Phenology changes are increasingly recognized as a common response of species to ongoing global change. Phenology can be influenced by environmental cues that impact the initiation or duration of life history events as well as intrinsic organismal traits that may affect how different species respond to such environmental cues. Despite the importance of phenology for biodiversity conservation as demonstrated by terrestrial and marine research, freshwater phenology is understudied. Therefore, we conducted a literature review on freshwater phenology research to summarize the spatial, taxonomic and temporal biases of studies; as well as relationships between phenology metrics, environmental cues and intrinsic species traits studied in these systems. We find that phenology research in freshwaters may be limited by a lack of long‐term time‐series data, especially in lotic habitats. Phenology metrics studied differed between lotic and lentic habitats, with limnological research focused on planktonic population growth whereas macroinvertebrate emergence and fish spawning seasons are the most frequently studied aspects of phenology in streams and rivers. Across habitats, temperature is the most investigated environmental cue, with additional research attention to resources and hydrology in influencing phenology events in lentic and lotic environments, respectively. Knowledge gaps in contemporary freshwater phenology research include relationships between phenology and environmental cues in tropical systems, understanding of non‐salmonid fish phenology and testing hypotheses related to intrinsic traits. We recommend that future research broaden the biological, spatial and temporal scales of phenology studies in these systems, and make use of novel data sources, methods and technologies to address contemporary research gaps.

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Abstract. Heterotrophic microbes play key roles in regulating fluxes of energy and nutrients, which are increasingly affected by globally changing environmental conditions such as warming and nutrient enrichment. While the effects of temperature and nutrients on microbial mineralization of carbon have been studied in some detail, much less attention has been given to how these factors are altering uptake rates of nutrients. We used laboratory experiments to simultaneously evaluate the temperature dependence of soluble reactive phosphorus (SRP) uptake and respiration by leaf-litter-associated microbial communities from temperate headwater streams. Additionally, we evaluated the influence of the initial concentration of SRP on the temperature dependence of P uptake. Finally, we used simple simulation models to extrapolate our results and estimate the effect of warming and P availability on cumulative gross uptake. We found that the temperature dependence of P uptake was lower than that of respiration (0.48 vs. 1.02 eV). Further, the temperature dependence of P uptake increased with the initial concentration of SRP supplied, ranging from 0.12 to 0.48 eV over an 11 to 212 µg L−1 gradient in initial SRP concentration. Finally, despite our laboratory experiments showing increases in mass-specific rates of gross P uptake with temperature, our simulation models predict declines in cumulative P uptake with warming, because the increased rates of respiration at warmer temperatures more rapidly depleted benthic carbon substrates and consequently reduced the biomass of the benthic microbial community. Thus, even though mass-specific rates of P uptake were higher at the warmer temperatures, cumulative P uptake was lower over the residence time of a pulsed input of organic carbon. Our results highlight the need to consider the combined effects of warming, nutrient availability, and resource availability and/or magnitude on carbon processing as important controls of nutrient processing in heterotrophic ecosystems.

Abstract. Heterotrophic microbes play key roles in regulating fluxes of energy and nutrients, which are increasingly affected by globally changing environmental conditions such as warming and nutrient enrichment. While the effects of temperature and nutrients on microbial mineralization of carbon have been studied in some detail, much less attention has been given to how these factors are altering uptake rates of nutrients. We used laboratory experiments to simultaneously evaluate the temperature dependence of soluble reactive phosphorus (SRP) uptake and respiration by leaf litter-associated microbial communities from temperate headwater streams. Additionally, we evaluated the influence of the initial concentration of SRP on the temperature dependence of P uptake. Finally, we used simple simulation models to extrapolate our results and estimate the effect of warming and P availability on cumulative gross uptake at the ecosystem level. We found that the temperature dependence of P uptake was lower than that of respiration (0.48 vs. 1.02 eV). Further, the temperature dependence of P uptake increased with the initial concentration of SRP supplied, ranging from 0.12–0.48 eV over a 11–212 µg L-1 gradient in initial concentrations. Finally, despite our laboratory experiments showing increases in mass-specific rates of gross P uptake with temperature, our simulation models found declines in cumulative P uptake with warming because the increased rates of respiration at warmer temperatures more rapidly depleted benthic carbon substrates and consequently reduced the biomass of the benthic microbial community. Thus, even though mass-specific rates of P uptake were higher at the warmer temperatures, cumulative ecosystem-level P uptake was lower over the residence time of a pulsed input of organic carbon. Our results highlight the need to consider the combined effects of warming, nutrient availability, and resource availability/magnitude on carbon processing as important controls of nutrient processing.

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