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

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.

A minimal Maxey–Riley model for the drift of Sargassum rafts

“…The Maxey-Riley theory for inertial particle dynamics in the ocean (Beron-Vera et al 2019b;Miron et al 2020;Olascoaga et al 2020) accounts for the combined effects of ocean currents and winds on the motion of floating finite-size particles. Elastic interaction among such particles unveils, as we show here, a mechanism for long-range transport that may be at the core of connectivity of Sargassum between accumulation regions in the Caribbean Sea and surroundings and possibly quite remote blooming areas in the tropical North Atlantic from the coast of Africa (Ody et al 2019) to the Amazon River mouth (Gower, Young & King 2013), along what has been dubbed (Wang et al 2019) the 'Great Sargassum belt'.…”

In situ observations and modelling revealed environmental factors favouring occurrence of Vibrio in microbiome of the pelagic Sargassum responsible for strandings

“…West Atlantic -Sargassum Expedition (http://dx.doi.org/10.17600/17004300) took place on board the N/O ANTEA from June 19 th to July 13 th 2017, and explored the new high Sargassum abundance region situated from off the Northeast coast of Brazil to the Caribbean arc, as well as the historical Sargasso Sea (25°N). The Transatlantic -Sargassum Expedition (http://dx.doi.org/10.17600/17016900) took place on board the M/V YERSIN from October 6 th to 24 th 2017 from Cape-Verde Islands, crossing the Atlantic between 8 and 12°N to the island of Martinique (Ody et al, 2019). The pelagic Sargassum rafts were composed with three morphotypes.…”

“…Stations « S » and « Y » correspond to June 19 th -July 13th, 2017 cruise and October 6th to 24th 2017 respectively. The color of dot corresponds to Sargassum raft type determined according to Ody et al (2019). Geographic coverage has been indicated according to Wang et al( 2019) (the great Atlantic Sargassum belt).…”

“…Winds, waves and currents aggregate these surface drifters into configurations that range from widely dispersed clumps to large neustonic rafts tens of meters wide and windrows that extend across the ocean surface for tens of kilometres (e.g. Marmorino et al, 2011;Ody et al, 2019). In the Sargasso Sea, the pelagic Sargassum taxa are part of the natural ecosystems in the mid-Atlantic.…”

Macroalgal diversity for sustainable biotechnological development in French tropical overseas territories

“…In Fiji, there is a very rapid proliferation of Gracilaria edulis and S. polycystum (N'Yeurt and Iese 2015a), but also in French Polynesia, where a proliferation of S. pacificum and T. ornata began in the 1990s (Stiger and Payri 1999a,b, Andréfouët et al 2004, Zubia et al 2008). On the Caribbean coasts, the problem of mass beaching of Sargassum species has not been solved yet (Smetacek and Zingone 2013, Gower et al 2013, Johnson et al 2013, Oyesiku and Egunyomi 2014, Wang and Hu 2016, Maréchal et al 2017, Ody et al 2019. Since 2011, the Caribbean has been suffering recurrently from massive strandings of pelagic Sargassum.…”