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

Type Article
Date 2019-09
Language English
Author(s) Ody Anouck1, Thibaut Thierry1, Berline Léo1, Changeux Thomas1, André Jean-Michel1, Chevalier Cristèle1, Blanfuné Aurélie1, Blanchot Jean1, Ruitton Sandrine1, Stiger-Pouvreau Valerie2, Connan Solène2, Grelet Jacques3, Aurelle Didier1, Guéné Mathilde4, Bataille Hubert5, Bachelier Celine3, Guillemain Dorian6, Schmidt Natascha1, Fauvelle Vincent1, Guasco Sophie1, Ménard Frédéric1
Affiliation(s) 1 : Aix-Marseille Univ., Université de Toulon, CNRS, IRD, MIO UM 110, Marseille, France
2 : Université de Bretagne Occidentale (UBO), Institut Universitaire Européen de la Mer (IUEM), LEMAR UMR 6539, Technopôle Brest-Iroise, Plouzané, France
3 : IRD DR-OUEST, US191 IMAGO, Technopole de Brest-Iroise—Site de la Pointe du Diable, Plouzané, France
4 : Université des Antilles, UMR BOREA, Campus de Fouillole, BP 592, Pointe-à-Pitre, Guadeloupe, France
5 : IRD, IRD-Images, Marseille, France
6 : Aix Marseille Univ, CNRS, IRD, IRSTEA, OSU PYTHEAS, Marseille, France
Source Plos One (1932-6203) (Public Library of Science (PLoS)), 2019-09 , Vol. 14 , N. 9 , P. e0222584 (29p.)
DOI 10.1371/journal.pone.0222584
WOS© Times Cited 21
Abstract

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 m2 area) nested within larger ones (of hundreds of km2). 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.

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Publisher's official version 29 6 MB Open access
S1 Fig. Comparison between MSI Sargassum maps computed using the (A) FAI and (B) MSI-MFAI floating algae index. 3 MB Open access
S2 Fig. Satellite sensors map (MODIS-A, MODIS-T, VIIRS, OLCI) and in situ pictures of Sargassum rafts observed for each station during the West Atlantic and Transatlantic cruise. 42 2 MB Open access
S3 Fig. MSI images matching stations S11 and S24b near Guadeloupe island (West Atlantic cruise) and station Y15 near Martinique island (Transatlantic cruise). 3 982 KB Open access
S1 Table. Fractional coverage estimates for each raft type and for MODIS, VIIRS and OLCI sensors. 765 KB Open access
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Ody Anouck, Thibaut Thierry, Berline Léo, Changeux Thomas, André Jean-Michel, Chevalier Cristèle, Blanfuné Aurélie, Blanchot Jean, Ruitton Sandrine, Stiger-Pouvreau Valerie, Connan Solène, Grelet Jacques, Aurelle Didier, Guéné Mathilde, Bataille Hubert, Bachelier Celine, Guillemain Dorian, Schmidt Natascha, Fauvelle Vincent, Guasco Sophie, Ménard Frédéric (2019). From In Situ to satellite observations of pelagic Sargassum distribution and aggregation in the Tropical North Atlantic Ocean. Plos One, 14(9), e0222584 (29p.). Publisher's official version : https://doi.org/10.1371/journal.pone.0222584 , Open Access version : https://archimer.ifremer.fr/doc/00514/62609/