Mixing behavior of dissolved organic matter at the Yukon and Kolyma land ocean interface

Unprecedented rates of climate change in the Arctic are causing altered land to ocean transport of dissolved organic matter (DOM) and subsequent processing in the Arctic Ocean. Low salinity waters have been suggested as hotspots for DOM dynamics. Although a wide range of biogeochemical processes have been observed in temperate and tropical estuaries, very little is known about DOM behavior at the Arctic land-ocean interface. Here, we use dissolved organic carbon (DOC) concentration, DOM absorption properties and ultrahigh resolution mass spectrometry to assess DOM mixing behavior at the Yukon and Kolyma land-ocean interface. Mixing behavior varied seasonally in the Yukon River. During freshet, despite high spatial variability, DOC concentration was depleted ~10% compared to conservative mixing, however aromatic DOM was enriched through mid-salinity (≤15). In late summer, DOC concentration was ~20% depleted at mid-salinity, yet DOM composition reflected enhanced in situ production compared to conservative mixing. In the Kolyma, DOC concentration suggested non-conservative loss at salinity <1 (~7%) with concurrent enrichment of aliphatics and heteroatoms before DOC enrichment of ~50% at mid-salinity. This relatively large addition of DOC at mid-salinity in the Kolyma, likely due to in situ primary production or sediment resuspension, deviates from the linear relationship previously observed between colored DOM and DOC concentration in large Arctic rivers. Ultimately, land-ocean transects from the Yukon and Kolyma Rivers represent a variety of DOM mixing behaviors in the near-shore coastal environment and highlight that land-ocean mixing in the Arctic is highly complex, with readily apparent spatial and temporal variability. Furthering our understanding of seasonal and system-specific controls of DOC concentration and DOM composition in Arctic River-Ocean mixing is critical in constraining riverine carbon fluxes on a global scale.