Those Seaweed Blobs Headed for Florida? See How Big They Are.

Scientists say they spotted more than 13 million tons of Sargassum, a yellowish-brown seaweed, drifting in the Atlantic Ocean last month — a record for the month of March.

Here’s what the so-called belt of Sargassum, which can stretch thousands of miles from the western coast of Africa to the Gulf of Mexico, usually looks like in March:

Source: Optical Oceanography Laboratory at the University of South Florida College of Marine Science

Note: Sargassum density detected via satellite imagery analysis by researchers at the University of South Florida. The legend values range from 0% to 0.5% and greater, referring to the percent of the ocean surface covered by Sargassum.

Tangles of the goopy, leafy seaweed have already begun to wash ashore beaches in southern Florida and Mexico. In the coming months, they could start emitting a rotting stench as they decay, potentially posing health risks to beachgoers.

Floating mats of seaweed accumulate in the central Atlantic Ocean for much of the year. But during the spring and summer, patches of it are carried by ocean currents toward the Caribbean, eastern Florida and elsewhere along the Gulf Coast.

It’s hard to predict how much will creep onto beaches in the coming months, but the right combination of ocean currents and wind conditions could push a lot of it ashore, experts said.

Some parts of the Florida Keys have already seen unusually large amounts of seaweed for this time of year, said Brian Lapointe, a research professor at Florida Atlantic University who has studied Sargassum for most of his career.

“I was amazed at what I saw driving along the Overseas Highway,” he said, referring to the main road that runs through the Keys.

Mats of Sargassum, which is technically algae, have been observed for centuries, but researchers started noticing abnormally large accumulations in 2011. The immense blooms have continued to grow almost every year, in large part because of excessive, nutrient-rich runoff from the Congo, Amazon and Mississippi rivers.

Alyson Crean, the public information officer for the City of Key West, Fla., said this year has been heavier than usual so far, though the seaweed hasn’t yet required raking more than once a day.

By law, the Sargassum can’t be harvested until it reaches the shore, because it provides crucial environmental benefits for many marine species. Floating Sargassum serves as shelter and breeding grounds for fish, crabs and endangered sea turtles.

Two beachgoers walk past a large pile of seaweed on a beach with the ocean on the right and high-rise buildings on the left in the distance.

Sargassum washed up on a beach in Fort Lauderdale, Fla., last month.

Joe Raedle/Getty Images

To estimate the amount of Sargassum in the ocean, researchers at the Optical Oceanography Laboratory at the University of South Florida analyzed satellite imagery from a number of sources including NASA’s Terra and Aqua satellites. For each pixel in the image, the researchers determined how much floating algae is present by comparing its color, a yellowish brown, to the blue color of the ocean.

The idea is simple: “If our human eyes can tell that there’s Sargassum, then the satellites can tell as well,” said Chuanmin Hu, a professor and director of the lab.

The precision of satellite analysis allows the researchers to quantify the actual change in color in a specific pixel, which can then be used to calculate the amount of biomass.

Dr. Hu and other researchers in the lab have validated the accuracy of their analyses by physically collecting Sargassum in specific areas, weighing what they’ve collected and comparing it with the estimate derived from the satellite picture.

As the Sargassum washes ashore and begins to decompose, it degrades the water quality and pollutes beaches, scientists say. The decaying algae releases ammonia and hydrogen sulfide, which has an unpleasant odor like rotten eggs and can irritate the eyes, nose and throat.

Some Caribbean islands have seen huge amounts of the brown algae inundate beaches during Sargassum season, posing worrisome respiratory health risks to residents exposed to the hydrogen sulfide, according to a 2018 study.