Nitrogen transfers off Walvis Bay: a 3-D coupled physical/biogeochemical modeling approach in the Namibian upwelling system

Abstract. Eastern boundary upwelling systems (EBUS) are regions of high primary production often associated with oxygen minimum zones (OMZs). They represent key regions for the oceanic nitrogen (N) cycle. By exporting organic matter (OM) and nutrients produced in the coastal region to the open ocean, EBUS can play an important role in sustaining primary production in subtropical gyres. However, losses of fixed inorganic N through denitrification and anammox processes take place in oxygen depleted environments such as EBUS, and can potentially mitigate the role of these regions as a source of N to the open ocean. EBUS can also represent a considerable source of nitrous oxide (N₂O) to the atmosphere, affecting the atmospheric budget of N₂O.

In this paper a 3-D coupled physical/biogeochemical model (ROMS/BioEBUS) is used to investigate the N budget in the Namibian upwelling system. The main processes linked to EBUS and associated OMZs are taken into account. The study focuses on the northern part of the Benguela upwelling system (BUS), especially the Walvis Bay area (between 22° S and 24° S) where the OMZ is well developed. Fluxes of N off the Walvis Bay area are estimated in order to understand and quantify (1) the total N offshore export from the upwelling area, representing a possible N source that sustains primary production in the South Atlantic subtropical gyre; (2) export production and subsequent losses of fixed N via denitrification and anammox under suboxic conditions (O₂ < 25 mmol O₂ m⁻³); and (3) the N₂O emission to the atmosphere in the upwelling area.

In the mixed layer, the total N offshore export is estimated as 8.5 ± 3.9 × 10¹⁰ mol N yr⁻¹ at 10° E off the Walvis Bay area, with a mesoscale contribution of 20%. Extrapolated to the whole BUS, the coastal N source for the subtropical gyre corresponds to 0.1 ± 0.04 mol N m⁻² yr⁻¹. This N flux represents a major source of N for the gyre compared with other N sources, and contributes 28% of the new primary production estimated for the South Atlantic subtropical gyre.

Export production (16.9 ± 1.3 × 10¹⁰ mol N yr⁻¹) helps to maintain an OMZ off Namibia in which coupled nitrification, denitrification and anammox processes lead to losses of fixed N and N₂O production. However, neither N losses (0.04 ± 0.025 × 10¹⁰ mol N yr⁻¹) nor N₂O emissions (0.03 ± 0.002 × 10¹⁰ mol N yr⁻¹) significantly impact the main N exports of the Walvis Bay area.

The studied area does not significantly contribute to N₂O emissions (0.5 to 2.7%) compared to the global coastal upwelling emissions. Locally produced N₂O is mostly advected southward by the poleward undercurrent.