A quantitative comparison of the physical supply and biological uptake of new nitrogen in the Arctic Ocean

Nitrogen constrains biomass across the Arctic Ocean, with nitrate (NO3) supply to the surface waters fuelling new primary production and net carbon drawdown. In this Review, we explore the physical mechanisms driving NO3 fluxes to the euphotic zone across the Arctic Ocean and how biological processes respond. The volume and inflow depth of Atlantic and Pacific Ocean waters, together with sea ice and halocline dynamics, govern internal physical mixing of NO3. Respectively, these inflows supply ~34 ± 5 kmol NO3 s−1 and 9 ± 1 kmol NO3 s−1, spreading at mid-depth. NO3 from below the euphotic zone is mixed upwards via several mechanisms. Overall, NO3 fluxes associated with diffusive and turbulent mixing, submesoscale fronts and cyclonic mesoscale eddies are relatively low (on the order of ~0.1–0.7 mmol m−2 per day) but cover a large area, with peaks associated with wind events or individual strong eddies. By comparison, upwelling-driven fluxes are much stronger (on the order of ~1 mmol m−2 per day) but are more localized. Near-inertial and tidal mixing over the Arctic Ocean’s complex bathymetry drives perhaps the strongest NO3 fluxes, for example, reaching 4.5 mmol m−2 per day in the Barents Sea. Comparing these fluxes with observed biological NO3 uptake rates indicates that the internal physical supply of NO3 only limits primary productivity in 9 of the 17 cases considered. Thereafter, light limitation and lagged growth responses can result in excess NO3 remaining in the surface waters. Future research should prioritize linking NO3 supply and uptake at corresponding spatiotemporal scales.