Permafrost Arctic streams are effective pathways for the emissions of allochthonous and autochthonous greenhouse gases to the atmosphere during the open water season: a holistic study in Ambolikha river, Northeast Siberia.

Abstract:

The carbon exchange between the land, aquatic and atmospheric interactions in the Arctic is being altered due to permafrost thawing. In this study, we analyzed the synergy between the land and river gas fluxes on a site in Ambolikha river (near Chersky, Northeast Siberia) and its associated floodplain influenced by permafrost thaw. Following a holistic approach, we measured the surface river O₂ concentration and the terrestrial as well as river CO₂ and CH₄ fluxes during the spring to summer transition (from 26^th June to 2^nd August, 2019). The river measurements were done with an array of unattended optical sensors, and the land carbon fluxes were measured with the eddy covariance method. Distinct diel cycles were observed in the river and land gas fluxes associated with day to night temperature changes. The downstream direction of the river water reversed frequently during the second half of the study, allowing for a longer residence time of the water within the stream. This reversing river water flow increased the retention time of leached material and gases from the soil which influenced the biogeochemistry in the river. The surface water and land reach areas of gas fluxes were estimated as 1 km² in each case. During the study period, the Ambolikha river was a net sink of O₂ (–0.9×10⁶ gC/d) and a net source of CO₂ (0.7×10⁶ gC/d) and CH₄ (0.6×10⁶ gC/d) to the atmosphere, but their concentration varied much until the end of the experiment. Interestingly, the CO₂ emissions to the atmosphere from land soil respiration were three times higher (2.1×10⁶ gC/d) than the river emissions, while CH₄ emissions from the river were one order of magnitude larger than the land CH₄ emissions (0.07×10⁶ gC/d). Complementary mass spectrometric analysis of water samples collected at the start and end of the study revealed changes in the dissolved organic matter (DOM) fingerprint during the observation period. This variation implies DOM leached from thawing permafrost soil at the start of the experiment along with potential changes in bacterial degradation processes. Microbial analysis showed that methanogens were not abundant in the river water, suggesting that the majority of the CH₄ in the river was allochthonous. Thus, the Ambolikha river might be an effective pathway for emission of gases to the atmosphere that were originally formed in the soil.