While the preindustrial ocean was assumed to be in equilibrium with the atmosphere, the modern ocean is a carbon sink, resulting from natural variability and anthropogenic perturbations, such as fossil fuel emissions and changes in riverine exports over the past two centuries. Here we use a suite of sensitivity experiments based on the ECCO-Darwin global-ocean biogeochemistry model to evaluate the response of air-sea CO2 flux and carbon cycling to present-day lateral fluxes of carbon, nitrogen, and silica. We generate a daily export product by combining point-source freshwater discharge from JRA55-do with the Global NEWS 2 watershed model, accounting for lateral fluxes from 5171 watersheds worldwide. From 2000 to 2019, carbon exports increase CO2 outgassing by 0.22 Pg C yr-1 via the solubility pump, while nitrogen exports increase the ocean sink by 0.17 Pg C yr-1 due to phytoplankton fertilization. On regional scales, exports to the Tropical Atlantic and Arctic Ocean are dominated by organic carbon, which originates from terrestrial vegetation and peats and increases CO2 outgassing (+10 and +20%, respectively). In contrast, Southeast Asia is dominated by nitrogen from anthropogenic sources, such as agriculture and pollution, leading to increased CO2 uptake (+7%). Our results demonstrate that the magnitude and composition of riverine exports, which are determined in part from upstream watersheds and anthropogenic perturbations, substantially impact present-day regional-to-global-ocean carbon cycling. Ultimately, this work stresses that lateral fluxes must be included in ocean biogeochemistry and Earth System Models to better constrain the transport of carbon, nutrients, and metals across the land-ocean-aquatic-continuum.

Arctic rivers are intricate water networks that chemically and biologically process carbon before releasing it as carbon dioxide (CO2) into the atmosphere or carrying it to the ocean. Primary producers use inorganic carbon to build biomass at the basis of the trophic chain. Little is known about how Arctic rivers adapt to climate warming, changes in hydrology and biogeochemical properties. To quantify net and gross biological productivity we measured the dissolved oxygen-to-argon (O2/Ar) ratios and O2 triple isotopologue composition in the river Kolyma and in its tributary Ambolikha during late freshet (June) and base-flow conditions (August) in 2019. We found that hydrological factors restricted river productivity. The river system released CO2 into the atmosphere in June and August, however August emissions were only 6 % of late freshet emissions. Also, the Ambolikha tributary emitted twice as much CO2 per area than the main Kolyma channel in June. Due to higher river flow and turbidity in June, river production was reduced, while lower flows in August permitted more light penetration and a phytoplankton bloom at the confluence of tributary and main Kolyma channel. Total CO2 emissions per area during June and August amounted to (5±11) % of the gross carbon uptake estimated at the bloom site. Thus, in-stream metabolism can exceed riverine CO2 emissions under specific flow and light conditions. Arctic climate change may promote biological productivity in particular locations and increase its contribution to carbon budgets in Arctic rivers as flow slows during longer open water periods.