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Abstract Permafrost thaw alters groundwater flow, river hydrology, stream‐catchment interactions, and the availability of carbon and nutrients in headwater streams. The impact of permafrost on watershed hydrology and biogeochemistry of headwater streams has been demonstrated, but there is little understanding of how permafrost influences fish in these ecosystems. We examined relations among permafrost characteristics, the resulting changes in water temperature, stream hydrology (e.g., discharge flashiness), and macroinvertebrates, with the abundance, biomass, and energy density of juvenile Dolly Varden (Salvelinus malma) and Arctic Grayling (Thymallus arcticus) across 10 headwater streams in northwestern Alaska. Macroinvertebrate density was driven by concentrations of dissolved carbon and nutrients supporting stream food webs. Dolly Varden abundance was primarily related to water temperature with fewer fish in warmer streams, whereas Dolly Varden energy density decreased with the flashiness of the headwater streams. Dolly Varden biomass was related to both temperature and bottom‐up food web effects. The energy density of Arctic Grayling decreased with warmer temperatures and discharge flashiness. These relations demonstrate the importance of terrestrial–aquatic connections in permafrost landscapes and indicate the complexity of landscape effects on fish. Because permafrost thaw is one of the most impactful changes occurring as the Arctic warms, an improved understanding of how stream temperature, hydrology, and bottom‐up food web processes influence fish populations can aid forecasting of future conditions across the Arctic. 

Abstract Climate change in the Arctic is altering watershed hydrologic processes and biogeochemistry. Here, we present an emergent threat to Arctic watersheds based on observations from 75 streams in Alaska’s Brooks Range that recently turned orange, reflecting increased loading of iron and toxic metals. Using remote sensing, we constrain the timing of stream discoloration to the last 10 years, a period of rapid warming and snowfall, suggesting impairment is likely due to permafrost thaw. Thawing permafrost can foster chemical weathering of minerals, microbial reduction of soil iron, and groundwater transport of metals to streams. Compared to clear reference streams, orange streams have lower pH, higher turbidity, and higher sulfate, iron, and trace metal concentrations, supporting sulfide mineral weathering as a primary mobilization process. Stream discoloration was associated with dramatic declines in macroinvertebrate diversity and fish abundance. These findings have considerable implications for drinking water supplies and subsistence fisheries in rural Alaska. 

Abstract In recent decades the habitat of North American beaver (Castor canadensis) has expanded from boreal forests into Arctic tundra ecosystems. Beaver ponds in Arctic watersheds are known to alter stream biogeochemistry, which is likely coupled with changes in the activity and composition of microbial communities inhabiting beaver pond sediments. We investigated bacterial, archaeal, and fungal communities in beaver pond sediments along tundra streams in northwestern Alaska (AK), USA and compared them to those of tundra lakes and streams in north‐central Alaska that are unimpacted by beavers.β‐glucosidase activity assays indicated higher cellulose degradation potential in beaver ponds than in unimpacted streams and lakes within a watershed absent of beavers. Beta diversity analyses showed that dominant lineages of bacteria and archaea in beaver ponds differed from those in tundra lakes and streams, but dominant fungal lineages did not differ between these sample types. Beaver pond sediments displayed lower relative abundances of Crenarchaeota and Euryarchaeota archaea and of bacteria from typically anaerobic taxonomic groups, suggesting differences in rates of fermentative organic matter (OM) breakdown, syntrophy, and methane generation. Beaver ponds also displayed low relative abundances of Chytridiomycota (putative non‐symbiotic) fungi and high relative abundances of ectomycorrhizal (plant symbionts) Basidiomycota fungi, suggesting differences in the occurrence of plant and fungi mutualistic interactions. Beaver ponds also featured microbes with taxonomic identities typically associated with the cycling of nitrogen and sulfur compounds in higher relative abundances than tundra lakes and streams. These findings help clarify the microbiological implications of beavers expanding into high latitude regions. 

Beavers build dams that change the way water moves between streams, lakes, and the land. In Alaska, beavers are moving north from the forests into the Arctic tundra. When beavers build dams in the Arctic, they cause frozen soil, called permafrost, to thaw. Scientists are studying how beavers and the thawing of permafrost are impacting streams and rivers in Alaska’s national parks. For example, permafrost thaw from beavers can add harmful substances like mercury to streams. Mercury can be taken up by stream food webs, including fish, which then become unhealthy to eat. Permafrost thaw can also move carbon (from dead plants) to beaver ponds. When this carbon decomposes, it can be released from beaver ponds into the air as greenhouse gases, which cause Earth’s climate to warm. Scientists are trying to keep up with these busy beavers to better understand how they are changing Arctic landscapes and Earth’s climate. 

