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Abstract Saprotrophic fungi play important roles in transformations of carbon (C), nitrogen (N), and phosphorus (P) in aquatic environments. However, it is unclear how warming will alter fungal cycling of C, N, and P. We conducted an experiment with four aquatic hyphomycetes (Articulospora tetracladia, Hydrocina chaetocladia, Flagellospora sp., and Aquanectria penicillioides), and an assemblage of the same taxa, to test how temperature alters C and nutrient use. Specifically, we evaluated biomass accrual, C:N, C:P, δ13C, and C use efficiency (CUE) over a 35-d experiment with temperatures ranging from 4ºC to 20ºC. Changes in biomass accrual and CUE were predominantly quadratic with peaks between 7ºC and 15ºC. The C:P of H. chaetocladia biomass increased 9× over the temperature gradient, though the C:P of other taxa was unaffected by temperature. Changes in C:N were relatively small across temperatures. Biomass δ13C of some taxa changed across temperatures, indicating differences in C isotope fractionation. Additionally, the 4-species assemblage differed from null expectations based on the monocultures in terms of biomass accrual, C:P, δ13C, and CUE, suggesting that interactions among taxa altered C and nutrient use. These results highlight that temperature and interspecific interactions among fungi can alter traits affecting C and nutrient cycling. 

Abstract Understanding the observed temperature dependence of decomposition (i.e., its apparent activation energy) requires separation of direct effects of temperature on consumer metabolism (i.e., the inherent activation energy) from those driven by indirect seasonal patterns in phenology and biomass, and by longer‐term, climate‐driven shifts in acclimation, adaptation, and community assembly. Such parsing is important because studies that relate temperature to decomposition usually involve multi‐season data and/or spatial proxies for long‐term shifts, and so incorporate these indirect factors. The various effects of such factors can obscure the inherent temperature dependence of detrital processing. Separating the inherent temperature dependence of decomposition from other drivers is important for accurate prediction of the contribution of detritus‐sourced greenhouse gases to climate warming and requires novel approaches to data collection and analysis. Here, we present breakdown rates of red maple litter incubated in coarse‐ and fine‐mesh litterbags (the latter excluding macroinvertebrates) for serial approximately one‐month increments over one year in nine streams along a natural temperature gradient (mean annual: 12.8°–16.4°C) from north Georgia to central Alabama, USA. We analyzed these data using distance‐based redundancy analysis and generalized additive mixed models to parse the dependence of decomposition rates on temperature, seasonality, and shredding macroinvertebrate biomass. Microbial decomposition in fine‐mesh bags was significantly influenced by both temperature and seasonality. Accounting for seasonality corrected the temperature dependence of decomposition rate from 0.25 to 0.08 eV. Shredder assemblage structure in coarse‐mesh bags was related to temperature across both sites and seasons, shifting from “cold” stonefly‐dominated communities to “warm” communities dominated by snails or crayfish. Shredder biomass was not a significant predictor of either coarse‐mesh or macroinvertebrate‐mediated (i.e., coarse‐ minus fine‐mesh) breakdown rates, which were also jointly influenced by temperature and seasonality. Unlike fine‐mesh bags, however, temperature dependence of litter breakdown did not differ between models with and without seasonality for either coarse‐mesh (0.36 eV) or macroinvertebrate‐mediated (0.13 eV) rates. We conclude that indirect (non‐thermal) seasonal and site‐level effects play a variable and potentially strong role in shaping the apparent temperature dependence of detrital breakdown. Such effects should be incorporated into studies designed to estimate inherent temperature dependence of slow ecological processes. 

Abstract. Heterotrophic microbes play key roles in regulating fluxes ofenergy and nutrients, which are increasingly affected by globally changingenvironmental conditions such as warming and nutrient enrichment. While theeffects of temperature and nutrients on microbial mineralization of carbonhave been studied in some detail, much less attention has been given to howthese factors are altering uptake rates of nutrients. We used laboratoryexperiments to simultaneously evaluate the temperature dependence of solublereactive phosphorus (SRP) uptake and respiration by leaf-litter-associatedmicrobial communities from temperate headwater streams. Additionally, weevaluated the influence of the initial concentration of SRP on thetemperature dependence of P uptake. Finally, we used simple simulationmodels to extrapolate our results and estimate the effect of warming and Pavailability on cumulative gross uptake. We found that the temperaturedependence of P uptake was lower than that of respiration (0.48 vs. 1.02 eV). Further, the temperature dependence of P uptake increased with theinitial concentration of SRP supplied, ranging from 0.12 to 0.48 eV over an11 to 212 µg L−1 gradient in initial SRP concentration.Finally, despite our laboratory experiments showing increases inmass-specific rates of gross P uptake with temperature, our simulationmodels predict declines in cumulative P uptake with warming, because theincreased rates of respiration at warmer temperatures more rapidly depletedbenthic carbon substrates and consequently reduced the biomass of thebenthic microbial community. Thus, even though mass-specific rates of P uptake were higher at the warmer temperatures, cumulative P uptake was lowerover the residence time of a pulsed input of organic carbon. Our resultshighlight the need to consider the combined effects of warming, nutrientavailability, and resource availability and/or magnitude on carbon processing asimportant controls of nutrient processing in heterotrophic ecosystems.