Storm-scale and seasonal dynamics of carbon export from a nested subarctic watershed underlain by permafrost

Subarctic catchments underlain by permafrost sequester a major stock of frozen organic carbon (C), which may be mobilized as the Arctic warms. Warming can impact C export from thawing soils by altering the depth and timing of runoff related to changing storm and fire regimes and altered soil thaw depths. We investigated C export in a first order headwater stream (West Twin Creek) and its receiving third order river (Beaver Creek) in interior Alaska using discrete sampling of dissolved organic and inorganic C (DOC and DIC) and 15-min collection of specific conductance (SC), fluorescent dissolved organic matter (fDOM) and water discharge (Q). Storm SC-Q relationships displayed negative slopes, indicating solute limitation and limited influence of seasonal soil thaw on storm runoff chemistry. Concurrently, fDOM-Q displayed positive slopes that decreased over the summer, indicating flushing of a limited fDOM pool. Baseflow DIC increased over the season concurrent with soil thaw, with higher DIC at the larger scale indicating greater influence of deeper, mineral-rich flow paths. Storm and seasonal trends were generally similar at both scales. The biggest difference was in fDOM, which displayed higher concentrations and slower depletion in the first order stream. Improved process understanding from this study can be used to better predict carbon export and cycling by stream networks as northern forests and arctic regions continue to warm.