Physiographic Controls and Wildfire Effects on Aquatic Biogeochemistry in Tundra of the Yukon‐Kuskokwim Delta, Alaska

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 CO2in 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. 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 CO2were 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. 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) Variation in hydrochemistry across tundra aquatic environments was more strongly coupled to physiographic factors than to wildfire effects Wildfire generally reduced dissolved organic carbon (DOC) and CO2, and enhanced ion mobility and nitrogen cycling, but effects varied Wildfire effects on some hydrochemical constituents persisted beyond 3 years (e.g., DOC, CO2) and were brief for others (conductivity)