Aquatic Productivity Signals in the Kolyma River (Northeastern Siberia) From O ₂ /Ar Ratios and O ₂ Triple Isotopologues

Arctic rivers are intricate water networks that chemically and biologically process carbon before releasing it as carbon dioxide (CO 2 ) into the atmosphere or carrying it to the ocean. Primary producers use inorganic carbon to build biomass at the base of the trophic chain. Little is known about how biogeochemical properties in Arctic rivers adapt to climate warming and changes in hydrology. To quantify net and gross biological productivity, we measured the dissolved oxygen‐to‐argon (O 2 /Ar) ratios and O 2 triple isotopologues composition in the river Kolyma and in its tributary Ambolikha during late freshet (June) and low‐flow conditions (August) in 2019. We found that hydrological factors restricted river productivity. The river system released CO 2 into the atmosphere in June and August, however August emissions were only 6% of late freshet emissions. In June, higher river flow and turbidity restricted river production, but in August, lower flows allowed more light penetration and a phytoplankton bloom at the tributary‐main Kolyma channel confluence. CO 2 emissions per area during June and August accounted for 5 ± 11% of the gross carbon uptake estimated during a bloom event. Thus, in‐stream metabolism can exceed riverine CO 2 emissions under certain flow and light conditions. Arctic climate change may promote biological productivity in particular locations along with changes in dissolved organic matter signature and microbiome, and contribute to Arctic river carbon budgets as flow slows during prolongued open water periods.