Sargassum Now Worldâs Largest Harmful Algal Bloom Due to Nitrogen A photo taken this month shows Sargassum piled up on a beach in Palm Beach County, Florida. (Photo credit: Brian Lapointe, Ph.D.) For centuries, pelagic Sargassum, floating brown seaweed, have grown in low nutrient waters of the North Atlantic Ocean, supported by natural nutrient sources like excretions from fishes and invertebrates, upwelling and nitrogen fixation. Using a unique historical baseline from the 1980s and comparing it to samples collected since 2010, researchers from Florida Atlantic Universityâs Harbor Branch Oceanographic Institute and collaborators have discovered dramatic changes in the chemistry and composition of