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 natans and S. fluitans using morphological characteristics; ITS2 sequences showed low divergence (0%–3%) with sequences of nine other Sargassum species. 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 Sargassum in 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 Sargassum was estimated during four events, peaking in 98 kg m−2 wet weight ...

Rhodolith beds are important marine benthic ecosystems, representing oases of high biodiversity among sedimentary seabed environments. They are found frequently and abundantly, acting as major carbonate 'factories' and playing a key role in the biogeochemical cycling of carbonates in the South Atlantic. Rhodoliths are under threat due to global change (mainly related to ocean acidification and global warming) and local stressors, such as fishing and coastal run-off. Here, we review different aspects of the biology of these organisms, highlighting the predicted effects of global change, considering the additional impact of local stressors. Ocean acidification (OA) represents a particular threat that can reduce calcification or even promote the decalcification of these bioengineers, thus increasing the eco-physiological imbalance between calcareous and fleshy algae. OA should be considered, but this together with extreme events such as heat waves and storms, as main stressors of these ecosystems at the present time, will worsen in the future, especially if possible interactions with local stressors like coastal pollution are taken into consideration. Thus, in Brazil there is a serious need for starting monitoring programs and promote innovative experimental infrastructure in order to improve our knowledge of these rich environments, optimize management efforts and enhance the needed conservation initiatives.