From In Situ to satellite observations of pelagic Sargassum distribution and aggregation in the Tropical North Atlantic Ocean

• Published in: PLOS ONE
• Main Authors: Ody, Anouck, Thibaut, Thierry, Berline, L., Changeux, T., André, Jean-Michel, Chevalier, Cristele, Blanfuné, Aurelie, Blanchot, Jean, Ruitton, Sandrine, Stiger-Pouvreau, Valérie, Connan, Solène, Grelet, Jacques, Aurelle, D., Guéné, Mathilde, Bataille, Hubert, Bachelier, Celine, Guillemain, Dorian, Schmidt, Natascha, Fauvelle, Vincent, Guasco, Sophie, Ménard, Frédéric
• Other Authors: Institut méditerranéen d'océanologie (MIO), Institut de Recherche pour le Développement (IRD)-Aix Marseille Université (AMU)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Toulon (UTLN)-Centre National de la Recherche Scientifique (CNRS), Laboratoire des Sciences de l'Environnement Marin (LEMAR) (LEMAR), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Institut Universitaire Européen de la Mer (IUEM), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS), Instrumentation, Moyens analytiques, Observatoires en Géophysique et Océanographie (IMAGO), Biologie des Organismes et Ecosystèmes Aquatiques (BOREA), Université de Caen Normandie (UNICAEN), Normandie Université (NU)-Normandie Université (NU)-Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université des Antilles (UA), Institut de Recherche pour le Développement (IRD), Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA), SAREDAs4 project, European Project: 1166-39417,FEDER
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2019
• Subjects: Algae; Wind; Islands; ACL; Biomass; Clouds; Atmospheric clouds; Sea; Sargasso; Ships; [SDE.BE]Environmental Sciences/Biodiversity and Ecology; [SDV.EE.ECO]Life Sciences [q-bio]/Ecology; environment/Ecosystems; North Atlantic
• Online Access: https://hal.archives-ouvertes.fr/hal-02291680; https://hal.archives-ouvertes.fr/hal-02291680/document; https://hal.archives-ouvertes.fr/hal-02291680/file/journal.pone.0222584.pdf; https://doi.org/10.1371/journal.pone.0222584

International audience The present study reports on observations carried out in the Tropical North Atlantic in summer and autumn 2017, documenting Sargassum aggregations using both ship-deck observations and satellite sensor observations at three resolutions (MSI-10 m, OLCI-300 m, VIIRS-750 m and MODIS-1 km). Both datasets reported that in summer, Sargassum aggre-gations were mainly observed off Brazil and near the Caribbean Islands, while they accumulated near the African coast in autumn. Based on in situ observations, we propose a five-class typology allowing standardisation of the description of in situ Sargassum raft shapes and sizes. The most commonly observed Sargassum raft type was windrows, but large rafts composed of a quasi-circular patch hundreds of meters wide were also observed. Satellite imagery showed that these rafts formed larger Sargassum aggregations over a wide range of scales, with smaller aggregations (of tens of m 2 area) nested within larger ones (of hundreds of km 2). Match-ups between different satellite sensors and in situ observations were limited for this dataset, mainly because of high cloud cover during the periods of observation. Nevertheless, comparisons between the two datasets showed that satellite sensors successfully detected Sargassum abundance and aggregation patterns consistent with in situ observations. MODIS and VIIRS sensors were better suited to describing the Sargas-sum aggregation distribution and dynamics at Atlantic scale, while the new sensors, OLCI and MSI, proved their ability to detect Sargassum aggregations and to describe their (sub-) mesoscale nested structure. The high variability in raft shape, size, thickness, depth and biomass density observed in situ means that caution is called for when using satellite maps of Sargassum distribution and biomass estimation. Improvements would require additional in situ and airborne observations or very high-resolution satellite imagery.