Weekly monitoring of dissolved methane (CH 4 ) at two sites along an , 2-km stretch of the Willamette River (Oregon) between October 2008 and November 2010 revealed persistent supersaturation (24-1703 atmospheric equilibrium). The CH 4 concentration difference (DCH 4 : 0-200 nmol L 21 ) between the two sites varied inversely with river flow, which ranged from 125 m 3 s 21 to 1500 m 3 s 21 over the time series. At the downstream site, an 'excess' of # 125% was observed, with groundwater input being the likely CH 4 source. Quasi-synoptic studies of spatial trends in summer (2010, 2011) revealed steady CH 4 decrease along a 12-km river stretch downstream of the time-series sites. The estimated loss due to air-water exchange for this stretch was , 93 greater than the perceived net loss, consistent with regionally widespread groundwater input of CH 4 . Bi-weekly dissolved nutrient measurements indicated that a distinct nitrate (NO { 3 ) source also existed between the upstream and downstream time-series sites. The excesses of NO { 3 and CH 4 were inversely correlated, with the greatest NO { 3 supply corresponding to periods of high rainfall in winter and highest river flow. Although groundwater input is also the probable source of NO { 3 , seasonal seepage of rainwater-saturated soils (shallow groundwater recharge) explains the NO { 3 input, while hyporheic exchange with a persistent deep aquifer best explains the CH 4 input. Improved understanding of groundwater input and exchange dynamics in the Willamette River will clarify the influence of human activities on river biogeochemistry and help to better constrain the magnitude of CH 4 and other greenhouse gas fluxes associated with inland waters.

Thermokarst lagoons represent the transition state from a freshwater lacustrine to a marine environment, and receive little attention regarding their role for greenhouse gas production and release in Arctic permafrost landscapes. We studied the fate of methane (CH4) in sediments of a thermokarst lagoon in comparison to two thermokarst lakes on the Bykovsky Peninsula in northeastern Siberia through the analysis of sediment CH4 concentrations and isotopic signature, methane‐cycling microbial taxa, sediment geochemistry, lipid biomarkers, and network analysis. We assessed how differences in geochemistry between thermokarst lakes and thermokarst lagoons, caused by the infiltration of sulfate‐rich marine water, altered the microbial methane‐cycling community. Anaerobic sulfate‐reducing ANME‐2a/2b methanotrophs dominated the sulfate‐rich sediments of the lagoon despite its known seasonal alternation between brackish and freshwater inflow and low sulfate concentrations compared to the usual marine ANME habitat. Non‐competitive methylotrophic methanogens dominated the methanogenic community of the lakes and the lagoon, independent of differences in porewater chemistry and depth. This potentially contributed to the high CH4 concentrations observed in all sulfate‐poor sediments. CH4 concentrations in the freshwater‐influenced sediments averaged 1.34 ± 0.98 μmol g−1, with highly depleted δ13C‐CH4 values ranging from −89‰ to −70‰. In contrast, the sulfate‐affected upper 300 cm of the lagoon exhibited low average CH4 concentrations of 0.011 ± 0.005 μmol g−1 with comparatively enriched δ13C‐CH4 values of −54‰ to −37‰ pointing to substantial methane oxidation. Our study shows that lagoon formation specifically supports methane oxidizers and methane oxidation through changes in pore water chemistry, especially sulfate, while methanogens are similar to lake conditions.

Thermokarst lagoons represent the transition state from a freshwater lacustrine to a marine environment, and receive little attention regarding their role for greenhouse gas production and release in Arctic permafrost landscapes. We studied the fate of methane (CH4) in sediments of a thermokarst lagoon in comparison to two thermokarst lakes on the Bykovsky Peninsula in northeastern Siberia through the analysis of sediment CH4 concentrations and isotopic signature, methane-cycling microbial taxa, sediment geochemistry, and lipid biomarkers. We specifically assessed whether sulfate-driven anaerobic methane oxidation (S-AOM) through anaerobic methanotrophic archaea (ANMEs), common in marine sediments with constant supply of sulfate and methane, establish after thermokarst lagoon development and whether sulfate-driven ANMEs consequently oxidize CH4 that would be emitted to the water column under thermokarst lake conditions. The marine-influenced lagoon environment had fundamentally different methane-cycling microbial communities and metabolic pathways compared to the freshwater lakes, suggesting a substantial reshaping of microbial and carbon dynamics during lagoon formation. Anaerobic sulfate-reducing ANME-2a/2b methanotrophs dominated the sulfate-rich sediments of the lagoon despite its known seasonal alternation between brackish and freshwater inflow. CH4 concentrations in the freshwater-influenced sediments averaged 1.34±0.98 μmol g-1, with highly depleted δ13C-CH4 values ranging from -89‰ to -70‰. In contrast, the sulfate-affected upper 300 cm of the lagoon exhibited low average CH4 concentrations of 0.011±0.005 μmol g-1 with comparatively enriched δ13C-CH4 values of -54‰ to -37‰ pointing to substantial methane oxidation. Non-competitive methylotrophic methanogens dominated the methanogenic community of the lakes and the lagoon, independent of porewater chemistry and depth. This potentially contributed to the high CH4 concentrations observed in all sulfate-poor sediments. Our study shows that S-AOM in lagoon sediments can effectively reduce sediment CH4 concentrations and we conclude that thermokarst lake to lagoon transitions have the potential to mitigate terrestrial methane fluxes before thermokarst lakes fully transition to a marine environment.

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