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How pollution and climate change may have caused the Florida seaweed blob

With a bank of seaweed on the beach in the foreground and a view of high-rises on the coast in the background, two women in bikinis holding surfboards talk to a man in red shorts and baseball cap.
Beachgoers walk past seaweed that washed ashore on March 16 in Fort Lauderdale, Fla. (Joe Raedle/Getty Images)

A 5,000 mile-long blob of slimy, smelly seaweed is headed for Florida’s beaches on the Gulf of Mexico, and it’s partly because of human activity, including water pollution and climate change.

The mass of sargassum — a large, brown seaweed — is moving west from the Caribbean, where it has left unsightly deposits on beaches near Cancun, Mexico and in Key West, Florida.

Sargassum is a microalgae, an organism that lives in the water and converts sunlight, water and carbon dioxide into a biomass.

Sargassum occurs naturally, but various forms of pollution have been causing it to grow more rapidly since 2011, when it was first large enough to be detected by satellite. The annual March-October algae season in the Atlantic Ocean has set a new record for size in each of the last five years, according to the Independent. This year’s bloom stretches all the way to Africa, is 200 to 300 miles wide, and has been doubling in size every month since November.

"This year could be the biggest year yet," Brian Lapointe, an algae specialist and research professor at Florida Atlantic University's Harbor Branch Oceanographic Institute, told USA Today.

It is not clear exactly what is causing this recent trend, but the most likely explanations relate to human activity.

Water pollution

A floating clump of sargassum seaweed.
Sargassum seaweed seen from under water. (Getty Images)

One potential cause is water pollution. As use of fertilizer increases and wastewater from sewers sends ammonium, nitrate and phosphate into rivers and out to the ocean, those chemicals do the same thing they do to crops on a farm: expand their growth.

A 2021 study co-authored by Lapointe looked at the chemistry from the 1980s to 2019 and found that sargassum collected in recent years contained nitrogen levels 35% higher on average than 30 years earlier, due to sewage and farm runoff. (Nitrogen is found in fertilizers and human and animal waste.)

"It's almost like sargassum is a barometer for how global nitrogen levels are changing,” Lapointe told NPR on Wednesday.

Ocean acidification

The combustion of fossil fuels releases carbon dioxide, which is increasing the concentrations of carbon dioxide in the air and the oceans. That may be feeding the sargassum’s growth.

“Algae need carbon dioxide to survive,” the Environmental Protection Agency explains. “Higher levels of carbon dioxide in the air and water can lead to rapid growth of algae, especially toxic blue-green algae that can float to the surface of the water.”

Climate change

Beachgoers walk past seaweed.
Beachgoers walk past seaweed on March 16 in Fort Lauderdale. (Joe Raedle/Getty Images)

Sargassum, like other algal blooms, peaks in the summer months, because it grows most rapidly in warmer water. As the increased atmospheric concentrations of greenhouse gases like carbon dioxide cause global warming, average ocean temperatures have risen 1.5°F since 1901. That may be contributing to the increased growth of sargassum and lengthening the growth season.

“The world is changing, and part of that is the oceans are getting warmer, and algae seems to be able to grow over a longer period of the calendar now than used to be the case,” Dave Tomasco, executive director of the Sarasota Bay Estuary Program, told ABC News.

The EPA also notes that “warmer water is easier for small organisms to move through and allows algae to float to the surface faster” and “algal blooms absorb sunlight, making water even warmer and promoting more blooms.”

It’s not just the warmer temperatures. Second-order effects of climate change may also play a role. For example, since warmer air causes more evaporation, rainstorms are growing more intense. Those storms bring flood waters filled with nitrogen-heavy runoff out to sea.

The same phenomena are also causing other kinds of algal blooms, like the toxic “red tides” that have caused dead fish to show up in droves on Florida’s coastline.

When sargassum washes up on beaches, it decomposes, releasing hydrogen sulfide, which gives off a smell like rotten eggs. Exposure to hydrogen sulfide can irritate the eyes, nose and throat. And although sargassum itself is not harmful to humans, “tiny sea creatures that live in Sargassum can cause skin rashes and blisters,” according to the Florida Department of Health.