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

A Ody ,Léo Berline,Didier Aurelle,Jacques Grelet,Thomas Changeux,Sophie Guasco,Vincent Fauvelle,Thierry Thibaut,Sandrine Ruitton,Solène Connan,Céline Bachelier,Natascha Schmidt,Jean-Michel André,Dorian Guillemain,Hubert Bataille,Jean Blanchot,Cristèle Chevalier,Mathilde Guene ,Aurelie Blanfuné ,Valérie Stiger‐Pouvreau ,Frédéric Ménard

doi:10.1371/journal.pone.0222584

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.

Highlights

Harmful macroalgal blooms have become a global concern, causing ecological, economic and health problems [1,2,3,4]
MODIS and VIIRS sensors are well designed to describe the Sargassum aggregation distribution patterns at Atlantic scale with a daily frequency, but we demonstrated that the OLCI sensor is better suited to describe their mesoscale structure
We reported in S1 Table the fractional coverage range reached by each Sargassum raft type for the MODIS, VIIRS and OLCI spatial resolution

Summary

Introduction

Harmful macroalgal blooms have become a global concern, causing ecological, economic and health problems [1,2,3,4]. On the European Atlantic coast and in the Mediterranean Sea, Sargassum muticum (Yendo) Fensholt can have an ecological and economic impact [16,17,18,19,20]), and Sargassum pacificum Bory in the South Pacific [7,8,21] These benthic species are unable to travel very long distances due to the degradation of their thalli. In the North Atlantic, pelagic Sargassum thalli float and grow at the sea surface during their entire lifetime. They can aggregate, forming Sargassum rafts which can travel long distances under the action of winds, waves and currents Pelagic Sargassum is common in the Sargasso Sea [24,25] as well as in the Gulf of Mexico where it recurrently strands in large quantities on the coasts [4,26]

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Discussion

Conclusion