A massive bloom of floating brown algae, stretching more than 8,800 kilometers across the Atlantic Ocean, is troubling scientists and coastal communities alike — and new research suggests it’s no fluke. The formation, known as the Great Atlantic Sargassum Belt, has grown dramatically in size over the past decade, choking beaches from West Africa to the Caribbean and the Gulf of Mexico. Once confined to the nutrient-poor waters of the Sargasso Sea, the brown macroalgae (Sargassum fluitans and S. natans) are now thriving in far wider tropical zones. According to satellite data published in Harmful Algae, this sprawling mass reached a record 37.5 million tonnes in May 2025. Scientists stress that eutrophication — once seen as a localised coastal issue — is now shaping global ocean patterns.