The distribution of pelagic Sargassum observed with OLCI

We present monthly maps and derived statistics to describe the large-area distribution of Sargassum in the equatorial Atlantic, Caribbean, and Gulf of Mexico, using image data from the OLCI (Ocean and Land Colour Instrument) on the Sentinel 3a satellite. OLCI has provided image data since October 2016 which can be compared with images produced by MERIS (Medium Resolution Imaging Spectrometer) in the period 2002 to 2012. Results show a significant and continuing increase of the area covered by pelagic Sargassum, starting from a first expansion into the Caribbean and equatorial Atlantic in 2011. We use results from a global detection scheme based on the MCI (Maximum Chlorophyll Index) derived from MERIS and OLCI and implemented by ESA's Grid Processing on Demand (GPOD) project. The scheme uses the OLCI image data set at the reduced resolution of 1200m to give monthly global composite images at 5km spatial resolution, designed to record location, extent and intensity of intense plankton blooms and floating vegetation. In the present study area, images are dominated by a growing and spreading distribution of pelagic Sargassum, whose origin and annual cycle remain unexplained. The composite images give no indication of similar populations in other parts of the global ocean. Results derived from MERIS and OLCI are consistent with those derived from MODIS and VIIRS.