Rachel Tucker | The Washington Post

13 March 2023

TAMPA, Fla. (WFLA) — Marine scientists are tracking a 5,000-mile-wide seaweed bloom that is so large, it can be seen from space.

These sargassum blooms are nothing new, but scientists say this one could be the largest in history. At last check, it was heading toward Florida’s Gulf coast.

The thick mat of algae drifts between the Atlantic coast of Africa and the Gulf of Mexico, providing habitat for marine life and absorbing carbon dioxide, but it can also wreak havoc when when it gets closer to shore. It blocks light from reaching coral and negatively impacts air and water quality as it decomposes.

Florida’s Gulf coast is already grappling with an algae bloom amid the busy spring break tourism season. Red tide has caused dead fish to wash ashore in droves, while the risk of respiratory irritation for humans has canceled events and driven beachgoers away.

With a blanket of sargassum approaching, spanning twice the width of the continental U.S., scientists warn that Florida beaches could soon be inundated with seaweed.

“It’s incredible,” Brian LaPointe, a research professor at Florida Atlantic University’s Harbor Branch Oceanographic Institute told NBC News. “What we’re seeing in the satellite imagery does not bode well for a clean beach year.”

LaPointe, who has studied the blooms for decades, said beaches in the Florida Keys are already being affected. Earlier this week, parts of Mexico were told to prepare for up to three feet of sargassum to build up on shore.

Chunks of brown plant matter may be unappealing to look at, but the impact on humans does not end there. Large pieces of sargassum can ensnare boats and other machinery in the water.

“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 told NBC News. “It can really threaten critical infrastructure.”

Rotting sargassum releases hydrogen sulfide, which can cause respiratory problems for tourists, residents and anyone who works on the water, LaPointe told NBC News.

“Following the big 2018 blooms, doctors in Martinique and Guadeloupe reported thousands of people going to clinics with breathing complications from the air that was coming off these rotting piles of sargassum,” LaPointe said.

Barnes and his colleagues at USF’s Optical Oceanography Laboratory track sargassum blooms. The blanket of seaweed appears to be growing each year, but 2018 and 2022 had the largest blooms, he said. This year could top last year’s record.

“Historically, as far back as we have records, sargassum has been a part of the ecosystem, but the scale now is just so much bigger,” Barnes told NBC News. “What we would have thought was a major bloom five years ago is no longer even a blip.”

Scientists have found that climate change is causing ocean temperatures to rise, creating a more ideal environment for the algae to thrive. Meanwhile, urban and agriculture runoff is sending nitrates from fertilizers and other nutrients flowing into the ocean, feeding the bloom.

Typically, rafts of sargassum gather in the Sargassum Sea region in the northern Atlantic Ocean. From there, the Gulf Stream pushes the algae around the Atlantic basin, which allows it to spread and grow in different areas.