Water from the Kolyma river was sampled during the Freshet spring thaw in June 2019 to understand microbial community dynamics.

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 percent 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 distribution of methane in the river could be linked statistically to T and k 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 k, 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.