Reply on RC2

Abstract

Large amounts of methane (CH4) could potentially be formed as a result of the gradual or abrupt thawing of Arctic permafrost due to global warming. Upon its release, this potent greenhouse gas can be emitted into the atmosphere, or transported laterally into aquatic ecosystems via hydrologic connectivity at surface or 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. In this study, we measured high-resolution continuous CH4 concentrations in a ~120 km section of the Kolyma River in Northeast Siberia navigated twice between 15–17 June 2019 (late freshet). 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. At key sites located near the riverbank and tributary confluences, pCH4 (41±7 µatm) and emissions (0.03±0.004 mmol m–2 d–1) were higher compared to other sites within the main channel. Warm waters (T>14.5 °C) and low specific conductivities (κ<88 µS cm–1) defined these key sites. The distribution of methane in the river could also be linked statistically to T and κ of the water, as well as to the distance to the shore z, as indicators used to predict CH4 concentrations in unsampled river areas. Similarly, the abundance of methane consuming bacteria and methane producing archaea strongly correlated mainly to T and κ, and less to the pCH4, and were similar to those previously detected in nearby soils, suggesting the source of CH4 to be associated with sites close to land. The average total CH4 flux densities in the investigated Kolyma River section were 0.02±0.006 mml m–2 d–1, equivalent to a total CH4 flux of 12.4 mmol m–2. Key sites with highest CH4 concentrations contributed from 13 to 20 % to the total flux. Our study highlights the importance of high-resolution continuous CH4 measurements in Arctic Rivers for identifying spatial and temporal variabilities, and offers a glimpse to the magnitude of riverine methane emissions in the Arctic and their potential relevance to regional methane budgets.