Significant Methane Emissions from Siberian Arctic Rivers: The Importance of Ebullition, a Regional Estimate and the Challenges of Broader Arctic Scaling

Abstract:

Recent studies suggest that the contribution of rivers and streams to global freshwater methane emissions may be underestimated by at least an order of magnitude. These systems are ubiquitous features of high latitude landscapes but emissions estimates are notably lacking and poorly constrained for this region. We quantified diffusive and ebullitive methane emissions from a range of streams and rivers (upland and floodplain; headwater to main stem) of the Kolyma River Lowlands, Northeastern Siberia, using floating chambers and submerged bubble traps. The highest magnitude emissions were observed in upland headwater streams (14 mmol m^-2 d^-1) with 96% of the flux via diffusion. Fluxes from larger floodplain rivers were smaller (1-2 mmol m^-2 d^-1) with approximately 50% of the flux via ebullition. Ebullition events in these rivers were triggered by falling barometric pressure and water depth, suggesting that ebullition may be predictable and that emissions from rivers may be significantly underestimated if ebullition is not explicitly considered. Scaling our fluxes to their corresponding stream size classes in a 1900 km² region of the Kolyma Lowlands, we estimate a total annual flux of 0.5 Gg CH₄-C per year from streams and rivers alone, which, when compared to 11 Gg CO₂-C emissions from streams and rivers in the same region, accounts for more than 50% of the combined radiative forcing. Our study highlights the regional importance of riverine methane emissions and the pressing need to quantify both high latitude stream area and open-water duration to facilitate broader scaling and the incorporation of field-based riverine methane fluxes into pan-arctic/subarctic and global methane budgets.