Images from space show a 5,000-mile bloom of seaweed threatening the beaches of Florida

• Stinky seaweed in the "Great Atlantic Sargassum Belt" threatens wildlife, infrastructure, and tourism.
• This year's 5,000-mile bloom of algae stretching across the Atlantic is one of the largest on record.
• The smell of rotting beached sargassum causes problems for tourism industries in Mexico and Florida.

An enormous stretch of seaweed measuring 5,000 miles wide is set to bring stench, pests, and bacteria to the beaches of Florida and Mexico.

The "Great Atlantic Sargassum Belt" is a massive bloom of brown algae that stretches from the coast of West Africa to the Gulf of Mexico. It is the largest seaweed bloom in the world — weighing approximately 20 million tons — and is visible from outer space.

This year's bloom is the biggest on record for the month of March, and it's expected to grow from here, peaking in June or July. Scientists are increasingly concerned about the impacts of the algae.

It's important to note that seaweed is usually fairly innocuous and actually has benefits like providing habitats for fish and absorbing carbon dioxide. But that's when it's out in the open ocean.

Sargassum, like the bloom spanning about twice the width of the US right now, could wreak havoc on beaches as ocean currents push the brown algae towards land.

Because once the seaweed reaches shore, "the [blooms] degrade water quality, they smell bad, they attract insects and bacteria, they chase away tourists. It's a bad impact on the economy," Chuanmin Hu, professor of oceanography at the University of Miami who leads a team to monitor and track sargassum blooms using satellites, told Insider.

The algae can also destroy coastal ecosystems, suffocate coral, harm wildlife, threaten infrastructure, and decrease air quality.

As beached sargassum dies and rots, it has a distinct rotten-egg smell, which has caused a huge problem for tourism in both Mexico and Florida.

Hotels and resorts in Mexico, for example, spend millions each year to get rid of beaches of sargassum, hiring workers to collect it and move it elsewhere.

"Increasing sargassum blooms are good for the ocean ecosystem, but pretty bad for some local residents," Hu said.

It's a mystery why sargassum blooms are growing

There are hundreds of different species of sargassum. Some of those that populate the Atlantic Ocean grow on the surface of the water, since they don't form roots to attach themselves to rocks like other algae.

This makes it easy for small clumps to move together and form larger clumps as winds between South Africa and the Gulf of Mexico push them together, Hu said. That's what makes the great seaweed belt across the Atlantic each spring and summer.

"We have a lot of such clumps, but only 0.1% of the ocean surface within this belt is covered by this plant," Hu said. "Sargassum does not fully cover any part of the ocean, and sargassum is not toxic."

Still, the consequences of the Sargassum Belt have concerned scientists for the past decade. Experts say this year's bloom is particularly alarming, according to reporting by Denise Chow for NBC News published Saturday.

"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 sargassum for four decades, told the news outlet that beaches in Key West are already being covered with the algae, despite the piles usually washing ashore in May. Beaches in Mexico — like in Cancun, Playa del Carmen, and Tulum — are also preparing for a large build-up of sargassum this week. 

Blooms have continued to grow, on average, larger and larger over the past five years. In 2018 and 2022 having record-breaking increases, Brian Barnes, an assistant research professor at the University of South Florida's College of Marine Science, told NBC News.

This year is approaching these records, and could surpass them, Hu said.

One study in 2019 suggested that deforestation and fertilizer use may be responsible for the alarming rate at which the mass is growing — the effects of which are all exacerbated by climate change.

"I think I've replaced my climate change anxiety with sargassum anxiety," Patricia Estridge, CEO of Seaweed Generation, told The Guardian.

"Both climate change and human activity play a role, but nobody can tell how much each one contributes to this. There are multiple factors because the Atlantic ocean is huge," Hu said. "It's a complex picture. That's all we can say now, and we're still doing research to understand why."

This post has been updated. It was originally published on March 15, 2023.