Fate of terrestrially-derived carbon in freshwaters across the Yukon Kuskokwim Delta, Alaska

Permafrost soils that underlay much of the Arctic, store large amounts of carbon (C) in the form of organic matter. As climate change warms the Arctic, soil temperatures are expected to rise and wildfires to increase in importance. Both factors are likely to accelerate permafrost thaws; releasing stored organic C. Natural processes, such as hydrological transport through soils, permit C to move freely through the landscape as dissolved organic carbon (DOC). Once DOC reaches freshwater systems, processes such as photo-mineralization, photo-degradation and microbial decomposition may transform or degrade DOC, and either release it into the atmosphere as greenhouse gasses, CO₂ and CH₄, or downstream as smaller, more labile organic forms. However, the factors that determine the fate of DOC entering freshwaters in the Arctic are still not well understood. To address these uncertainties, we investigated patterns of DOC concentration, and the impacts of light and microbes on DOC dynamics in ponds in the Yukon-Kuskokwim Delta in southwestern Alaska. We collected 146 water samples from pore water wells and surface water in fens, ponds and lakes; in both unburned areas and areas that burned in 1972 and 2015. All samples were analyzed for DOC, colored dissolved organic matter (CDOM), nutrients and dissolved gas concentrations. We also conducted and experiment with water samples collected from several fens that were filter sterilized, then irradiated under artificial sunlight (36 hours) with dark controls; afterwards, irradiated and dark treated waters were inoculated with ambient microbial communities. For these samples dissolved inorganic C was measured in acidified samples to quantify DOC loss. Burned areas showed a greater rate of photodegradation than those of unburned nature, as well as microbial respiration. These results highlight the importance of understanding how fire in tundra ecosystems, which is predicted to increase with climate change, can fundamentally change how light and microbes interact to process organic carbon released from thawing permafrost. Given the size of the organic carbon pool stored in permafrost, predicting its fate as permafrost thaws is critical to assessing the global impacts of climate change.