Temperature accelerates decomposition and controls carbon use efficiency for microbes and shredding caddisflies

Abstract Predicted increases in stream temperature are expected to accelerate organic matter decomposition in rivers. In detrital stream food webs, carbon use efficiency (CUE)—the proportion of consumed carbon (C) retained as biomass—determines whether C is stored in organisms or rapidly lost as CO 2 . We investigated how temperature affects CUE for microbes and shredding macroinvertebrates as they process decomposing leaf litter in replicated artificial stream experiments. Across two consecutive years, we maintained six temperature treatments (2.4–15.4°C) in flow‐through stream chambers, using two leaf species labeled with 13 C to trace C through multiple decomposition pathways. We measured microbial respiration and biomass, caddisfly feeding rates, C assimilation, net growth, and total litter C loss. As predicted, warming accelerated litter C loss and controlled CUE for both decomposer groups. In microbes, temperature‐driven increases in respiration exceeded gains in biomass, reducing microbial CUE. Caddisfly consumption and C assimilation increased with temperature, but biomass gains plateaued above ~9°C. Consequently, CUE peaked near this intermediate temperature and declined at higher stream temperatures, a pattern consistent across three independent metrics of CUE. Temperature sensitivity of leaf litter breakdown, assessed using Arrhenius‐based activation energies, was higher in the presence of invertebrates—particularly on recalcitrant litter—highlighting the role of animal activity in regulating thermal responses. Our results suggest that even modest stream warming can decrease decomposer efficiency, reducing C retention in stream biota and limiting energy transfer to higher trophic levels.