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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. 

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 Human activities have dramatically altered global patterns of nitrogen (N) and phosphorus (P) availability. This pervasive nutrient pollution is changing basal resource quality in food webs, thereby affecting rates of biological productivity and the pathways of energy and material flow to higher trophic levels.Here, we investigate how the stoichiometric quality of basal resources modulates patterns of material flow through food webs by characterizing the effects of experimental N and P enrichment on the trophic basis of macroinvertebrate production and flows of dominant food resources to consumers in five detritus‐based stream food webs.After a pre‐treatment year, each stream received N and P at different concentrations for 2 years, resulting in a unique dissolved N:P ratio (target range from 128:1 to 2:1) for each stream. We combined estimates of secondary production and gut contents analysis to calculate rates of material flow from basal resources to macroinvertebrate consumers in all five streams, during all 3 years of study.Nutrient enrichment resulted in a 1.5× increase in basal resource flows to primary consumers, with the greatest increases from biofilms and wood. Flows of most basal resources were negatively related to resource C:P, indicating widespread P limitation in these detritus‐based food webs. Nutrient enrichment resulted in a greater proportion of leaf litter, the dominant resource flow‐pathway, being consumed by macroinvertebrates, with that proportion increasing with decreasing leaf litter C:P. However, the increase in efficiency with which basal resources were channelled into metazoan food webs was not propagated to macroinvertebrate predators, as flows of prey did not systematically increase following enrichment and were unrelated to basal resource flows.This study suggests that ongoing global increases in N and P supply will increase organic matter flows to metazoan food webs in detritus‐based ecosystems by reducing stoichiometric constraints at basal trophic levels. However, the extent to which those flows are propagated to the highest trophic levels likely depends on responses of individual prey taxa and their relative susceptibility to predation. 

Abstract Increases in nitrogen (N) and phosphorus (P) availability are changing animal communities, partly by altering stoichiometric imbalances between consumers and their food. Testing relationships between resource stoichiometry and consumer assemblage structure requires ecosystem‐level manipulations that have been lacking to date.We analysed patterns of macroinvertebrate community composition in five detritus‐based headwater streams subject to experimental whole‐stream N and P additions that spanned a steep gradient in dissolved N:P ratio (2:1, 8:1, 16:1, 32:1, 128:1) over 2 years, following a 1‐year pre‐treatment period.We predicted that shifts in leaf litter stoichiometry would drive overall patterns of community composition via greater responses of shredders to enrichment than other taxa, as shredders dominate primary consumer biomass and experience larger consumer–resource elemental imbalances than other functional groups in stream ecosystems. Specifically, we expected litter C:P to be a significant predictor of shredder biomass given the greater relative imbalances between shredder and litter C:P than C:N. Finally, we tested whether shredder responses to enrichment were related to other taxon‐level traits, including body size and stoichiometry, larval life span and growth rate.Whole‐community composition shifted similarly across the five streams after enrichment, largely driven by increased shredder and predator biomass. These shifts were limited to the autumn/winter seasons and related to decreased leaf litter C:P, highlighting important links between the quality of seasonal litter subsidies and community phenology.Among 10 taxa that drove structural shifts, two declined while other taxa from the same functional/taxonomic groups responded positively, suggesting that specific life‐history traits may determine sensitivity to enrichment.Increases in total shredder biomass, and in biomass of several common shredders, were associated with lower litter C:P. Body C:P did not predict shredder response to enrichment. However, weak negative relationships between shredder response and body size, and larval life span, suggest that small‐bodied and short‐lived taxa may be more responsive to shifting resource stoichiometry.Moderate anthropogenic increases in N and P availability affect resource stoichiometry and can alter animal communities, influencing additional food web and ecosystem properties. We provide support for ecological stoichiometry as a framework for predicting such outcomes based on changes in the elemental composition of resource pools. Aplain language summaryis available for this article.