Abstract
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.
Highlights
Eastern boundary upwelling systems (EBUS) represent less than 1 % of the global areas of the oceans, they contribute about 11 % of the global oceanic new primary production (Chavez and Toggweiler, 1995; Carr, 2002; Carr and Kearns, 2003; Monteiro, 2010) and 20 % of the global catches (Freon et al, 2009)
The Angola cyclonic gyre or “Angola Dome” represents a pool of nutrient-enriched and oxygen-depleted waters (Monteiro et al, 2006; Mohrholz et al, 2008), which are advected poleward by the warm and saline Angola Current to the Angola-Benguela front (15–18◦ S) before being relayed by the subsurface undercurrent in the northern part of the Benguela upwelling system (BUS) (Mohrholz et al, 2008). This large-scale forcing associated with high local primary production and subsequent export production allows the maintenance of an oxygen minimum zones (OMZs) in the northern part of the BUS, especially in the Namibian upwelling system between 20◦ S and 25◦ S (Monteiro et al, 2006, 2008 and 2011; Hutchings et al, 2009)
In our study of the Namibian upwelling system, the following questions are addressed: (1) What is the significance of the N export from the coastal upwelling area to the eastern part of the South Atlantic subtropical gyre? Does it sustain primary production in the open ocean as compared to the other N sources? (2) What are the losses of fixed N via denitrification/anammox in the OMZ off Namibia? (3) What are the N2O emissions to the atmosphere in the upwelling area? To answer these questions, we investigated the N fluxes in the Namibian upwelling system, and off the Walvis Bay area where the OMZ is well developed
Summary
Eastern boundary upwelling systems (EBUS) represent less than 1 % of the global areas of the oceans, they contribute about 11 % of the global oceanic new primary production (Chavez and Toggweiler, 1995; Carr, 2002; Carr and Kearns, 2003; Monteiro, 2010) and 20 % of the global catches (Freon et al, 2009). The Angola cyclonic gyre or “Angola Dome” represents a pool of nutrient-enriched and oxygen-depleted waters (Monteiro et al, 2006; Mohrholz et al, 2008), which are advected poleward by the warm and saline Angola Current to the Angola-Benguela front (15–18◦ S) before being relayed by the subsurface undercurrent in the northern part of the BUS (Mohrholz et al, 2008) This large-scale forcing associated with high local primary production and subsequent export production allows the maintenance of an OMZ in the northern part of the BUS, especially in the Namibian upwelling system between 20◦ S and 25◦ S (Monteiro et al, 2006, 2008 and 2011; Hutchings et al, 2009).
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