Highest methane concentrations in an Arctic river linked to local terrestrial inputs

Abstract

Large amounts of methane (CH4) could be released as a result of the gradual or abrupt thawing of Arctic permafrost due to global warming. Once available, this potent greenhouse gas is emitted into the atmosphere or transported laterally into aquatic ecosystems via hydrologic connectivity at the surface or via groundwaters. While high northern latitudes contribute up to 5 % of total global CH4 emissions, the specific contribution of Arctic rivers and streams is largely unknown. We analyzed high-resolution continuous CH4 concentrations measured between 15 and 17 June 2019 (late freshet) in a ∼120 km transect of the Kolyma River in northeast Siberia. The average partial pressure of CH4 (pCH4) in tributaries (66.8-206.8 μatm) was 2-7 times higher than in the main river channel (28.3 μatm). In the main channel, CH4 was up to 1600 % supersaturated with respect to atmospheric equilibrium. Key sites along the riverbank and at tributary confluences accounted for 10 % of the navigated transect and had the highest pCH4 (41 ± 7 μatm) and CH4 emissions (0.03 ± 0.004 mmolm-2d-1) compared to other sites in the main channel, contributing between 14 % to 17 % of the total CH4 flux in the transect. These key sites were characterized by warm waters (T>14.5 °C) and low specific conductivities (κ<88 μScm-1). The distribution of CH4 in the river could be linked statistically to T and κ of the water and to their proximity to the shore z, and these parameters served as predictors of CH4 concentrations in unsampled river areas. The abundance of CH4-consuming bacteria and CH4-producing archaea in the river was similar to those previously detected in nearby soils and was also strongly correlated to T and κ. These findings imply that the source of riverine CH4 is closely related with sites near land. The average total CH4 flux density in the river section was 0.02 ± 0.006 mmolm-2d-1, equivalent to an annual CH4 flux of 1.24×107 gCH4yr-1 emitted during a 146 d open water season. Our study highlights the importance of high-resolution continuous CH4 measurements in Arctic rivers for identifying spatial and temporal variations, as well as providing a glimpse of the magnitude of riverine CH4 emissions in the Arctic and their potential relevance to regional CH4 budgets.