Ody Anouck auteur aut IRD Thibaut T. auteur aut IRD Berline L. auteur aut IRD Changeux Thomas auteur aut IRD André Jean-Michel auteur aut IRD Chevalier Cristèle auteur aut IRD Blanfune A. auteur aut IRD Blanchot Jean auteur aut IRD Ruitton S. auteur aut IRD Stiger-Pouvreau V. auteur aut IRD Connan S. auteur aut IRD Grelet Jacques auteur aut IRD Aurelle D. auteur aut IRD Guene M. auteur aut IRD Bataille Hubert auteur aut IRD Bachelier Céline auteur aut IRD Guillemain D. auteur aut IRD Schmidt N. auteur aut IRD Fauvelle V. auteur aut IRD Guasco S. auteur aut IRD Ménard Frédéric auteur aut IRD text journalArticle eng text/pdf reformatted digital access 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 aggregations 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 Sargassum 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. specialized 034 036 126 030 14 9 e0222584 [29 ] 2019 1932-6203 https://www.documentation.ird.fr/hor/fdi:010078059 10.1371/journal.pone.0222584 1932-6203 [F B010078059] https://www.documentation.ird.fr/hor/fdi:010078059 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers20-06/010078059.pdf IRD - Base Horizon / Pleins textes 2020-07-02 2025-02-24 fdi:010078059 fre