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Intensifying permafrost thaw alters carbon cycling by mobilizing large amounts of terrestrial substrate into aquatic ecosystems. Yet, few studies have measured aquatic carbon fluxes and constrained drivers of ecosystem carbon balance across heterogeneous Arctic landscapes. Here, we characterized hydrochemical and landscape controls on fluvial carbon cycling, quantified fluvial carbon fluxes, and estimated fluvial contributions to ecosystem carbon balance across 33 watersheds in four ecoregions in the continuous permafrost zone of the western Canadian Arctic: unglaciated uplands, ice‐rich moraine, and organic‐rich lowlands and till plains. Major ions, stable isotopes, and carbon speciation and fluxes revealed patterns in carbon cycling across ecoregions defined by terrain relief and accumulation of organics. In previously unglaciated mountainous watersheds, bicarbonate dominated carbon export (70% of total) due to chemical weathering of bedrock. In lowland watersheds, where soil organic carbon stores were largest, lateral transport of dissolved organic carbon (50%) and efflux of biotic CO2 (25%) dominated. In watersheds affected by thaw‐induced mass wasting, erosion of ice‐rich tills enhanced chemical weathering and increased particulate carbon fluxes by two orders of magnitude. From an ecosystem carbon balance perspective, fluvial carbon export in watersheds not affected by thaw‐induced wasting was, on average, equivalent to 6%–16% of estimated net ecosystem exchange (NEE). In watersheds affected by thaw‐induced wasting, fluvial carbon export approached 60% of NEE. Because future intensification of thermokarst activity will amplify fluvial carbon export, determining the fate of carbon across diverse northern landscapes is a priority for constraining trajectories of permafrost region ecosystem carbon balance. Plain Language Summary: Freshwaters are a main component of the global carbon cycle and climate. Yet, their role in climate change is uncertain in permafrost regions, where thaw is releasing large amounts of carbon and enabling production of climate‐warming greenhouse gases. To reduce uncertainty, we measured stream chemistry and carbon fluxes across four ecoregions including a global hotspot of permafrost thaw in the western Canadian Arctic. Comparing across ecoregions, lowlands were strong sources of biological carbon dioxide and methane to the atmosphere; mountain rivers ferried products from chemical rock weathering downstream; and streams affected by permafrost thaw‐induced wasting transported large amounts of particulate carbon. Typical of northern ecosystems, carbon fluxes in non‐thaw‐affected streams were equivalent to 6%–16% of carbon uptake by terrestrial vegetation. However, in watersheds affected by thaw‐induced wasting, carbon fluxes were up to 100 times higher and approached 60% of vegetation carbon uptake. Together, these findings reveal that thermokarst‐susceptible terrains are poised to emerge as a significant component of the Arctic ecosystem carbon balance and global climate feedbacks, due to hydrologic carbon fluxes that will intensify as permafrost thaw accelerates. Key Points: Permafrost landscape history regulates fluvial carbon (C) sources and export across a transect of continental glaciation and thermokarstFluvial C flux equaled 6%–16% of vegetation C uptake in most watersheds and 60% in those affected by permafrost thaw‐driven mass wastingThaw‐susceptible terrains will be significant to Arctic ecosystem carbon balance as permafrost thaw and hydrologic carbon fluxes intensify [ABSTRACT FROM AUTHOR]

Northern high‐latitude deltas are hotspots of biogeochemical processing, terrestrial‐aquatic connectivity, and, in Alaska's Yukon‐Kuskokwim Delta (YKD), tundra wildfire. Yet, wildfire effects on aquatic biogeochemistry remain understudied in northern delta regions, thus limiting a more comprehensive understanding of high latitude biogeochemical cycles. In this study, we assess wildfire impacts on summertime aquatic biogeochemistry in YKD tundra using a multi‐year (2015–2019) data set of water chemistry measurements (n = 406) from five aquatic environments: peat plateau ponds, fen ponds, fen channels, lakes, and streams. We aimed to (a) characterize variation in hydrochemistry among aquatic environments; (b) determine wildfire effects on hydrochemistry; and (c) assess post‐fire multi‐year patterns in hydrochemistry in lakes (lower terrestrial‐freshwater connectivity) and fen ponds (higher connectivity). Variation in hydrochemistry among environments was more strongly associated with watershed characteristics (e.g., terrestrial‐aquatic connectivity) than wildfire. However, certain hydrochemical constituents showed consistent wildfire effects. Decreases in dissolved organic carbon (DOC) and CO2, and increases in pH, specific conductance, NH4+, and NO3− indicate that, by combusting soil organic matter, wildfire reduces organics available for hydrologic transport and microbial respiration, and mobilizes nitrogen into freshwaters. Multi‐year post‐fire variation in specific conductance, DOC, and CO2 in lakes and fen ponds suggest that watershed characteristics underlie ecosystem response and recovery