The timing and the scale of the proliferation of Sargassum polycystum in Funafuti Atoll, Tuvalu

The biomass of the algal bloom of Sargassum polycystum along the lagoon shores of Funafuti Atoll (Tuvalu) was estimated in November 2014 using a multispectral spaceborne Pleiades image and concurrent field data. Field work established (1) the geographical limit of the distribution of the algae in the lagoon, (2) the different types of S. polycystum beds that developed in the lagoon, and (3) the range of biomass per square meter for 12 stations. Extrapolation of field data using the green spectral band of the image over the distribution domain and manual correction to remove outliers suggested a standing biomass of 852 t of wet biomass (±95% confidence interval = 420–1747 t) over 0.74 km of reef. Six months later, the processing of a Pleiades image from April 2015 suggested a decrease of algal biomass by 17% (710 ± 333–1542 t). The examination of a multi-sensor series of very high-resolution multispectral images (acquired in 2002, 2003, 2004, 2010, 2011, 2012, 2014, 2015) suggested that algal proliferation started in 2010, with a strong unambiguous signal 1 year later in 2011. The time-series of images also suggested that overall, the biomass had remained fairly stable since 2012, but spatial variations occur, with algae disappearing in some areas, and developing in others. Perspectives for future monitoring are discussed.