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Abstract
Temperature is an important control on biological processes and affects ecosystem structure and function across scales. The Metabolic Theory of Ecology (MTE) provides a framework for predicting how temperature effects on metabolism scale up to ecosystem processes, but the conformity of biological rates to MTE predictions varies due to system-specific characteristics, antecedent conditions, and species' thermal preferences. Headwater streams, where detritivorous invertebrates (detritivores) and microorganisms contribute to carbon (C) storage, transport, and emissions, are hotspots of organic matter processing and thus, essential to study in the context of global change. Here, I examined temperature effects on (1) leaf litter breakdown mediated by detritivores versus microorganisms, (2) invertebrate community structure, and (3) detritivore thermal physiology in streams across a 2185-hectare basin in the southern Appalachian mountains (North Carolina, USA). I also quantified how leaf characteristics (namely carbon-to-nutrient (C:N) ratios) modulate temperature effects on leaf breakdown and detritivore physiology. I found that the temperature dependence of leaf breakdown mediated by detritivores was higher than both MTE predictions and the temperature dependence of microbial breakdown. In accordance with theoretical predictions, the breakdown of higher-C:N leaf litter had a higher temperature dependence than the breakdown of lower C:N litter. When examining temperature effects on aquatic invertebrate community structure across a landscape gradient, I found that temperature was a significant predictor of community dissimilarity in the summer, but not in the winter. Finally, measuring thermal responses of the stonefly Tallaperla (Plecoptera: Peltoperlidae), I found that survival probability declined with temperature, development rate increased with temperature, growth rate was highest at intermediate temperatures (n.s.), and temperature effects on consumption rate depended on leaf characteristics. These results imply that C routing to detritivores may increase relative to current conditions as stream temperatures rise. However, summer aquatic invertebrate communities, particularly long-lived detritivore taxa, may be vulnerable in the face of global change due to periods of increasingly high temperatures, increases in detrital C:N, and depletion of detrital resources earlier in the year.