Signatures of Arctic Change: Molecular‐Level Composition and Bioavailability of Shifting Dissolved Organic Matter Sources

Abstract The Arctic is experiencing unprecedented rates of climate change, leading to numerous disturbances on the terrestrial landscape, including shrubification, increased frequency of wildfires, and permafrost thaw. These changes may impact the mobilization of terrestrial organic carbon into Arctic rivers and are hypothesized to lead to distinct alterations to the molecular composition and thus the reactivity of riverine dissolved organic matter (DOM). To understand how these three major perturbations may impact DOM dynamics in Arctic fluvial and coastal systems, we examined the concentration and bioavailability of dissolved organic carbon (DOC) together with the molecular‐level DOM composition of different source endmember leachates from the Yukon River watershed using biodegradation incubation experiments and Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS). Simulated climate‐related landscape perturbations generally led to increased leachate DOC concentrations. Incubations demonstrated that the biodegradability of leachate DOC was lowest for vegetation endmembers, particularly for shrubs (12.3% DOC loss), and highest for thawing Yedoma permafrost (64.9% loss) and organic‐rich tundra soil (70.9% loss). FT‐ICR MS highlighted that aliphatic and high‐H/C molecular formulas were preferentially biodegraded, whereas condensed aromatic and polyphenolic compounds were relatively enriched post‐biodegradation in all endmember leachates. Together these findings suggest that with continued climate change and landscape perturbation, larger amounts of less bioavailable DOC will be mobilized into Arctic rivers leading to higher relative amounts of highly aromatic, biologically stable DOM being exported into receiving ecosystems and the Arctic Ocean, potentially altering the rates and mechanisms of carbon turnover in the coastal zone.