Ocean color satellites show extensive lines of floating sargassum in the Gulf of Mexico

We present satellite imagery that is interpreted as showing extensive lines of floating Sargassum in the western Gulf of Mexico in the summer of 2005. In spite of frequent reports of floating weed covering extended areas in different parts of the world's ocean, this appears to be the first observation of Sargassum from space. Satellite observations were made with the Medium Resolution Imaging Spectrometer (MERIS) on the Envisat satellite launched by the European Space Agency, and subsequently with the Moderate Resolution Imaging Spectro-radiometer (MODIS) launched on both the Terra and Aqua satellites by the National Aeronautics and Space Administration. Both instruments cover wide swaths, providing near-daily images. Both have optical spectral bands in the range 670 to 750 nm, which detect the chlorophyll red-edge characteristic of land and marine vegetation, but only MERIS has a band at 709 nm, which was critical to the initial discovery. The combined satellite data from both sensors show the seasonal cycle of weed density in different areas of the Gulf. A wider ranging study is now needed to map its occurrence in other areas, including the Sargasso Sea (named for the weed, but not so far covered in our survey). The satellite observations suggest that Sargassum biomass is greater than previously estimated, and hence plays a more important part in oceanic productivity.