Gutknecht, E., Dadou, I., Le Vu, B., Cambon, G., Sudre, J., Garçon, V., Machu, E., Rixen, Tim, Kock, A., Flohr, A., Paulmier, A. and Lavik, G. (2013) Nitrogen transfers and air-sea N₂O fluxes in the upwelling off Namibia within the oxygen minimum zone: a 3-D model approach. Biogeosciences Discussions, 8 (2). pp. 3537-3618. DOI https://doi.org/10.5194/bgd-8-3537-2011.

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Abstract

Eastern boundary upwelling systems (EBUS) areregions of high primary production often associated withoxygen minimum zones (OMZs). They represent key regionsfor the oceanic nitrogen (N) cycle. By exporting organic mat-ter (OM) and nutrients produced in the coastal region to theopen ocean, EBUS can play an important role in sustainingprimary production in subtropical gyres. However, losses offixed inorganic N through denitrification and anammox pro-cesses take place in oxygen depleted environments such asEBUS, and can potentially mitigate the role of these regionsas a source of N to the open ocean. EBUS can also representa considerable source of nitrous oxide (N2O) to the atmo-sphere, affecting the atmospheric budget of N2O.In this paper a 3-D coupled physical/biogeochemicalmodel (ROMS/BioEBUS) is used to investigate the N bud-get in the Namibian upwelling system. The main processeslinked to EBUS and associated OMZs are taken into account.The study focuses on the northern part of the Benguela up-welling system (BUS), especially the Walvis Bay area (be-tween 22◦S and 24◦S) where the OMZ is well developed.Fluxes of N off the Walvis Bay area are estimated in orderto understand and quantify (1) the total N offshore exportfrom the upwelling area, representing a possible N sourcethat sustains primary production in the South Atlantic sub-tropical gyre; (2) export production and subsequent losses offixed N via denitrification and anammox under suboxic con-ditions (O2<25 mmol O2m−3); and (3) the N2O emission tothe atmosphere in the upwelling area.In the mixed layer, the total N offshore export is estimatedas 8.5±3.9×1010mol N yr−1at 10◦E off the Walvis Bayarea, with a mesoscale contribution of 20 %. Extrapolated tothe whole BUS, the coastal N source for the subtropical gyrecorresponds to 0.1±0.04 mol N m−2yr−1. This N flux rep-resents a major source of N for the gyre compared with otherN sources, and contributes 28 % of the new primary produc-tion estimated for the South Atlantic subtropical gyre. Export production (16.9±1.3×1010mol N yr−1) helpsto maintain an OMZ off Namibia in which coupled ni-trification, denitrification and anammox processes lead tolosses of fixed N and N2O production. However, neitherN losses (0.04±0.025×1010mol N yr−1) nor N2O emis-sions (0.03±0.002×1010mol N yr−1) significantly impactthe main N exports of the Walvis Bay area.The studied area does not significantly contribute to N2Oemissions (0.5 to 2.7 %) compared to the global coastal up-welling emissions. Locally produced N2O is mostly advectedsouthward by the poleward undercurrent.

Document Type:	Article
Programme Area:	UNSPECIFIED
Research affiliation:	Biogeochemistry and Geology > Carbon and Nutrient Cycling
Refereed:	Yes
Open Access Journal?:	Yes
DOI:	https://doi.org/10.5194/bgd-8-3537-2011
ISSN:	1810-6285
Date Deposited:	05 Sep 2019 10:24
Last Modified:	01 Oct 2020 13:00
URI:	http://cris.leibniz-zmt.de/id/eprint/2864

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