Masses of tangled brown seaweed that typically spend most of the year floating around in the open waters of the Atlantic Ocean are once again drifting toward shore, where they could pose a significant threat to coastal environments and beach communities.
Known as the great Atlantic Sargassum belt, the seaweed clusters collectively weigh about 13 million tons, according to most recent calculations — and are large enough to be seen from space.
Though it’s been widely referred to as a “blob,” the belt is a collection of individual patches, said Brian Barnes, a researcher with the University of South Florida’s College of Marine Science.
“The projection that we have for this year is that it’s going to be at least as big, if not a little bit bigger, than last year’s bloom,” said Barnes, who’s been tracking the belt’s size and path. “But last year was pretty bad already.”
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Here’s what you need to know:
Loads of seaweed are threatening the Caribbean’s sea life and tourism. Experts say we may be to blame
What is the great Atlantic Sargassum belt?
Sargassum is a large, leafy brown seaweed that’s abundant in the ocean. Its ability to float comes from the small berrylike orbs sprouting from the plant — gas-filled structures mostly containing oxygen.
The seaweed tends to bob around in island-like masses that can stretch for miles. Out in the open ocean, these massive rafts can be key floating habitats that provide food, shelter and breeding grounds for an array of sea life including fish, sea turtles, crabs and marine birds. Sargassum is also a primary nursery for a variety of commercially important fish, such as mahi mahi, jacks and amberjacks.
The great Atlantic Sargassum belt refers to a formation of the floating seaweed that typically extends from the west coast of Africa toward Brazil and around the Caribbean region, said Brian Lapointe, a research professor at Florida Atlantic University who has studied sargassum for more than 40 years. Lapointe estimates that the belt is about 5,000 miles long and 300 miles wide.
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Experts emphasize that it is not a single, giant blob, but rather a collection of blobs.
“You can’t take a boat from Africa to the Caribbean and see it the entire way, absolutely not,” Barnes said. “It’s extremely patchy.”
The patches can vary widely in size, Lapointe said. Some clumps might be the size of a basketball while others can extend for hundreds of meters or even miles, depending on ocean conditions.
Where are the masses of seaweed now?
Recent satellite images show patches just south of islands in the northern Caribbean and offshore of the eastern Yucatán Peninsula, Barnes said. The seaweed has also reached the Gulf of Mexico, he added.
Experts anticipate that the belt will move farther into the Caribbean over the coming months and begin to wash up more frequently along coastlines in the region throughout the spring, peaking in the summer months.
Worrisome amounts of sargassum have already started to show up on beaches in Mexico. “But that is really the tip of the iceberg,” Lapointe said. “There’s more coming from that sargassum belt.”
What will happen when the sargassum reaches shore?
Small quantities washing up on beaches probably aren’t reason for concern, Barnes said. “The effects can be pretty minimal,” he said. “People just walk over it.”
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But some of the bigger patches can inundate beaches if they float ashore. “If they come on to your beach, it can cause pileups six feet tall,” he said.
There’s also a big difference between having blobs of sargassum floating in the ocean’s open waters and it coming closer to the shore, said Francis Chan, director of the Cooperative Institute for Marine Ecosystem and Resources Studies at Oregon State University.
“You’re essentially dumping continent-worths of nutrients in an ecosystem that is used to having very little of them,” Chan said. “It’s the same as throwing a ton of mass into a large pool versus doing so inside a water bottle.”
When sargassum decomposes in an open-water scenario, the vast volume of liquid is able to dissolve the biomass efficiently. But when it does along beaches where water isn’t as plentiful, the nutrients from decomposing algae can suck up the water’s oxygen and alter the ecosystem.
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“There’s not a lot of nutrients in these tropical beaches with pristine water quality and now they have to grapple with a ton of algae that’s going to decompose and change the nutrient regime,” Chan said. “You’re potentially changing the food web in this scenario.”
Last year’s record-setting bloom caused serious problems for various communities. The U.S. Virgin Islands, for instance, declared a state of emergency after high concentrations of sargassum in St. Croix clogged a desalination plant, a primary source of water for the island.
More broadly, excessive amounts of sargassum can threaten and even kill critical near-shore ecosystems, such as coral reefs, sea grass beds and mangroves. Decomposing sargassum can also affect water quality, Barnes said, likening the process to brewing tea. As the seaweed decomposes, organic matter will “seep out” into the surrounding water, he said.
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The seaweed poses economic and human health threats as well. Tourists aren’t likely to frequent beaches clogged with sargassum, where the unsightly piles can block access to the water and create unpleasant swimming conditions.
On land, rotting sargassum releases hydrogen sulfide, which can be a hazard to people with asthma or exacerbate other breathing conditions — and fills the air with a rotten egg smell.
“It’s there until it’s cleaned up, and it requires a lot of resources to do that in a timely fashion,” Barnes said.
Will these blooms keep getting bigger?
Since 2011, when researchers first observed abnormally large accumulations of sargassum in the tropical Atlantic Ocean, the blooms have typically continued to grow year after year.
Why are they getting bigger? One theory is that as the human population grows, more nutrient-rich runoff enters the ocean from activities such as deforestation, fertilizer use, agriculture and waste water, Lapointe said.
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The bloom’s growth appears to be associated with seasonal river discharges, he said. Runoff from these major waterways provides a source of nutrients for the seaweed.
Some communities have attempted to capitalize on the masses of seaweed washing up on their beaches, working to turn it into biofuels, fertilizers and building bricks. But because sargassum can contain heavy metals, including arsenic, Lapointe urged caution about repurposing the plant.
“In the long run, this is going to be the new normal,” he said. “As this problem worsens, I think we are going to see a number of different management and mitigation steps being taken.”
