A massive seaweed blob is headed to Florida — rotten stench included

A blob of sargassum seaweed thousands of miles long is headed to Florida, the Caribbean and eventually the Gulf of Mexico, scientists say.

Dubbed the Great Atlantic Sargassum Belt by scientists, the swath of tendrilled, somewhat rubbery seaweed has become an annual, often stinky occurrence in the last few years, swamping beaches and resorts from South Florida, through the Caribbean and into the Gulf of Mexico with mats of the macroalgae, which then decomposes, releasing hydrogen sulfide. The gas has an odor reminiscent of rotten eggs, and can cause respiratory problems.

Early chunks of the grass mats may start arriving in South Florida in the coming weeks, but the belt is so long the mats are expected to pile up and waft their stench over local beaches through October.

Though common for centuries — Christopher Columbus spotted sargassum floating in the middle of the Atlantic on his way to the Americas in the 15th century — 2011 saw a surge in the algae’s surface area that has continued to grow, according to a 2019 paper by scientists at the University of South Florida.

Brian Lapointe, of Florida Atlantic University, who worked on the paper, said 2018 produced the largest belt, a 5,500-mile-long swath of the stuff that stretched from the coast of West Africa to the Caribbean Sea. This year’s bloom is not quite as large.

The sargassum zone also has shifted from the Sargasso Sea, a massive area of the Atlantic off the U.S. southern seaboard, south to tropical waters, which may help explain the growth.

“What’s bizarre about this story is that we never historically had sargassum growing in that area until 2011. The Sargasso Sea is to the north of that, so we have a new center of distribution of sargassum. … Once it got there, it liked it — there were nutrients to support its growth.”

The belt starts growing in winter and builds up as it moves west on equatorial currents, and peaks in July, Lapointe said.

Though the seaweed mass forms in waters south of Florida, as it moves west into the Gulf of Mexico in the coming months, Gulf Stream currents will carry chunks of it north along Florida’s east coast.

Lapointe has studied nutrient sources that could fuel what is now the largest algae bloom on the planet, and said that as it travels west, it encounters the Amazon plume, essentially a nutrient bomb due to flooding and extreme runoff from the Amazon Basin exacerbated by deforestation and farming practices.

Starting in 2014, there was a large uptick in nitrogen and phosphate coming out of the Amazon, he said. Other nutrient sources include the Congo River and the Mississippi River.

His data suggests that the plant has 35% more nitrogen in its tissue than it did in the 1980s. “I was able to say, ‘ah-ha,’ this plant is seeing more nitrogen. … It’s going to be producing more biomass.”

2014 is when he began to see a significant uptick in sargassum coming ashore in the Florida Keys, where he has lived since the early 1980s.

Sargassum mats in the open ocean can benefit wildlife. Whole food chains, from microorganism, crabs and small fish, on up to mahi-mahi, tuna, wahoo, marlin and sharks, can gather under and around them to feed on one another.

But once the mats hit shore they can cause damage, both to human endeavors and shallow water environments.

The concentration of algae has been so heavy in some parts of the eastern Caribbean that the French island of Guadeloupe issued a health alert in late July of last year. It warned some communities about high levels of hydrogen sulfide emanating from huge rotting clumps of seaweed.

The Biden administration declared a federal emergency after the U.S. Virgin Islands warned last summer of “unusually high amounts” of sargassum affecting water production at a desalination plant near St. Croix that is struggling to meet demand amid a drought.

The problem is so odorous in Mexico that “resorts without sargassum” is a popular travel search term on google.

Stacked up mats of sargassum can also damage seagrasses and coral.

The mats can cover seagrass and prevent the photosynthesis the seagrass needs to survive, then, as the sargassum decomposes, it sucks up oxygen and turns the water brown, limiting light and reducing photosynthesis once again. The toxic hydrogen sulfide can also harm corals.

Information from The Associated Press was used to supplement this news article.