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Abstract:This study represents the efforts of a network of researchers to characterise the large, floating Sargassum(Fucales, Phaeophyceae) biomass that had reached the Brazilian coast in 2014 and 2015. Material collected during these events was identified as Sargassum natansand S. fluitansusing morphological characteristics; ITS2 sequences showed low divergence (0%–3%) with sequences of nine other Sargassumspecies. Several epiphytic macroalgae, invertebrates and fishes were associated with the floating Sargassum. Satellite images did not support the hypothesis of slicks moving south from the Sargasso Sea in the northern Atlantic Ocean. This strengthens the hypothesis that there is a matrix of pelagic Sargassumin the central Atlantic Ocean and that biomass accumulation should be considered a result of the combination of physicochemical seawater conditions and biological interactions. The biomass accumulation of the stranded Sargassumwas estimated during four events, peaking in 98 kg m−2wet weight on a beach on the Amazonian coast. The landing of huge Sargassumbiomass represents a potential source of environmental stress, as it can lead to an increase in oxygen demand and eventually synthesise and release chemical compounds with allelopathic properties. On the other hand, these floating islands are a fundamental element of the biogeography and macroecology of tropical environments in the Atlantic Ocean, providing connectivity south/north and east/west among marine biodiversity from Atlantic reef environments. Studies concerning Sargassumeffects on local communities are necessary for the proper management of this phenomenon.