Engineered nanoparticle (NP) size and natural organic matter (NOM) composition play important roles in determining NP environmental behaviors. The aim of this work was to investigate how NP size and NOM composition influence the colloidal stability of polyvinylpyrrolidone coated platinum engineered nanoparticles (PVP-PtNPs). We evaluated PVP-PtNP aggregation as a function of the NP size (20, 30, 50, 75, and 95 nm, denoted as PVP-PtNP 20–95 ) in moderately hard water (MHW). Further, we quantified the effect of the hydrophobic organic acid (HPOA) fraction of NOM on the aggregation of PVP-PtNP 20 and PVP-PtNP 95 using 6 NOM samples from various surface waters, representing a range of NOM compositions and properties. NOM samples were characterized for bulk elemental composition ( e.g. , C, H, O, N, and S), specific ultraviolet absorbance at 254 nm (SUVA 254 ), and molecular level composition ( e.g. , compound classes) using ultrahigh resolution mass spectrometry. Single particle-inductively coupled plasma-mass spectrometry (sp-ICP-MS) was employed to monitor the aggregation of PVP-PtNPs at 1 μg PVP-PtNP per L and 1 mg NOM per L concentrations. PVP-PtNP aggregate size increased with decreasing primary PVP-PtNP size, likely due to the lower zeta potential, the higher number concentration, and the higher specific surface area of smaller NPs compared to larger NPs at the same mass concentration. No aggregation was observed for PVP-PtNP 95 in MHW in the presence and absence of the different NOM samples. PVP-PtNP 20 formed aggregates in MHW in the presence and absence of the six NOM samples, and aggregate size increased in the presence of NOM likely due to interparticle bridging of NOM-coated PVP-PtNPs by divalent counterions. PVP-PtNP 20 aggregate size increased with the increase in NOM elemental ratio of H to C and the relative abundance of lignin-like/carboxyl rich-alicyclic molecules (CRAM)-like compounds. However, the aggregate size of PVP-PtNP 20 decreased with the increase in NOM molecular weight, NOM SUVA 254 , elemental ratio of O to C, and the relative abundance of condensed hydrocarbons and tannin-like compounds. Overall, the results of this study suggest that the composition and sources of NOM are key factors that contribute to the stability of PVP-PtNPs in the aquatic environment. 

Abstract. Repeated sampling of spatially distributed riverchemistry can be used to assess the location, scale, and persistence ofcarbon and nutrient contributions to watershed exports. Here, we provide acomprehensive set of water chemistry measurements and ecohydrologicalmetrics describing the biogeochemical conditions of permafrost-affectedArctic watersheds. These data were collected in watershed-wide synopticcampaigns in six stream networks across northern Alaska. Three watershedsare associated with the Arctic Long-Term Ecological Research site at ToolikField Station (TFS), which were sampled seasonally each June and August from2016 to 2018. Three watersheds were associated with the National ParkService (NPS) of Alaska and the U.S. Geological Survey (USGS) and weresampled annually from 2015 to 2019. Extensive water chemistrycharacterization included carbon species, dissolved nutrients, and majorions. The objective of the sampling designs and data acquisition was tocharacterize terrestrial–aquatic linkages and processing of material instream networks. The data allow estimation of novel ecohydrological metricsthat describe the dominant location, scale, and overall persistence ofecosystem processes in continuous permafrost. These metrics are (1)subcatchment leverage, (2) variance collapse, and (3) spatial persistence.Raw data are available at the National Park Service Integrated Resource Management Applications portal (O'Donnell et al., 2021, https://doi.org/10.5066/P9SBK2DZ) and within the Environmental Data Initiative (Abbott, 2021, https://doi.org/10.6073/pasta/258a44fb9055163dd4dd4371b9dce945). 

Abstract Most terrestrial allochthonous organic matter enters river networks through headwater streams during high flow events. In headwaters, allochthonous inputs are substantial and variable, but become less important in streams and rivers with larger watersheds. As allochthonous dissolved organic matter (DOM) moves downstream, the proportion of less aromatic organic matter with autochthonous characteristics increases. How environmental factors converge to control this transformation of DOM at a continental scale is less certain. We hypothesized that the amount of time water has spent travelling through surface waters of inland systems (streams, rivers, lakes, and reservoirs) is correlated to DOM composition. To test this hypothesis, we used established river network scaling relationships to predict relative river network flow‐weighted travel time (FWTT) of water for 60 stream and river sites across the contiguous United States (3090 discrete samples over 10 water years). We estimated lentic contribution to travel times with upstream in‐network lake and reservoir volume. DOM composition was quantified using ultraviolet and visible absorption and fluorescence spectroscopy. A combination of FWTT and lake and reservoir volume was the best overall predictor of DOM composition among models that also incorporated discharge, specific discharge, watershed area, and upstream channel length. DOM spectral slope ratio (R² = 0.77) and Freshness Index (R² = 0.78) increased and specific ultraviolet absorbance at 254 nm (R² = 0.68) and Humification Index (R² = 0.44) decreased across sites as a function of FWTT and upstream lake volume. This indicates autochthonous‐like DOM becomes continually more dominant in waters with greater FWTT. We assert that river FWTT can be used as a metric of the continuum of DOM composition from headwaters to rivers. The nature of the changes to DOM composition detected suggest this continuum is driven by a combination of photo‐oxidation, biological processes, hydrologically varying terrestrial subsidies, and aged groundwater inputs.