Rapidly, increasing air temperatures across the Arctic are thawing permafrost and exposing vast quantities of organic carbon, nitrogen, and phosphorus to microbial processing. Shifts in the absolute and relative supplies of these elements will likely alter patterns of ecosystem productivity and change the way carbon and nutrients are delivered from upland areas to surface waters such as rivers and lakes. The ultra-oligotrophic nature of surface waters across the Arctic renders these ecosystems particularly susceptible to changes in productivity and food web dynamics as permafrost thaw alters terrestrial-aquatic linkages. The objectives of this study were to evaluate decadal-scale patterns in surface water chemistry and assess potential implications of changing water chemistry to benthic organic matter and aquatic food webs. Data were collected from the upper Kuparuk River on the North Slope of Alaska by the U.S. National Science Foundation's Long-Term Ecological Research program during 1978-2014. Analyses of these data show increases in stream water alkalinity and cation concentrations consistent with signatures of permafrost thaw. Changes are also documented for discharge-corrected nitrate concentrations (+), discharge-corrected dissolved organic carbon concentrations (-), total phosphorus concentrations (-), and δ C isotope values of aquatic invertebrate consumers (-). These changes show that warming temperatures and thawing permafrost in the upland environment are leading to shifts in the supply of carbon and nutrients available to surface waters and consequently changing resources that support aquatic food webs. This demonstrates that physical, geochemical, and biological changes associated with warming permafrost are fundamentally altering linkages between upland and aquatic ecosystems in rapidly changing arctic environments.

Imbalances in phosphorus (P) intake relative to demand negatively affect animal growth, but their consequences are less understood for vertebrates, in which bone represents a significant and potentially flexible pool of P. Flexibility in body-P content could buffer vertebrates from the effects of imbalances between P intake and demand, reducing the likelihood of a sharp stoichiometric ''knife-edge'' in the relationship between growth rate and diet-P level. We conducted a meta-analysis of published aquaculture experiments that tested effects of diet %P on fish growth rate (49 studies, 28 species) and body-P content (27 of the studies in the main data set, 20 species). Our meta-analysis revealed significant P limitation of growth, as well as significant negative effects of excess P on growth rate. Diet-P thresholds for these effects occurred at ecologically relevant levels (optimal diet-P of 1.2% 6 0.45%, mean 6 SD, under experimental conditions of high ration). Finally, the analysis also suggested a pattern of relatively shallow relationships between growth rate and diet-P level, coupled with surprisingly flexible body-P content in fishes. This result is consistent with fish using flexible body-P content (presumably mediated through bone P) to buffer imbalances between P intake and demand. Together, our results provide evidence for a relatively ''dull'' stoichiometric ''knife-edge'' in fishes, driven in part by flexible body-P content.

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Summary Significant changes in the timing of thawing and freezing, annual discharge and nutrient supply are anticipated for arctic river ecosystems due to climate warming. Our understanding of the effects of such changes, however, is poor. Since biofilms are both critical to ecosystem processes and subject to control by combinations of freezing dynamics, discharge and nutrient supply, the effects of climate warming on arctic river ecosystems may be substantial. We studied biofilm accrual and metabolism in the Kuparuk River of Alaska with three objectives. First, we assessed biofilm activity during the open‐water season, including the poorly known ‘shoulder seasons’ (periods shortly after the spring thaw and then before autumn freezing). Second, we assessed how discharge and freezing dynamics may affect temporal patterns in biofilm activity. Third, we assessed biofilm response to increases in PO43− concentrations predicted as the climate warms. Our study consisted of an upstream reference reach and a downstream treatment reach where H3PO4 was experimentally added for two years (2011–2012; ˜0.3 μm increase). We used archived reference‐reach data from 1983 to 2014 to assess discharge–chlorophyll relationships. Mean biofilm gross primary production (GPP) in the reference reach during 2011 and 2012 was 15.4 ± 2.2 (mean ± S.E.) and 13.4 ± 2.5 mg C m−2 h−1, respectively, with spring shoulder season values being significantly higher than the summer values that are more routinely measured. An analysis of a 31‐year record from the reference reach revealed that biofilm chlorophyll biomass was significantly (R2 = 0.39, P < 0.001) related to the discharge regime of the previous summer. This ‘legacy effect’ is attributable to bedfast ice within the river channel during winter that protects biofilms from scour during the spring thaw. Before–after‐control–impact (BACI) analysis showed significant and positive PO43− effects on mean biofilm chl‐a biomass in the treatment reach. This treatment effect was associated with a parallel increase in GPP. High biofilm activity in the Kuparuk River during the shoulder seasons indicates its potential significance to the energetics of arctic river ecosystems, particularly in the context of climate change. The related ‘legacy effect’ probably underlies the scope for rapid recovery of biofilm activity during the spring. Finally, increases in PO43− at concentrations predicted as the climate warms are likely to result in increases in summer biofilm biomass and metabolism. Further studies of biofilm processes during the shoulder seasons, and how these are affected by shifts in discharge, freezing dynamics and nutrient supply, are required for a comprehensive understanding of the effects of climate warming on arctic river ecosystems.

Summary Regimes of temperature and nutrient availability are undergoing rapid modification at global scales. Both temperature and nutrients can influence consumer physiology and growth via several mechanisms. We examined how temperature and the nutrient content of food interact to affect consumption, growth and body stoichiometry of a detritivorous consumer (the caddisfly Pycnopsyche gentilis). In a 7‐week growth study, P. gentilis larvae were reared at two different temperatures (5 and 10 °C) while fed conditioned red maple (Acer rubrum) litter at one of two stoichiometric qualities (manipulated by raising phosphorus supply in one litter conditioning treatment; Amb: mean litter P = 0.03%, mean litter N = 0.79%; Hi‐P: mean litter P = 0.14%, mean litter N = 1.2%). Temperature and litter quality had differential effects on bulk consumption, element‐specific (N and P) consumption, growth and elemental body content of P. gentilis larvae. Temperature was the only factor affecting bulk feeding rates. Larvae in the Warm/Hi‐P treatment had by far the highest growth rates; the negligible growth in the Cold/Amb treatment was increased by either higher temperature (Warm/Amb) or higher food quality (Cold/Hi‐P). Higher temperature had no effect on body P content in Hi‐P treatments, but decreased body P content in the Amb treatments. Shifts in temperature and resource quality are both important components of global change and our results show that these factors can have interactive effects on detrital food webs, through which most primary production flows.