North Atlantic fjords are minor sources of nitrous oxide to the atmosphere

Nitrous oxide (N₂O) distribution and dynamics in high latitude fjords are relatively unknown. Surface water N₂O concentrations were measured in six fjords located in Sweden, Iceland, and Greenland, which represent highly diverse environmental conditions in terms of oxygen, eutrophication and climate. This study provides one of the few high spatial resolution observations of N₂O sea-air fluxes currently available in fjords. The two Icelandic fjords showed highest emissions (97.6±10.5 μg N₂O m⁻² day⁻¹), likely driven by aquaculture-induced nutrient enrichment and not fully oxygenated subsurface waters. The three Swedish fjords, characterized by inputs from nutrient-rich rivers and by poor water circulation, exhibited relatively high N₂O emissions averaging 19.9±19.3 μg N₂O m⁻² day⁻¹, with subsurface water anoxia enhancing emissions in By Fjord (64.4±24.0 µg N₂O m⁻² day⁻¹). In contrast, the Greenland fjord displayed net N₂O uptake (–8.3±7.8 μg N₂O m⁻² day⁻¹), likely due to glacier meltwater dilution. Each fjord appeared to be influenced by distinct N₂O drivers, including temperature, salinity, chlorophyll, and pH, but no single, unifying driver was found across all fjords. As a preliminary global upscaling effort, we integrated our measured fluxes from six fjords with literature data from thirteen additional fjords. We estimate that global fjords emit 7.9±1.7 Gg N₂O yr⁻¹, accounting for 2–13% of global coastal ecosystem emissions and do not significantly offset (3.5%) CO₂ sequestration in fjords. These findings underscore the role of fjords in greenhouse gas dynamics and highlight the need for further spatial and seasonal studies to refine global N₂O emissions from coastal ecosystems.