First observations of seasonal bottom water deoxygenation off the Gironde estuary (Bay of Biscay, North East Atlantic)

• Published in: Frontiers in Marine Science
• Main Authors: Dubosq, Nicolas, Schmidt, Sabine, Sudre, Joël, Rigaud, Sylvain, Lamarque, Bastien, Danilo, Martin, Grémare, Antoine, Deflandre, Bruno
• Other Authors: Environnements et Paléoenvironnements OCéaniques (EPOC), Observatoire aquitain des sciences de l'univers (OASU), Université Sciences et Technologies - Bordeaux 1 (UB)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Sciences et Technologies - Bordeaux 1 (UB)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-École Pratique des Hautes Études (EPHE), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Centre National de la Recherche Scientifique (CNRS), Coordination Pôles de données et de services pour le Système Terre (CPST), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Institut National de l'Information Géographique et Forestière IGN (IGN)-Centre National de la Recherche Scientifique (CNRS)-Centre National d’Études Spatiales Paris (CNES)-Météo-France, Détection, évaluation, gestion des risques CHROniques et éMErgents (CHROME) / Université de Nîmes (CHROME), Université de Nîmes (UNIMES), This work was supported by: (1) the JERICO-NEXT project (European Union’s Horizon 2020 Research and Innovation program under grant agreement no. 654410), (2) the LEFE- EC2CO VOG project), and (3) the MAGMA project cofounded by the COTE Cluster of Excellence (ANR-10-LABX-45), the Nouvelle-Aquitaine Regional Council, and the French Biodiversity Agency. Cruises were funded by the French Oceanographic Fleet. Doctoral fellowships were provided to ND and BL by the french Ministry of Higher Education, Research and Innovation., ANR-10-LABX-0045,COTE,COntinental To coastal Ecosystems: evolution, adaptability and governance(2010), European Project: 654410,H2020,H2020-INFRAIA-2014-2015,JERICO-NEXT(2015)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2022
• Subjects: deoxygenation; continental shelf; seasonal evolution; stratification; Bay of Biscay; dissolved oxygen; [SDE.MCG]Environmental Sciences/Global Changes; North East Atlantic
• Online Access: https://hal.science/hal-03824842; https://hal.science/hal-03824842/document; https://hal.science/hal-03824842/file/fmars-09-1006453.pdf; https://doi.org/10.3389/fmars.2022.1006453

International audience Although not systematically considered as oxygen minimum zones, River-dominated Ocean Margins are sensitive to oxygen depletions. For example, the continental shelf off the Gironde, which flows into the Bay of Biscay, had not yet been studied from this perspective although recent simulations suggested that this area should already have experienced deoxygenations in recent decades. To fill this gap, profiles of temperature, salinity and dissolved oxygen were performed in the water column of the continental shelf off the Gironde during seven cruises distributed over the different seasons between 2016 and 2021. Turbidity, chlorophyll- a and pH were also measured during some of these cruises. In winter, the water column was slightly stratified due to high river flows. Then, a seasonal thermal stratification was present from spring to autumn. Similarly, dissolved oxygen showed a seasonal dynamic with: a well-oxygenated water column in winter, an oxygen oversaturation in the first 20 meters during the spring bloom, and then a progressive oxygen depletion in bottom waters until reaching an oxygen saturation minimum down to 45% in autumn. These deoxygenations are explained by the seasonal stratification that isolates the bottom waters from spring to autumn, and are likely enhanced by the advection of deoxygenated waters from the north of the Bay of Biscay and the settling of the organic matter produced in surface waters. A better understanding of these processes in the context of global warming undoubtedly requires better documentation of dissolved oxygen variability through the implementation of a long-term and continuous in situ monitoring.