 2-38 cm s-1 range. Experiment 3. Across a range of near-bed velocity, trophic treatment, current velocity, and the interaction between these factors had significant effects on epilithic algae accrual (ANCOVA, p < 0.0001). A separation of effects was evident at velocities above 25 cm s-1. r2 = 0.66 r2 = 0.36 Acknowledgements Many thanks to the Rocky Mountain Biological Laboratory for allowing us access to Copper Creek and the use of its facilities. This work was funded by the UW-Eau Claire Center of Excellence for Faculty and Undergraduate Student Research Collaboration, the National Science Foundation (DEB-06-42512), and NSF’s Research Experience for Undergraduates program. Special thanks to Nia Gibson, Megan Ring, Shantee Scheel, Brandon Trimbell and Katie Weber, the undergraduate researchers who helped make this project possible and fun.