Predicted increases in stream temperature are expected to accelerate organic matter decomposition in rivers by up to 21%. Although increased breakdown rates are likely, less is known about their effects on stream food webs. Carbon (C) use efficiency for organisms measures the ratio of carbon incorporated as biomass relative to the amount consumed. This efficiency is crucial for decomposer organisms (microbes and macroinvertebrates) in detrital food webs because it determines how much C is retained in stream biota rather than respired as CO₂.
We investigated how temperature affects C use efficiency in microbes and shredding macroinvertebrates as they process decomposing leaf litter in artificial stream chambers. We maintained six distinct temperature treatments and used two leaf species labeled with ¹³C to trace C through multiple decomposition pathways.
We measured microbial respiration and biomass, caddisfly feeding rates, C assimilation, biomass, and total litter C loss across a temperature gradient ranging from 2.4 to 15.4 °C. We hypothesized that increasing temperature would accelerate decomposition and decrease C use efficiency in stream biota. We expected both microbes and shredding macroinvertebrates to consume and respire more C at higher temperatures, but that a smaller proportion would be retained as biomass.
Our results showed that warming accelerated C loss from litter and reduced C use efficiency for both decomposer groups. For microbes, temperature-driven increases in respiration exceeded increases in biomass. For caddisflies, higher temperatures increased consumption, C assimilation, and biomass. C use efficiency initially increased with temperature for caddisflies, but beyond ~9 °C, further increases in consumption and assimilation were not matched by additional biomass gain. This imbalance resulted in reduced C use efficiency at higher temperatures. Our findings provide new insights into how climate change may reshape river food webs by altering decomposer efficiency.