Equilibrium unveiled: Carbon fluxes of a natural Fennoscandian river

According to recent data-driven upscaling, emissions from rivers in the Arctic-boreal region offset between half to all of the wetland carbon dioxide (CO₂) sink, and release substantial amounts of methane (CH₄) despite their much smaller surface area. However, observational data in natural riverine ecosystems in this region remain scarce, and carbon emission estimates show large uncertainties.

In order to estimate the carbon budget of a natural river, we determined chamber-derived water-air fluxes and surface partial pressures of CO₂ and CH₄ on four transects along the Fennoscandian river Teno. In addition, hydrochemical parameters were measured. CO₂ fluxes were low and ranged from -0.1 to 0.9 µmol m^-2 s^-1 without significant spatial differences. CH₄ fluxes were highest in an area of shallow water and pronounced benthic vegetation, but overall showed small emissions (0.02 to 13.66 nmol m^-2 s^-1). Partial pressures of CO₂ and CH₄ were mostly in equilibrium with the atmosphere, and pCH₄ was only slightly elevated where highest CH₄ fluxes were found.

On each transect, the carbon fluxes were driven by different local factors including dissolved oxygen saturation, water temperature, pH and turbidity. Therefore, knowledge about heterogeneous patterns across landscapes is important to understand spatial variability of riverine biogeochemistry and enhance process understanding. Results from this study can be used to benchmark Earth System and process-based models, and to evaluate the impact of disturbances on river carbon dynamics.