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

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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S1 Fig. Comparison between MSI Sargassum maps computed using the (A) FAI and (B) MSI-MFAI floating algae index.
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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.
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S3 Fig. MSI images matching stations S11 and S24b near Guadeloupe island (West Atlantic cruise) and station Y15 near Martinique island (Transatlantic cruise).
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S1 Table. Fractional coverage estimates for each raft type and for MODIS, VIIRS and OLCI sensors.
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How to cite
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.).,

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