A 5,500-mile blob of seaweed in the Atlantic Ocean that has menaced beaches across the Caribbean and Florida in recent years is exploding in size, while a second patch farther north is declining rapidly, driven by rapid changes in the region’s climate.
A study published Thursday in the journal Nature Geoscience finds a big shift in the growth patterns of sargassum, a type of floating macroalgae that provides food and shelter for fish, turtles, seabirds and other marine life.
The southern patch, known as the Great Atlantic Sargassum Belt, has now reached 38 million metric tons, a 40 percent increase from its record year of 2022.
“Usually we have a 10 percent to 20 percent fluctuation year to year,” said Chuanmin Hu, a professor of physical oceanography at the University of South Florida and an author of the paper. “But this year was crazy, and we do not have an answer of why.”
Scientists hadn’t detected sargassum in this equatorial region before 2011.
Since then, winds and ocean currents have pushed blobs of sargassum west toward coastal waters in the springtime. Beaches along Mexico’s Yucatán Peninsula have suffered from smelly piles of seaweed, while the president of the Dominican Republic said in June that his nation faced a “regional emergency” from sargassum and called for action by the United Nations.
In 2023, about 13 million metric tons of sargassum covered some Florida beaches and decomposed into smelly piles before disappearing in the summer. A study that year in The Journal of Global Health by doctors on the island of Martinique reported respiratory ailments from decomposing seaweed that produces hydrogen sulfide gas.
While experts have not pinpointed the cause of the massive sargassum bloom, Saharan dust storms and smoke from African wildfires blowing west across the Atlantic are playing a role, scientists say. These tiny particles drift into the ocean and act as a kind of fertilizer for the sargassum, according to Brian Lapointe, principal investigator at Harbor Branch Oceanographic Institute at Florida Atlantic University and an author of the new paper.
Runoff from the Amazon and Orinoco Rivers in South America and the Congo River in Africa also provide pulses of nutrient-laden water that feed the growth along the equatorial seaweed belt. After a two-year drought in the Amazon, Dr. Lapointe noted that big floods in the spring caused a corresponding boost in sargassum.
“The climate is having a very significant effect on this bloom, but there are many underlying drivers of climate change,” Dr. Lapointe said. “Everything from high temperature, extreme rainfall, droughts and rainfall events, and winds. ”
Dr. Hu and his colleagues used satellite imagery of the ocean surface as well as shipboard observations to document 25 years of sargassum movement. The rapid decline of the northern patch, known as the Northern Sargasso Sea, started in 2015. The Gulf Stream current system has carried seaweed from its nursery in the western Gulf of Mexico into the Northern Sargasso Sea for centuries. However, rising ocean temperatures and a series of marine heat waves have begun making the gulf too warm for Sargassum, Dr. Hu said.
In the past 20 years, the gulf on average has warmed 0.4 degrees Celsius, or 0.72 degrees Fahrenheit, he added.
“In the summer, the water is already warm, adding another 0.7 degrees is very bad,” Dr. Hu said. “Plus we have more frequent marine heat waves in the past 10 years, so that will add to more stress to native sargassum.”
Dani Cox, an independent marine microbiologist based in Miami, said rising ocean temperatures was just one of many causes for the decline of one patch and the explosion of the other, noting that some lab experiments found that sargassum thrive in warm water while others have found the opposite.
“There’s so much going on which is why everyone is not sold on one answer,” Dr. Cox said. “All of us can only look at one or two things at a time. I absolutely think the inundations will stay the same or get worse.”
As for next year’s seaweed invasion, Dr. Lapointe said beach-lovers should stay vigilant before booking a spring break vacation, noting that the Bahamas might be a safer bet than the Yucatán.
“I would be very careful in choosing a destination,” he said.
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