Abstract
The invasive species of brown algae Sargassum gathers in large aggregations in the Caribbean Sea, and has done so especially over the last decade. These aggregations wash up on shores and decompose, leading to many socio-economic issues for the population and the coastal ecosystem. Satellite ocean color data sensors such as Sentinel-3/OLCI can be used to detect the presence of Sargassum and estimate its fractional coverage and biomass. The derivation of Sargassum presence and abundance from satellite ocean color data first requires atmospheric correction; however, the atmospheric correction procedure that is commonly used for oceanic waters needs to be adapted when dealing with the occurrence of Sargassum because the non-zero water reflectance in the near infrared band induced by Sargassum optical signature could lead to Sargassum being wrongly identified as aerosols. In this study, this difficulty is overcome by interpolating aerosol and sunglint reflectance between nearby Sargassum-free pixels. The proposed method relies on the local homogeneity of the aerosol reflectance between Sargassum and Sargassum-free areas. The performance of the adapted atmospheric correction algorithm over Sargassum areas is evaluated. The proposed method is demonstrated to result in more plausible aerosol and sunglint reflectances. A reduction of between 75% and 88% of pixels showing a negative water reflectance above 600 nm were noticed after the correction of the several images.
Highlights
Sargassum is an algal invasive species originally found in the Sargasso Sea, which is located in the northwest of the Atlantic Ocean facing the State of Florida, USA between 20◦N and 35◦ N
Atmosphericover corSince the determination of aerosol sunglint reflectance (ρ ) is challenging rection procedure, which will hereafter be named. This method relies on the rection procedure, which will hereafter be named. This method relies on ext Sargassum-dominated areas, we propose an extension of the POLYMER atmospheric corthe idea of exploiting the value of ρag, which can be estimated over Sargassum-free waters rection procedure, which will hereafter be named POLYMERext
Lands pixels are provided by the POLYMER algorithm
Summary
Sargassum is an algal invasive species originally found in the Sargasso Sea, which is located in the northwest of the Atlantic Ocean facing the State of Florida, USA between 20◦N and 35◦ N. Sargassum is an algal invasive species originally found in the Sargasso Sea, which is located in the northwest of the Atlantic Ocean facing the State of Florida, USA between 20◦. Since 2010, Sargassum has been found in large quantities elsewhere, typically in the south of the Caribbean Sea as far as Brazil, and as far as West Africa. The spatial extent of Sargassum presence can possibly be explained by global climate changes, especially by the increase of oceanic water temperatures, by the modifications of hydrodynamical conditions and by the farming of deforested zones [1,2]. Once washed up on beaches, Sargassum decomposes, producing a gas impacting inhabitants’ health, tourism and the coastal environment. Scientific researches are conducted to gain an understanding of the evolution of the Sargassum [3,4]. Remote sensing techniques can provide interesting information regarding standing stock forecasts in terms of spatial location, period of occurrence and abundance [5]
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