A new Sargassum (Phaeophyceae) stranding hotspot: high-magnitude pelagic Sargassum spp. strandings on the Amazon coast of Brazil

Large-scale strandings of pelagic Sargassum species have increasingly impacted coastal regions of the tropical Atlantic Ocean, generating severe ecological and socioeconomic consequences. This study provides an event-specific quantification of a large Sargassum stranding along part of the Amazon coast, a region often impacted by the Great Atlantic Sargassum Belt. A spatially explicit, event-scale biomass estimate was produced by integrating high-resolution UAV-derived orthomosaics with in situ wet-biomass measurements obtained from quadrat sampling. High-resolution drone orthoimages (3.3 cm pixel−1) covering 2.3 km of coastline were used to map the affected area. Field biomass was estimated through the analysis of ten 1 m2 quadrats distributed along a 1.5 km transect. The algae were identified as Sargassum fluitans var. fluitans, S. natans var. natans and S. natans var. wingei. Mean wet biomass density was 125.6 kg m−2 (range 108.9–144.3 kg m−2). Based on image classification, total wet biomass was estimated at 10.5 thousand tons, and approximately 42.8 thousand tons when extrapolated to the entire 9.4 km beach coastline. The estimated amount for this single event is approximately double the mean annual amount estimated for the entire Brazilian coast between 2018 and 2023, based on available published assessments. Only 4.9% (2,112 tons) was removed by local authorities in this event. These findings illustrate the region’s exposure to high‑magnitude strandings and underscore the potential value of systematic monitoring and coordinated response strategies.