Seaweed is one of the most variable, sustainable substances on earth. Scientists have used it to make new plastics, medical devices, food, biofuels, and more. But right now, one variety of aquatic plant is also making a giant toxic bloom that can be seen from space.
Meet the Great Atlantic Sargassum Belt—a nearly 5,000-mile-long, thickly matted sheet of sargassum algae floating between Mexico and West Africa. Sargassum, a genus of large brown seaweed, is pretty much harmless —or even beneficial—out in the open ocean. But when it creeps up on beaches, it can be a serious problem. And it’s growing.
While these seaweed mounds may serve as carbon sinks and fish habitats when floating asea, as the mass inches closer to land, it can diminish water and air quality, smother coral reefs, and restrict oxygen for coastal fish. Huge piles of the seaweed typically turn up on Florida beaches around May, but the seaweed is already starting to swamp beaches in Key West, Brian LaPointe, a research professor at Florida Atlantic University’s Harbor Branch Oceanographic Institute, tells NBC. As of last week, 200 tons of the marine plant are expected to wash up on beaches in the Mexican Caribbean.
[Related: This fossilized ‘ancient animal’ might be a bunch of old seaweed.]
With these pile-ups come even more pile-ups—of dead fish. According to the Independent, around 1,000 pounds of fish were cleared from Florida’s St. Pete Beach this month, and 3.5 tons of dead fish have already been removed in the past two weeks from the state’s Manatee County Parks.
The seaweed can be a huge problem for infrastructure. “Even if it’s just out in coastal waters, it can block intake valves for things like power plants or desalination plants, marinas can get completely inundated and boats can’t navigate through,” Brian Barnes, an assistant research professor at the University of South Florida’s College of Marine Science, tells NBC. Not to mention, one 2022 paper linked the hydrogen sulfide that rotting seaweed emits to serious pregnancy complications, alongside headaches and eye irritation.
[Related: Horrific blobs of ‘plastitar’ are gunking up Atlantic beaches.]
While some types of seaweed make for awesome, sustainable products, this kind of sargassum is virtually useless. Using it as a fertilizer or compost is tricky, thanks to its high heavy metal content. Some scientists have argued for sinking the massive carpet of algae to the bottom of the ocean to use as carbon capture and storage.
“There is a lot of carbon biomass associated with sargassum–about 3m tonnes in the Great Sargassum Belt,” Columbia University oceanographer Ajit Subramaniam tells The Guardian.
For now, it’s probably best to keep an eye out for beach closures, event cancellations, and warnings as the season attracts more people—and smelly seaweed—toward the coast.