to wildfire in the YKD. Together, these results indicate that increasing tundra wildfire occurrence at northern high latitudes could drive multi‐year shifts toward stronger aquatic inorganic nutrient cycling, and that variation in terrain characteristics is likely to underlie wildfire effects on aquatic ecosystems across broader scales. Plain Language Summary: Warming at northern high latitudes is expected to increase wildfire activity, permafrost thaw, and hydrologic connectivity. Yet, contrasting evidence for wildfire effects on aquatic biogeochemistry hinders a more concrete understanding of potential future changes to northern ecosystems, especially in delta environments, where land‐freshwater linkages are strong. To address this, we made ∼400 measurements of hydrochemistry within the Yukon‐Kuskokwim Delta, Alaska, following high wildfire activity in 2015. Variation in hydrochemistry among five aquatic environments—peat plateau ponds, fen ponds, fen channels, lakes, and streams—was more strongly associated with watershed characteristics (e.g., land‐freshwater linkages) than wildfire. However, wildfire consistently affected some parameters. For instance, dissolved organic carbon and CO2 were lower in burned watersheds, likely from combustion of organic matter, whereas, pH, specific conductance, and nitrogen were higher, reflecting solute release from combusted organics. Multi‐year measurements (2015–2019) allowed us to assess post‐fire variation in hydrochemistry among lakes and fen ponds, and indicate that ecosystem response and recovery to wildfire depend on watershed characteristics. Results indicate that increasing tundra wildfire at northern high latitudes could drive multi‐year shifts toward stronger aquatic inorganic nutrient cycling. Across broader spatiotemporal scales, variation in terrain characteristics is likely to underlie wildfire effects on aquatic ecosystems. Key Points: Variation in hydrochemistry across tundra aquatic environments was more strongly coupled to physiographic factors than to wildfire effectsWildfire generally reduced dissolved organic carbon (DOC) and CO2, and enhanced ion mobility and nitrogen cycling, but effects variedWildfire effects on some hydrochemical constituents persisted beyond 3 years (e.g., DOC, CO2) and were brief for others (conductivity) [ABSTRACT FROM AUTHOR]

Permafrost degradation is delivering bioavailable dissolved organic matter (DOM) and inorganic nutrients to surface water networks. While these permafrost subsidies represent a small portion of total fluvial DOM and nutrient fluxes, they could influence food webs and net ecosystem carbon balance via priming or nutrient effects that destabilize background DOM. We investigated how addition of biolabile carbon (acetate) and inorganic nutrients (nitrogen and phosphorus) affected DOM decomposition with 28‐day incubations. We incubated late‐summer stream water from 23 locations nested in seven northern or high‐altitude regions in Asia, Europe, and North America. DOM loss ranged from 3% to 52%, showing a variety of longitudinal patterns within stream networks. DOM optical properties varied widely, but DOM showed compositional similarity based on Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) analysis. Addition of acetate and nutrients decreased bulk DOM mineralization (i.e., negative priming), with more negative effects on biodegradable DOM but neutral or positive effects on stable DOM. Unexpectedly, acetate and nutrients triggered breakdown of colored DOM (CDOM), with median decreases of 1.6% in the control and 22% in the amended treatment. Additionally, the uptake of added acetate was strongly limited by nutrient availability across sites. These findings suggest that biolabile DOM and nutrients released from degrading permafrost may decrease background DOM mineralization but alter stoichiometry and light conditions in receiving waterbodies. We conclude that priming and nutrient effects are coupled in northern aquatic ecosystems and that quantifying two‐way interactions between DOM properties and environmental conditions could resolve conflicting observations about the drivers of DOM in permafrost zone waterways. Key Points: Dissolved organic matter (DOM) from diverse circumpolar regions showed compositional and optical commonalitiesAdded acetate and nutrients suppressed background DOM degradation but accelerated colored DOM breakdownDisconnect between DOM composition and function indicates intrinsic and extrinsic conditions interact to regulate DOM dynamics [ABSTRACT FROM AUTHOR]

