Sargassum: The Golden Tide

The Florida Keys are currently experiencing a familiar golden-brown visitor: floating mats of sargassum seaweed. Scientists often refer to the annual influx of sargassum as a “golden tide” because of the macroalgae’s distinctive golden-brown color and the way vast floating mats resemble oceanic tides as they drift toward shore. The golden tide is composed of visible masses of floating seaweed, Sargassum natans and Sargassum fluitans; these two brown macroalgae species spend their entire lifecycle drifting on the surface of the open ocean far out at sea. Satellite images frequently show the algae stretching across thousands of miles of the tropical Atlantic Ocean, creating a striking ribbon of gold visible from space. While these drifting algae have long been part of the Atlantic Ocean ecosystem, scientists say the scale of recent blooms represents a dramatic shift that began in 2011 with the emergence of the Great Atlantic Sargassum Belt. Before 2011, large concentrations of pelagic sargassum were typically confined to the Sargasso Sea. Beginning in 2011, scientists observed a massive and recurring sargassum accumulation below the equator and stretching thousands of miles across the tropical Atlantic Ocean. This phenomenon became known as the Great Atlantic Sargassum Belt.

The Florida Keys sit directly in the path of these drifting sargassum mats. We start to see sargassum piling up on beaches, canals, groins and jetties in late spring and this can continue into September. Ocean currents and prevailing winds transport Sargassum like a conveyor belt from the tropical Atlantic into the Caribbean, then riding along the Yucatan Current between Mexico and Cuba and finally into the Gulf Stream next to the Florida Keys and eastern Florida. It takes several days of prevailing south to southeast winds and ocean currents that are favorable to break these rafts free from the Gulf Stream, sending them landward. Favorable ocean currents can include: a) proximity of the Gulf Stream to shore; b) the presence of meanders and eddies along the north wall of the Gulf Stream that can assist in the northward transport of Sargassum; and c) during high tide when the flood current pushes water from south to north and the sargassum travels landward. During the spring tides that occur with a new moon or full moon, we also experience the highest high tides and lowest low tides, and these highest monthly tides can leave a lot of sargassum stranded on sandy beaches.

In the open ocean, sargassum provides valuable habitat and shelter for fish, crustaceans, loggerhead turtles and seabirds. But once on land, heavy amounts of sargassum creates challenges for coastal communities. When large quantities are pushed into residential canals and strand on beaches, the seaweed begins to decompose. The process can generate unpleasant odors associated with hydrogen sulfide gas and require costly beach removal. Heavy accumulations may also affect seagrass beds, coral reef habitats, and nearshore water quality. There may be fish kills as the decomposing seaweed depletes the oxygen in the water.

World’s Largest Algal Bloom

Researchers describe the Great Atlantic Sargassum Belt as the world’s largest algal bloom extending from coastal West Africa to South America and spans 5,000 miles. Since 2025, the algal bloom has contained more than 30 million metric tons of biomass, making it the largest macroalgal bloom ever documented. Scientists at the University of South Florida College of Marine Science have been monitoring these blooms through the Sargassum Watch System (SaWS), a satellite-based program that tracks the location, abundance, and movement of sargassum across the Atlantic, Caribbean Sea, and the Gulf. Using satellite imagery and ocean circulation models, SaWS provides near-real-time forecasts that help coastal communities prepare for potential beach landings. These products are available online through the University of South Florida Optical Oceanography Lab.

Additional resources can be found at NOAA, including Sargassum: From Sea to Shore, and Coordinating across NOAA to address management and disaster issues around Sargassum.

9