Nitrous Oxide Dynamics in the Southern Benguela Upwelling System

Abstract

St Helena Bay in the southern Benguela upwelling system (SBUS) is characterized by seasonal upwelling, water mass and nutrient retention, and persistent cyclonic circulation that limits oxygen exchange, creating ideal conditions for subsurface N2O production and subsequent ventilation at the ocean-atmosphere interface. However, due to a paucity of observations, little is known about N2O dynamics in the SBUS. Here we use a coupled physical-biogeochemical model and new observations to investigate the magnitude and seasonality of the N2O source and ocean-atmosphere flux terms along a transect from St Helena Bay into offshore waters. Both the model and observations indicate that significant N2O production occurs at depth in nearshore waters, with ΔN2O concentrations (i.e., the difference between the simulated or observed concentration of N2O and the equilibrium value) exceeding 14 μmol m−3. Equatorward advection of this N2O occurs at a maximum rate of 4.50 μmol N2O m−2 s−1 in the upper 200 m. By contrast, the SBUS poleward undercurrent hosts low N2O concentrations on the shelf slope. Modeled fluxes range from −0.02 to 0.2 nmol m−2 s−1, consistent with reports from other upwelling systems. The ocean-atmosphere N2O flux reaches 0.21 g N m−2 yr−1 in nearshore St Helena Bay, and follows a distinct seasonal cycle driven by ΔN2O disequilibrium in winter and prevailing south-easterly winds and associated upwelling in spring and summer. We calculate a mean N2O flux for the whole SBUS of 4 ± 2 × 10−3 Tg N yr−1, representing 0.1% of the estimated global ocean annual flux.