Wildfire Effects on Aquatic Biogeochemistry in the Yukon-Kuskokwim Delta, Alaska

Northern high-latitude deltas are hotspots of ecological diversity, biogeochemical processing, and, in Alaska’s Yukon-Kuskokwim Delta (YKD), tundra wildfire. Despite strong terrestrial-aquatic connectivity in the YKD, wildfire effects on aquatic biogeochemistry are relatively unknown. Here, we assess the impacts of recent (2015) wildfire on aquatic biogeochemistry in YKD tundra using a multiyear (2015–2019) dataset of water chemistry (n= 398) from five aquatic environments: peat plateaus, fen ponds, fen channels, lakes, and streams. Objectives were to: (i) determine wildfire effects on hydrochemistry; (ii) compare wildfire effects on carbon and nutrient cycling in environments with relatively lower connectivity (lakes) and higher connectivity (fens) with the terrestrial landscape; and (iii) characterize multiyear post-fire trends in aquatic biogeochemistry. Hydrochemistry varied significantly among aquatic environments and wildfire was associated with consistent increases in pH (mean = +0.25 pH units), conductivity (6 µS cm-1, +20%), NH4+ (2.4 µM, +140%), and NO3– (1.4 µM, +70%) concentrations, and decreases in dissolved organic carbon (320 µM, –25%), CO2(90 µM, –57%) and d-excess of water isotopes (1.7, –50%). Considering hydrochemical constituents together using redundancy analysis, separation between lakes (high pH, dissolved oxygen, NH4+) and fens (high DOC, CO2, CH4) appeared more strongly driven by underlying watershed characteristics (e.g. landscape connectivity) than by wildfire. Moderation of wildfire effects by watershed characteristics was reflected in contrasting multiyear post-fire trends in conductivity, DOC, and CO2 for lakes and fen ponds. Broadly, wildfire in the YKD enhances inorganic biogeochemical cycles via soil organic matter combustion, reduced biotic CO2 production, enhanced nitrogen export to freshwaters, increased water residence time in soils, and greater mineral weathering. These results highlight the important of landscape characteristics and connectivity in driving biogeochemical cycling in northern aquatic ecosystems, and the potential for intensifying wildfire regimes to substantially alter the functioning of these systems.