Oxygen variability over the southern Senegalese shelf [résumé] - fdi:010072123 - Horizon

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Machu Eric, Tall A.W., Capet X., Ndoye S., Lazar A., Baurand F., Auger Pierre-Amaël, Brehmer Patrice. (2017). Oxygen variability over the southern Senegalese shelf [résumé]. In : Brehmer Patrice (ed.), Ba B. (ed.), Kraus G. (ed.) International conference ICAWA 2016 : extended book of abstract : the AWA project : ecosystem approach to the management of fisheries and the marine environment in West African waters. Dakar : SRFC/CSRP ; IRD, 75-76. ICAWA : International Conference AWA, 3., Dakar (SEN), 2016/12/13-15. ISBN 978-2-9553602-0-5

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Titre
Oxygen variability over the southern Senegalese shelf [résumé]
Année de publication2017
Type de documentColloque
AuteursMachu Eric, Tall A.W., Capet X., Ndoye S., Lazar A., Baurand F., Auger Pierre-Amaël, Brehmer Patrice.
InBrehmer Patrice (ed.), Ba B. (ed.), Kraus G. (ed.) International conference ICAWA 2016 : extended book of abstract : the AWA project : ecosystem approach to the management of fisheries and the marine environment in West African waters
SourceDakar : SRFC/CSRP ; IRD, 2017, p. 75-76. ISBN 978-2-9553602-0-5
ColloqueICAWA : International Conference AWA, 3., Dakar (SEN), 2016/12/13-15
RésuméThe shelf of the southern part of the Canary upwelling system along Senegal and Mauritania is bordered by an oxygen minimum zone whose center is located at about 400m depth above which is superimposed another minimum shallower than 200m. As a consequence, dynamic upwelling bring hypoxic waters (60 µmol/l or 1.34 ml/l; 15°C) over the shelf during the upwelling season from December to May. An oceanographic survey was conducted in the heart of the upwelling season (7th-17th March 2012) over the southern Senegalese coast. Integrated physical and biogeochemical measurements were acquired and allowed us to measure for the first time to our knowledge an anoxic event in Senegalese waters with dissolved oxygen concentrations falling to zero. Very high concentrations of nitrite (11 mmol/m3) mirroring nitrate reduction and nitrate/nitrite deficit to phosphate highlight intense denitrification taking place in this oxygen-depleted water body. The record of this event by 30m depth occurs after a relaxation phase of the upwelling associated with a surface northward advection of warm waters from the south of the plateau. A subsurface north-westward advection of water consistent with wind relaxation was recorded near the anoxia event but inertial wave can not be dismissed for driving this northward flow. A diatom biomass was measured at 18 mgChl/m3 along the shore two days earlier. Apparent Oxygen Utilization and oxygen concentrations support the decay of phytoplankton biomass at the end of the survey. Whether the anoxia is due to the local decay of fast sinking dead diatoms or to a specific 3D history of the water body is difficult to establish. However, recently upwelled waters south of Cape Verde peninsula are advected southward and cover the entire shelf with undersaturated waters. Frequent occurrence of high phytoplankton biomass along the coast suggests that anoxia and denitrification could occur recurrently in this region during the degradation of organic matter. These observations take place in a long term monitoring of bottom oxygen concentrations in this very productive coastal region of the world.
Plan de classementLimnologie physique / Océanographie physique [032] ; Sciences du milieu [021]
LocalisationFonds IRD [F A010071832]
Identifiant IRDfdi:010072123
Lien permanenthttp://www.documentation.ird.fr/hor/fdi:010072123

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