great atlantic sargassum belt

The Great Atlantic Sargassum Belt

October 17, 2025 By EarthWise Leave a Comment

Sargassum is a free-floating brown seaweed that can drift together in vast mats, sometimes stretching for miles across the ocean’s surface. For centuries, these blooms have been a natural part of the Atlantic, creating food and shelter for many marine creatures. Sargassum was once thought to be confined mainly to the Sargasso Sea in the western Atlantic, but scientists now know it grows rapidly and spreads widely, driven both by natural forces and by nutrients from human activities.

Scientists at Florida Atlantic University have reviewed 40 years of data on sargassum seaweed. Their study, which was recently published in the journal Harmful Algae, points to the rise of what is now called the Great Atlantic Sargassum Belt – a massive seaweed bloom stretching from West Africa to the Gulf of Mexico. First spotted in 2011, it has since appeared every year except in 2013. This past May, the bloom reached a record 37.5 million tons, a number that does not include the 7.3 million tons in the Sargasso Sea.

Sargassum thrives in nutrient-rich waters. Under ideal conditions, controlled studies found sargassum can double its biomass in just eleven days! According to the scientists, nutrient pollution from agriculture and wastewater has fueled these record blooms.

In the ocean, sargassum provides habitat for fish, turtles, and other marine life. But along the coast, it piles up in smelly heaps, clogs waterways, disrupts tourism, and can even threaten power plants.

Understanding why sargassum is spreading so rapidly is key to tackling a problem that now spans an entire ocean.

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A monster seaweed bloom is taking over the Atlantic

Photo, posted December 24, 2014, courtesy of Roban Kramer via Flickr.

Earth Wise is a production of WAMC Northeast Public Radio

A Giant Seaweed Bloom

August 19, 2019 By EarthWise Leave a Comment

Scientists using data from NASA satellites have discovered and documented the largest bloom of seaweed in the world, stretching all the way from West Africa to the Gulf of Mexico. The gigantic macroalgae bloom has been dubbed the Great Atlantic Sargassum Belt.

The brown seaweed floats in surface water and in recent years has become a problem to shorelines lining the tropical Atlantic, Caribbean Sea, Gulf of Mexico, and east coast of Florida. The stuff carpets popular beach destinations and crowded coastal waters. In 2018, more than 20 million tons of it floated on the ocean surface.

Scientists have been studying the Sargassum algae using satellites since 2006, but the major blooms have only started appearing since 2011. They have occurred every year between 2011 and 2018 except for 2013. Before 2011, most of the free-floating Sargassum in the ocean was primarily found in patches around the Gulf of Mexico and the Sargasso Sea located on the western edge of the central Atlantic Ocean.

Sargassum provides habitat for turtles, crabs, fish and birds, and produces oxygen via photosynthesis. However, too much of it can crowd out many marine species.

According to researchers, the ocean’s chemistry must have changed in order for the bloom to get so out of hand. The factors involved include a large seed population left over from a previous bloom, nutrient input from West Africa, and nutrient input from the Amazon River. The increase in nutrients may be a result of deforestation and fertilizer use.

Climate-change effects on precipitation and ocean currents ultimately do play a role in this, but increased ocean temperatures do not. Unfortunately, these giant seaweed blooms are probably here to stay.

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NASA Satellites Find Biggest Seaweed Bloom in the World

Photo courtesy of NASA.

Earth Wise is a production of WAMC Northeast Public Radio.