Dissolved organic matter composition of winter stream flow in the Yukon River basin

In the Yukon River Basin (YRB), groundwater-to-stream discharge has increased by 0.7-0.9% yr^-1 over the last three decades, and is likely in response to regional climate warming and permafrost thaw. This recent shift in watershed hydrology has important implications for the flux of dissolved organic matter (DOM) from terrestrial to freshwater and marine ecosystems and its composition. For instance, it has been hypothesized that permafrost thaw and increased groundwater discharge may account for the long-term decline in discharge-normalized dissolved organic carbon (DOC) export in the main stem of the Yukon River. However, the response of DOC dynamics in YRB subcatchments to recent warming and thaw will likely vary over space and time as a function of vegetation, parent material, ground ice content and disturbance history. To evaluate spatial patterns of groundwater DOM composition, we collected under-ice samples during winter flow from 68 streams in the YRB. Using a suite of conservative tracers (specific conductivity, base cations), we also separated the relative contribution of supra- and sub-permafrost groundwaters to winter flow. In general, DOC concentration in winter stream flow was low relative to summer flow, averaging 3.94 ± 0.46 and 18.39 ± 1.39 mg L^-1. However, DOM composition varied widely across the YRB, indicating a broad range of organic matter quality and reactivity present for different groundwater sources. In streams receiving inputs primarily from sub-permafrost groundwater, we observed low specific ultraviolet absorbance (SUVA₂₅₄) values (0.4-1.1 L mgC^-1 m^-1), a high proportion of hydrophilic compounds (35-50%), and a large proportion of protein-like compounds (13-35%, as determined by fluorescence spectroscopy). In streams where winter flow was a mixture of supra- and sub-permafrost groundwater sources, we observed higher SUVA₂₅₄ values (2.0-3.6 L mgC^-1 m^-1), high hydrophobic acid content (43 ± 1%), and small proportion of protein-like compounds (< 5%). Based on these data, we present a new conceptual model of DOM responses to regional warming, characterized by early-stage (deeper active layer thickness) and late-stage (wholesale permafrost thaw) soil processes.