Terrestrial-Aquatic Coupling of River Foodwebs

about_3River networks are characterized by predictable downstream changes in environmental conditions such as habitat structure, temperature, disturbance frequency, solar radiation, and the importance of terrestrial resource subsidies. These gradients present species with physiological and behavioral opportunities and challenges, and can influence resource use, movement, survival, and ultimately the ecological role species play in river food webs. Despite nearly two decades of research demonstrating the primacy of predation by stream-dwelling fishes, we have little ability to predict how the role of such top predators should change across these spatial gradients within river networks or with the addition of single or multiple stressors. Over the past 5 years we have worked on a series of collaborative projects with graduate and undergraduate students at SFU (W. Atlas, R. Munshaw, D. Courcelles, Z. Monteith) and faculty colleagues at other institutions (J. Finlay, C. McNeely, M. Tsu, M. Power, M. Limm) combining manipulative experiments of stream food webs with broad-scale surveys and models to estimate how the strength of trophic interactions between juvenile steelhead (Oncorhynchus mykiss) and aquatic consumers change with the degree of terrestrial subsidy across river networks. We find from surveys that isotope mixing models of juvenile steelhead diets and tissue stable isotope (d13C, d15N, dD) composition predict strong changes in the reliance on terrestrial-derived carbon sources at drainage areas of 20-100km2, a position previously predicted to be a key threshold in algal productivity and the hypothesized transition from heterotrophic to autotrophic stream ecosystems. Field-based experiments in small tributaries (<20km^2) suggest that terrestrial invertebrates directly subsidize juvenile steelhead growth and abundance at the top of aquatic food webs, but do not intensify top-down control of aquatic invertebrates or primary production in these locations. We have also found that the consumptive activities of top stream predators (juvenile steelhead, coastal giant salamanders) in subsidized tributaries can also exert large effects on the availability of labile nutrients through consumer driven nutrient recycling. These results from tributaries are in contrast to experiments in rivers >100km2 demonstrating that juvenile steelhead initiate cascading strong interactions leading to changes in ecosystem productivity in years with seasonal winter disturbance. Our results collectively suggest that two key attributes that change down river networks control the importance of juvenile steelhead interactions in stream food webs, 1) the magnitude of direct terrestrial subsidy (in the form of terrestrial invertebrate prey), and 2) the proportion of aquatic consumers (herbivores, detritivores) that are vulnerable to predation by fish. We find that the ubiquitous distribution of juvenile steelhead in many river networks provides a unique opportunity to make spatially explicit predictions about top-down control in river food webs.

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