The Temperature Sensitivity of Leaf Litter Breakdown in Streams and Rivers with Variable nutrient Supply

A significant fraction of terrestrial carbon is processed and stored in surface waters. Elevated global temperature and eutrophication are major changes that influence the processing of carbon in streams and rivers through the breakdown of organic matter. Independently, increases in temperature and moderate levels of nutrient availability stimulate rates of leaf litter breakdown. However, it is unclear how these two factors interact to influence the breakdown of leaf litter in streams and rivers worldwide. Recent studies show that the temperature sensitivity of leaf litter breakdown in streams and rivers varies from 0.34-0.46 eV when mediated by microbes and 0-0.34 eV when mediated by microbes and detritivores combined. These values are lower than the canonical value predicted by metabolic theory (0.65 eV). We assembled a dataset containing 610 paired observations of leaf litter breakdown in fine and coarse mesh bags amassed from 57 studies conducted between the years 1982-2017 along a broad latitudinal gradient (19° S and 65° N). We isolated the effect of detritivore- and microbe-mediated breakdown from total breakdown and then calculated the Ea of each breakdown coefficient for oligotrophic, mesotrophic, and eutrophic streams and rivers. We are the first to compare the temperature sensitivity of leaf litter breakdown mediated by microbes and detritivores in isolation from one another and to assess whether nutrient enrichment affects the temperature sensitivity of leaf litter breakdown. Apparent temperature sensitivities were derived from measures of activation energy using the Arrhenius equation normalized by a standard temperature (10 °C). We found an activation energy of 0.51 ± 0.04 eV (95% CI: 0.42 - 0.59 eV) common amongst breakdown coefficients and trophic categories. The similarity between temperature sensitivities of leaf litter breakdown mediated by microbes and detritivores suggests that the balance of carbon respired to the atmosphere versus transported downstream should not shift as global temperatures increase, regardless of the trophic status of lotic ecosystems. This consistency also suggests that leaf litter assays have broad utility for assessing thermal pollution in streams and rivers or evaluating the efficacy of efforts to mitigate thermal pollution.