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Coupled physical/biogeochemical modeling including O2-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela

Coupled physical/biogeochemical modeling including O2-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela
Coupled physical/biogeochemical modeling including O2-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela
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 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.

In the OMZ off Namibia, the magnitude of N2O 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.
1726-4170
3559-3591
Gutknecht, E.
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Dadou, I.
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Le Vu, B.
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Cambon, G.
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Sudre, J.
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Garçon, V.
e8d06683-c40a-484a-9744-cd9fd8ed156b
Machu, E.
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Rixen, T.
dc65e23d-a115-4dbd-8aa8-7b1d402d6c66
Kock, A.
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Flohr, A.
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Paulmier, A.
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Lavik, G.
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Gutknecht, E.
1742cd3b-a48a-4602-952b-ca8c8207789d
Dadou, I.
7bb564ca-e9a7-4b43-a809-ec25c69d4c75
Le Vu, B.
7e6ffcd2-117d-4b0d-bd90-e4c0f0ebe683
Cambon, G.
a2066749-6e7f-431b-b536-46c215f66b28
Sudre, J.
8f950b51-3660-4da9-a76d-fc98f3359a0b
Garçon, V.
e8d06683-c40a-484a-9744-cd9fd8ed156b
Machu, E.
c6ec8b3d-ff72-41ee-8510-412261ee3b38
Rixen, T.
dc65e23d-a115-4dbd-8aa8-7b1d402d6c66
Kock, A.
7548b650-3832-4816-9fa6-3d2eb9570e37
Flohr, A.
1e293a22-bdba-408e-9608-fed8b65e4e79
Paulmier, A.
22dfe8e7-3872-40d7-859c-8923eda52441
Lavik, G.
50c5fdc6-5137-4d83-b3b9-ffe522b905d1

Gutknecht, E., Dadou, I., Le Vu, B., Cambon, G., Sudre, J., Garçon, V., Machu, E., Rixen, T., Kock, A., Flohr, A., Paulmier, A. and Lavik, G. (2013) Coupled physical/biogeochemical modeling including O2-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela. Biogeosciences, 10 (6), 3559-3591. (doi:10.5194/bg-10-3559-2013).

Record type: Article

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 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.

In the OMZ off Namibia, the magnitude of N2O 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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Published date: 3 June 2013
Organisations: Geochemistry

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Local EPrints ID: 400920
URI: http://eprints.soton.ac.uk/id/eprint/400920
ISSN: 1726-4170
PURE UUID: c37b0523-6a51-4af7-a261-1340deac6eed
ORCID for A. Flohr: ORCID iD orcid.org/0000-0002-5018-5379

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Date deposited: 29 Sep 2016 16:07
Last modified: 27 Apr 2022 01:59

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Contributors

Author: E. Gutknecht
Author: I. Dadou
Author: B. Le Vu
Author: G. Cambon
Author: J. Sudre
Author: V. Garçon
Author: E. Machu
Author: T. Rixen
Author: A. Kock
Author: A. Flohr ORCID iD
Author: A. Paulmier
Author: G. Lavik

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