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

[摘要] Eastern boundary upwelling systems (EBUS) are regions of high primaryproduction often associated with oxygen minimum zones (OMZs). They representkey regions for the oceanic nitrogen (N) cycle. By exporting organic matter(OM) and nutrients produced in the coastal region to the open ocean, EBUScan play an important role in sustaining primary production in subtropicalgyres. However, losses of fixed inorganic N through denitrification andanammox processes take place in oxygen depleted environments such as EBUS,and can potentially mitigate the role of these regions as a source of N tothe open ocean. EBUS can also represent a considerable source of nitrousoxide (N₂O) to the atmosphere, affecting the atmospheric budget ofN₂O.

In this paper a 3-D coupled physical/biogeochemical model (ROMS/BioEBUS) isused to investigate the N budget in the Namibian upwelling system. The mainprocesses linked to EBUS and associated OMZs are taken into account. Thestudy 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 areestimated in order to understand and quantify (1) the total N offshoreexport from the upwelling area, representing a possible N source thatsustains primary production in the South Atlantic subtropical gyre; (2)export production and subsequent losses of fixed N via denitrification andanammox 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 as8.5 ± 3.9 × 10¹⁰ mol N yr⁻¹ at 10° E offthe Walvis Bay area, with a mesoscale contribution of 20%. Extrapolatedto the whole BUS, the coastal N source for the subtropical gyre correspondsto 0.1 ± 0.04 mol N m⁻² yr⁻¹. This N flux represents amajor source of N for the gyre compared with other N sources, and contributes28% of the new primary production estimated for the South Atlanticsubtropical 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 N₂Oproduction. However, neither N losses(0.04 ± 0.025 × 10¹⁰ mol N yr⁻¹) nor N₂Oemissions (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 (0.5to 2.7%) compared to the global coastal upwelling emissions. Locallyproduced N₂O is mostly advected southward by the poleward undercurrent.

[学科分类] 地球化学与岩石