The carbon balance of a pristine subarctic river in northern Fennoscandia

Abstract. Carbon emission estimates from riverine ecosystems at high northern latitudes are rarely assessed, resulting in significant uncertainty in the carbon budgets of this region. To close this gap, we determined chamber-derived carbon dioxide (CO₂) and methane (CH₄) fluxes alongside surface partial pressure measurements within the Teno river catchment. The Teno river represents an undisturbed, natural ecosystem at the border between northern Finland and Norway. Chamber measurements were conducted on the water and at river banks (i.e. vegetation-free parafluvial and partially vegetated areas) and yielded predominantly low fluxes. Water-air CO₂ fluxes amounted to 0.25 ± 0.27 μmol m⁻² s⁻¹ (mean ± standard deviation), and water-air CH₄ fluxes to 1.77 ± 3.19 nmol m⁻² s⁻¹. Soil-air CO₂ fluxes from banks reached 1.45 ± 1.38 μmol m⁻² s⁻¹ for ecosystem respiration (Reco), and 0.55 ± 1.34 μmol m⁻² s⁻¹ for net ecosystem exchange (NEE), while corresponding CH₄ fluxes were 1.03 ± 3.14 nmol m⁻² s⁻¹. Using hydrographic datasets, we upscaled our observations for water-air fluxes from the river channel and for soil-air fluxes from the banks. Accordingly, the total vertical carbon emissions amounted to 3,017 tC yr⁻¹ for CO₂ and 3.3 tC yr⁻¹ for CH₄, while banks overall were estimated to emit more than four times the amount of CO₂ than the river channel. Furthermore, we determined the lateral carbon export of the river towards the ocean and estimated a total annual export of roughly 70,000 tC yr⁻¹ from Teno river, mostly in the form of dissolved organic carbon and bicarbonates. As a result, the river network acted as a small carbon source to the atmosphere and the ocean. Carbon dioxide dominated the net carbon balance of the river, while CH₄ emissions were minimal. Although the measured fluxes and the carbon balance of the river network were small in comparison with the landscape-scale carbon balance, the study provides important baseline data for pristine, subarctic rivers, a data-poor ecosystem in current literature. As previous flux measurement sites may be biased towards high-emission ecosystems, it is important to highlight the significance of carbon dynamics for low-emitting sites in order to achieve a balanced understanding. In addition, this study provides insight into the land-water-atmosphere carbon dynamics of northern rivers, and can be used as benchmark for Earth System and process-based models.