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| Titel |
Nitrogen transfers off Walvis Bay: a 3-D coupled physical/biogeochemical modeling approach in the Namibian upwelling system |
| VerfasserIn |
E. Gutknecht, I. Dadou, P. Marchesiello, G. Cambon, B. Vu, J. Sudre, V. Garçon, E. Machu, T. Rixen, A. Kock, A. Flohr, A. Paulmier, G. Lavik |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 10, no. 6 ; Nr. 10, no. 6 (2013-06-21), S.4117-4135 |
| Datensatznummer |
250018304
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| Publikation (Nr.) |
 copernicus.org/bg-10-4117-2013.pdf |
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| Zusammenfassung |
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. |
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