Stranding events of drift organisms (Arribadas) in southern Brazil and the spread of invasive bryozoan in South America

The accumulation of drift organisms (Arribadas, constituted by benthic microalgae, macroalgae and bryozoans) on Central Beach in Balneário Camboriú (SC) has drawn attention since the early 2000s. However, historical surveys suggest an ancient phenomenon that has been increasing in intensity, promoted by urbanization and the eutrophication of the bay in recent years, leading to changes in the taxonomic composition. Previously, these Arribadas were comprised of two species of benthic microalgae (Amphitetras antediluviana and Biddulphia biddulphiana) and the bryozoan Arbocuspis ramosa. However, since 2019, a substantial increase in biomass has been observed on the beach and the dominance of the macroalgae Bryopsis plumosa strongly suggests an increase in the load of organic matter and nutrients in the bay. Recently (2022) the presence of a new invasive bryozoan species (Amathia alternata) was detected, highlighting the need to continue investigating the Arribadas to monitor the ecological evolution of this process. [Display omitted] •Arribadas are formed by organisms stranded on the beach due to tidal/drift mechanisms.•Historical records suggest an ancient phenomenon enhanced by recent eutrophication.•The composition of Arribadas has changed from benthic microalgae to macroalgae.•Amathia alternata, an invasive bryozoan, is spreading southwards in the SW Atlantic.