Revisited Estimation of Moderate Resolution Sargassum Fractional Coverage Using Decametric Satellite Data (S2-MSI)

Audrey Minghelli,Jacques Descloitres,François Steinmetz,Malik Chami,Cristèle Chevalier,Léo Berline

doi:10.3390/rs13245106

Abstract

Since 2011, massive stranding of the brown algae Sargassum has regularly affected the coastal waters of the West Caribbean, Brazil and West Africa, leading to significant environmental and socio-economic impacts. The AFAI algal index (Alternative Floating Algae Index) is often used with remote sensing data in order to estimate the Sargassum coverage, and more precisely the AFAI deviation, which consists of the difference between AFAI and AFAI of the Sargassum-free background. In this study, the AFAI deviation is computed using NASA’s 1 km Terra/MODIS (Moderate-Resolution Imaging Spectroradiometer) and ESA/Copernicus’s 20 m Sentinel-2/MSI (Multi Spectral Instrument) for the same sites and at the same time. Both MODIS and MSI AFAI deviations are compared to confirm the relevance of AFAI deviation technique for two very different spatial resolutions. A high coefficient of determination was found, thus confirming a satisfactory downsampling from 20 m (MSI) to 1 km (MODIS). Then, AFAI deviations are used to estimate the fractional coverage of Sargassum (noted FC). A new linear relationship between the MODIS AFAI deviation and FC is established using the dense Sargassum aggregations observed by MSI data. The AFAI deviation is proportional to FC with a factor of proportionality close to 0.08. Finally, it is shown that the factor is dependent on the Sargassum spectral reflectance, submersion or physiological state.

Highlights

Since 2011, unprecedented massive stranding of the brown floating macro-algaeSargassum has been observed along the coastline of French Guyana, the Antilles Islands and the Caribbean Sea
High values ofcoefficient the coefficient of determination found between MODIS/δAFAI
Since the theoretical analysis established that both MODIS/δAFAI and m resolution (MSI)/δAFAI are proportional to the Sargassum fractional coverage, finding (i) corroborates theory

Summary

Introduction

Since 2011, unprecedented massive stranding of the brown floating macro-algaeSargassum has been observed along the coastline of French Guyana, the Antilles Islands and the Caribbean Sea. Satellite data are highly suitable to monitor Sargassum spatial distribution. ESA/Copernicus’s Envisat/MERIS (Medium Resolution Imaging Spectrometer) satellite sensor (300 m resolution), the so-called Maximum Chlorophyll Index (MCI), which is based on the water leaving radiance peak induced by Sargassum optical signature at 709 nm [1,2]. It has been shown that this peak reveals the presence of a high concentration of chlorophyll a at the surface, allowing the detection of extensive areas of pelagic vegetation (Sargassum spp.) [2]). The MCI index was used to determine the spatial distribution of Sargassum aggregations in the Gulf of Mexico and in the western Atlantic waters [3,4]

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Discussion

Conclusion