Terrestrial-aquatic transfers of carbon dioxide, methane, and organic carbon from riparian wetlands to an arctic headwater stream

Abstract

Transfers of dissolved gases from land contribute to gas evasion from surface waters. Although headwater streams may contribute strongly to overall gas evasion in a river network, the dynamics of CO2 and CH4 transfers between riparian wetlands and headwater streams are poorly understood. Imnavait Creek, a peat-bottom, beaded arctic headwater stream, was studied to determine the relative importance of CO2, CH4, and dissolved organic carbon (DOC) fluxes from surface inflow, in-stream processing such as photo-mineralization, and subsurface lateral inflows draining riparian wetland soils at different depths. Although concentrations of CO2 and CH4 were 1-2 orders of magnitude higher in subsurface lateral inflows than in surface waters, subsurface discharge was >0.1% of surface discharge along the studied reach of Imnavait. This means that 91-97% of the C entering and leaving this reach was introduced by surface inflows. Integrating the subsurface inflows per meter of stream reach above the study site provided the distances required to account for surface water concentrations at the upstream entry to the study reach. Assuming that subsurface inflows were similar along the entire stream, these distances were 120-363 m for CO2, 41-47 m for CH4, and 36 m for DOC. All of these distances were much less than the distance to Imnavait’s headwater source (~430 m), implying a loss of gases through evasion to the atmosphere and a loss of DOC through photochemical or biological conversion to CO2 from upstream to downstream. Furthermore, concentrations of CO2 were significantly higher upstream (p=0.02), while CH4 concentrations were significantly higher downstream (p<0.001). CO2 evasion from the surface of Imnavait pools was 14-30% of the CO2 imported within subsurface lateral inflows, while CH4 evasion was only 4-6%. Both CO2 and CH4 evasion increased significantly during periods of thermal stratification in the pools (p<0.001 and p=0.02, respectively), when concentrations of these gases increased in bottom waters influenced primarily by C-rich subsurface lateral inflows. Photo-mineralization of aromatic DOC from subsurface lateral inflows further increased CO2 in Imnavait. Deeper subsurface lateral inflows (10 cm depth to the permafrost boundary) tended to have significantly higher CO2 and CH4 concentrations than shallow subsurface lateral inflows (0 to 10 cm depth). Deeper subsurface lateral inflows of C-rich soil water to Imnavait increased over the summer proportionately with thaw depth. Current scenarios of global change for Alaska’s North Slope indicate that while terrestrial-aquatic C transfers via subsurface lateral inflows of soil water may increase in the future, in-stream stratification and thus greater gas evasion may decrease.