In the summer of 2011, a major ‘Sargassum event’ brought large amounts of seaweed onto the beaches of the islands of the eastern Caribbean with significant effects on local tourism. We present satellite observations showing that the event had its origin north of the mouth of the Amazon in an area not previously associated with Sargassum growth. A significant concentration of Sargassum was detected in April, when it was centred at about 7° N latitude and 45° W longitude. By July it had spread to the coast of Africa in the east and to the Lesser Antilles and the Caribbean in the west. We have previously used images from MERIS (Medium Resolution Imaging Spectrometer) and MODIS (Moderate Resolution Imaging Spectroradiometer) to show the value of satellite observations in tracking patterns of Sargassum. For the years 2003–2010, we were able to determine the seasonal distribution over the range of 20°–40° N latitude and 100°–40° W longitude covering the ‘Sargasso Sea’ region of the North Atlantic and the Gulf of Mexico. In 2011, satellite data showed a large shift in the distribution, whose cause is unclear.

We used satellite imagery from the European Space Agency (ESA) Medium Resolution Imaging Spectrometer (MERIS) optical sensor to make the first mapping of the full distribution and movement of the population of Sargassum in the Gulf of Mexico and the western Atlantic. For most of the years for which we have data (2002 to 2008), the results show a seasonal pattern in which Sargassum originates in the northwest Gulf of Mexico in the spring of each year, and is then advected into the Atlantic. The Sargassum appears east of Cape Hatteras as a ‘Sargassum jet’ in July and ends northeast of the Bahamas in February of the following year. This pattern is consistent with historical surveys from ships. MERIS provides a spectral band that greatly improves the discrimination of floating vegetation from confusing signals such as cloud and sunglint.

The MERIS maximum chlorophyll index (MCI), measuring the radiance peak at 709 nm in water‐leaving radiance, indicates the presence of a high surface concentration of chlorophyll a against a scattering background. The index is high in ‘red tide’ conditions (intense, visible, surface, plankton blooms) and is raised when aquatic vegetation is present. A bloom search based on the MCI has resulted in the detection of a variety of events in Canadian, Antarctic and other waters round the world, as well as detection of extensive areas of pelagic vegetation (Sargassum spp.), previously unreported in the scientific literature. Since 1 June 2006, global MCI composite images, at a spatial resolution of 5 km, are being produced daily from all MERIS (daylight) passes of reduced resolution (RR) data. The global composites significantly increase the area now being searched for events, although the reduced spatial resolution may cause smaller events to be missed. This paper describes the composites and gives examples of plankton bloom events that they have detected. It also shows how the composites are affected by the South Atlantic anomaly (SAA), where cosmic rays impact the detectors of the MERIS instrument.

There is an increasing need for satellite-derived accurate chlorophyll-a concentration (chla) products to improve fisheries management in coastal regions. However, the methods used to derive these products have to be evaluated, so the associated uncertainties are known. The performance of three atmospheric correction methods, the near infrared (NIR), the shortwave infrared (SWIR), and the Management Unit of the North Seas Mathematical Models with an additional modification (MUMM + SWIR), and derived chla products based on the Moderate Resolution Imaging Spectroradiometer AQUA (MODIS) images acquired from 2002 to 2014 over the west coast of Canada and the United States were evaluated. The atmospherically corrected products and above-water reflectance were compared with in situ AERONET (N ~ 650) and above-water reflectance (N ~ 34) data, and the Ocean Color 3 MODIS (OC3M)-derived chla were compared with in situ chla measurements (N ~ 82). The statistical analysis indicated that the MUMM + SWIR method was the most appropriate for this region, with relatively good retrievals of the atmospheric products, improved retrieval of remote sensing reflectance with bias lower than 20% for the OC3M bands, and improved retrievals of chla (r = 0.83, slope = 0.89, _logRMSE = 0.33 mg m⁻³ for ±1 h). The poorest chla retrievals were achieved with the SWIR and NIR methods. These results represent the most comprehensive satellite data analysis of MODIS retrievals for this region and provide a framework for the MUMM + SWIR method that can be further tested in other coastal regions of the world.

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.

The question of the origin, distribution and fate of the floating seaweed Sargassum has fascinated sailors and scientists from the time of Columbus. Observations from ships are hampered by the large and variable area over which Sargassum is dispersed. Here we use satellite imagery to present the first mapping of the full distribution and movement of the population of Sargassum in the Gulf of Mexico and western Atlantic in the years 2002 to 2008. For the first time, we show a seasonal pattern in which Sargassum originates in the northwest Gulf of Mexico in spring of each year, is advected into the Atlantic in about July, appearing east of Cape Hatteras as a "Sargassum jet", and ending northeast of the Bahamas in February of the following year. This pattern appears consistent with historical surveys. Future satellite observations will show whether this pattern repeats in all or most years.