Coupled physical/biogeochemical modeling including O-2-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela

Type Article
Date 2013
Language English
Author(s) Gutknecht E.1, Dadou Isabelle1, Le Vu B.1, Cambon Gildas1, Sudre J.1, Garcon V.1, Machu Eric2, Rixen T.3, Kock A.4, Flohr A.3, Paulmier A.1, 5, Lavik G.6
Affiliation(s) 1 : IRD, UPS, CNES, CNRS,Lab Etud Geophys & Oceanog Spatiales,UMR5566, Toulouse, France.
2 : UBO, IRD, IFREMER, Lab Phys Oceans,CNRS,UMR6523, Plouzane, France.
3 : Leibniz Ctr Trop Marine Ecol, Bremen, Germany.
4 : Helmholtz Zentrum Ozeanforsch, Forsch Bereich Marine Biogeochem, Kiel, Germany.
5 : Inst Mar Peru, Lima, Peru.
6 : Max Plank Inst Marine Microbiol, Bremen, Germany.
Source Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2013 , Vol. 10 , N. 6 , P. 3559-3591
DOI 10.5194/bg-10-3559-2013
WOS© Times Cited 32
Abstract 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, N2O) 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. 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 N2O parameterizations. The impact of parameter values on the OMZ off Namibia, on N loss, and on N2O concentrations and emissions is detailed. The model realistically reproduces the vertical distribution and seasonal cycle of observed oxygen, nitrate, and chlorophyll a concentrations, and the rates of microbial processes (e. g, NH4+ and NO2- oxidation, NO3- 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 N2O 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 300m isobath and by the biogeochemical activity inshore of this isobath, highlighting a spatial shift of dominant processes maintaining the minimum oxygen concentrations off Namibia. In the OMZ off Namibia, the magnitude of N2O out-gassing 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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Gutknecht E., Dadou Isabelle, Le Vu B., Cambon Gildas, Sudre J., Garcon V., Machu Eric, Rixen T., Kock A., Flohr A., Paulmier A., Lavik G. (2013). Coupled physical/biogeochemical modeling including O-2-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela. Biogeosciences, 10(6), 3559-3591. Publisher's official version : https://doi.org/10.5194/bg-10-3559-2013 , Open Access version : https://archimer.ifremer.fr/doc/00152/26314/