Thermokarst can rapidly mobilize vast amounts of sediment, solutes, and organic carbon previously maintained in frozen soils to inland waters. Streams provide a critical pathway for transforming these materials into carbon dioxide (CO2) and methane (CH4), yet the direct effects of thermokarst on fluvial C gas efflux from streams to the atmosphere are largely unknown. Working on the Peel Plateau in the western Canadian Arctic, we show that CO2 efflux in rill runoff thaw streams (runoff) within retrogressive thaw slumps (RTSs) was four times greater than in adjacent streams and contributed modestly but disproportionately to efflux at the landscape scale. In contrast, CH4 efflux was generally greater in adjacent streams than in RTS runoff and, overall, was within the range of values reported for other northern streams. While RTS occurrence was a primary driver of CO2 efflux, CH4 efflux was more strongly associated with conditions reflective of biological activity. Transects downstream of two RTSs revealed that CH4 consistently and rapidly degassed to the atmosphere, while elevated CO2 was sustained downstream of one RTS feature. At the watershed scale, streams adjacent to RTSs rather than runoff streams within RTSs dominated fluvial CO2 and CH4 efflux. Intensifying thermokarst activity in the western Canadian Arctic will likely amplify contributions from runoff streams in RTSs to watershed‐scale fluvial C gas efflux. Plain Language Summary: Rapid Arctic warming is thawing perennially frozen ground (permafrost) and driving terrain subsidence in ice‐rich areas (thermokarst). In glaciated permafrost landscapes, thermokarst features called retrogressive thaw slumps, which are caused by thawing of ice‐rich permafrost, release sediment, solutes, and organic materials that have been preserved in permafrost for millennia. Following thaw, permafrost carbon can rapidly transform into the greenhouse gas carbon dioxide and methane in streams. Yet the effects of thermokarst on fluvial gas flux are largely unknown. Working on the Peel Plateau in northwest Canada, we found that carbon dioxide efflux to the atmosphere from runoff draining from thaw slumps was four times greater than in streams adjacent to thaw slumps. In contrast, methane efflux in adjacent streams was twice that in runoff streams and both were similar to streams in other northern regions. While thaw slump occurrence was a primary driver of carbon dioxide efflux, methane efflux was more strongly associated with conditions reflective of biological activity. Generally, both carbon dioxide and methane were rapidly degassed downstream. While efflux of carbon dioxide and methane in thaw slumps were a small component of watershed‐scale fluvial C gas efflux, these fluxes will likely increase as regional thermokarst activity accelerates. Key Points: CO2 efflux in thaw slump runoff streams was four times greater than in adjacent streams, but a small part of watershed‐scale effluxCH4 efflux was generally small in runoff streams, variable in adjacent streams, and associated with conditions reflecting biological activityElevated CO2 partial pressures persisted downstream of a large thaw slump but dissipated downstream of a smaller thaw slump [ABSTRACT FROM AUTHOR]

Permafrost thaw in the Arctic enables the biogeochemical transformation of vast stores of organic carbon into carbon dioxide (CO2). This CO2 release has significant implications for climate feedbacks, yet the potential counterbalance from CO2 fixation via chemical weathering of minerals exposed by thawing permafrost is entirely unstudied. We show that thermokarst in the western Canadian Arctic can enable rapid weathering of carbonate tills, driven by sulfuric acid from sulfide oxidation. Unlike carbonic acid‐driven weathering, this caused significant and previously undocumented CO2 production and outgassing in headwater streams. Increasing riverine solute fluxes correspond with long‐term intensification of thermokarst and reflect the regional predominance of sulfuric acid‐driven carbonate weathering. We conclude that thermokarst‐enhanced mineral weathering has potential to profoundly disrupt Arctic freshwater carbon cycling. While thermokarst and sulfuric acid‐driven carbonate weathering in the western Canadian Arctic amplify CO2 release, regional variation in sulfide oxidation will moderate the effects on the permafrost carbon‐climate feedback. Plain Language Summary: In the Arctic, perennially frozen ground (permafrost) in previously glaciated regions stores abundant minerals and is often ice‐rich. Therefore, this permafrost can rapidly thaw and collapse, resulting in thermokarst and exposing minerals to breakdown by chemical weathering. Mineral weathering by carbonic acid fixes CO2, making it less likely to enter the atmosphere. However, the effect of thermokarst on mineral weathering, carbon cycling, and rising atmospheric CO2 levels is unknown. We show thermokarst enhances weathering in streams in the western Canadian Arctic can rapidly produce significant and previously undocumented CO2 because carbonate weathering in this region is driven by sulfuric acid (from weathering of sulfide minerals) instead of carbonic acid. Long‐term river chemistry reveals that this weathering is intensifying as thermokarst accelerates. Across the Arctic, increasing thermokarst will profoundly impact freshwater carbon cycling, yet the influence of weathering on climate feedbacks will depend on regional variation in the mineral composition of permafrost soils. Key Points: Permafrost thaw‐driven ground collapse (thermokarst) in the western Canadian Arctic enhances carbonate weathering locally and regionallyThis weathering is driven by sulfuric acid and rapidly produces significant, previously undocumented CO2Carbonic acid‐driven weathering elsewhere can consume CO2, yet thermokarst effects on weathering, carbon cycle, and climate are unstudied [ABSTRACT FROM AUTHOR]