The study confirmed that Sargassum can be accurately detected with MODIS and confirm the relevance of the Alternative Floating Algae Index (AFAI) for various spatial scales, namely between 1 km resolution (MODIS) and 20 m resolution (MSI). A new relationship between the AFAI deviation and the Sargassum fractional coverage is then proposed based on satellite observations of large Sargassum aggregations using MSI sensors and using radiative transfer simulations. Finally, this study has not only investigated floating algae coverage but provides insights on the influence of the Sargassum reflectance spectrum and the Sargassum submersion using an original adaptation of the Lee's radiative transfer model.