BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-10-3559-2013 Coupled physical/biogeochemical modeling including O<sub>2</sub>-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela Gutknecht E. 1 Dadou I. 1 Le Vu B. 1 Cambon G. 1 Sudre J. 1 Garçon V. 1 Machu E. 2 Rixen T. 3 Kock A. 4 Flohr A. 3 Paulmier A. 1 5 Lavik G. 6 Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (UMR5566, CNRS/CNES/UPS/IRD), Toulouse, France Laboratoire de Physique des Océans (UMR6523, CNRS/Ifremer/IRD/UBO), Plouzané, France Leibniz Center for Tropical Marine Ecology, Bremen, Germany Forschungsbereich Marine Biogeochemie, Helmholtz-Zentrum für Ozeanforschung, Kiel, Germany Instituto del Mar del Per&uacute;, Esquina Gamarra y General Valle S/N chucuito Callao (Lima), Peru Max Plank Institut for Marine Microbiology, Bremen, Germany 03 06 2013 10 6 3559 3591 Copyright: © 2013 E. Gutknecht et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/10/3559/2013/bg-10-3559-2013.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/10/3559/2013/bg-10-3559-2013.pdf

The Eastern Boundary Upwelling Systems (EBUS) contribute to one fifth of the global catches in the ocean. Often associated with Oxygen Minimum Zones (OMZs), EBUS represent key regions for the oceanic nitrogen (N) cycle. Important bioavailable N loss due to denitrification and anammox processes as well as greenhouse gas emissions (e.g, N<sub>2</sub>O) occur also in these EBUS. However, their dynamics are currently crudely represented in global models. In the climate change context, improving our capability to properly represent these areas is crucial due to anticipated changes in the winds, productivity, and oxygen content. <br><br> We developed a biogeochemical model (BioEBUS) taking into account the main processes linked with EBUS and associated OMZs. We implemented this model in a 3-D realistic coupled physical/biogeochemical configuration in the Namibian upwelling system (northern Benguela) using the high-resolution hydrodynamic ROMS model. We present here a validation using in situ and satellite data as well as diagnostic metrics and sensitivity analyses of key parameters and N<sub>2</sub>O parameterizations. The impact of parameter values on the OMZ off Namibia, on N loss, and on N<sub>2</sub>O concentrations and emissions is detailed. The model realistically reproduces the vertical distribution and seasonal cycle of observed oxygen, nitrate, and chlorophyll <i>a</i> concentrations, and the rates of microbial processes (e.g, NH<sub>4</sub><sup>+</sup> and NO<sub>2</sub><sup>&minus;</sup> oxidation, NO<sub>3</sub><sup>&minus;</sup> reduction, and anammox) as well. Based on our sensitivity analyses, biogeochemical parameter values associated with organic matter decomposition, vertical sinking, and nitrification play a key role for the low-oxygen water content, N loss, and N<sub>2</sub>O concentrations in the OMZ. Moreover, the explicit parameterization of both steps of nitrification, ammonium oxidation to nitrate with nitrite as an explicit intermediate, is necessary to improve the representation of microbial activity linked with the OMZ. The simulated minimum oxygen concentrations are driven by the poleward meridional advection of oxygen-depleted waters offshore of a 300 m isobath and by the biogeochemical activity inshore of this isobath, highlighting a spatial shift of dominant processes maintaining the minimum oxygen concentrations off Namibia. <br><br> In the OMZ off Namibia, the magnitude of N<sub>2</sub>O outgassing and of N loss is comparable. Anammox contributes to about 20% of total N loss, an estimate lower than currently assumed (up to 50%) for the global ocean.

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