The Sargassum Scourge

Monday, August 12, 2019
Say you’re settling in for a day at the beach: Sunblock? Check. Towels? Check. A good book? Check. Maybe some cold drinks, your favorite tunes, a lounge chair and umbrella? Perfect. Giant mounds of stinking seaweed? Not so much. But that’s the nasty surprise many beachgoers have encountered in recent years, as massive clusters of a brown macroalgae called sargassum have washed up on shore. Scientists point to a range of environmental factors as the possible culprit, and some think the seaweed invasion could mark a “new normal.”


Sargassum comes in many shapes and sizes and lives in oceans around the world, but two species—Sargassum fluitans and Sargassum natans—have been wreaking particular havoc in the Greater Caribbean. These are pelagic, or free-floating, species that multiply through vegetative reproduction and do not take root.

Floating on the open seas, sargassum provides a lush, vital habitat for marine life—what one oceanographer, Sylvia Earle, calls the “golden floating rainforest of the Atlantic Ocean.”

But when the seaweed drifts toward shore, it can trap small boats, tangle up fishing gear, smother coral reefs, and block sea turtles’ access to nesting sites. And in areas that depend on tourism, it can have a devastating economic impact, amassing on beaches in unsightly piles and emitting hydrogen sulfide, which smells like rotten eggs.

Not all countries—and certainly not all beaches—have been affected, but the problem has plagued areas from southeastern Florida to the Caribbean coasts of Central America to the Greater and Lesser Antilles. Last year, the influx of sargassum led Barbados to declare a national emergency. Cancún and other popular tourist destinations in the Mexican state of Quintana Roo have been particularly hard-hit this summer.

Quintana Roo Governor Carlos Joaquín convened a high-level international meeting on June 27 in Cancún to explore avenues of increased cooperation to address the problem. Participants included representatives of 13 governments: Belize, Cuba, Dominican Republic, Guadeloupe (French territory), Guatemala, Guyana, Haiti, Honduras, Jamaica, Mexico, Nicaragua, Panama, and Trinidad and Tobago. A new website launched after the meeting ( provides a platform for sharing information.


A Changing Phenomenon


Sargassum is certainly nothing new; in fact, sailors on Christopher Columbus’ expedition of 1492 reported seeing thick mats of it. The Sargasso Sea—a region of the North Atlantic Ocean defined by a system of currents—got its name from the seaweed in its waters. (As to how sargassum itself got its name, a recent National Geographic article explained it this way: “Early explorers noted that the air bladders keeping the seaweed afloat reminded them of a grape they called sargazo.”)

Sargassum still floats in the Sargasso Sea, but in 2011 it began to emerge in the tropical Atlantic too, according to Dr. Chuanmin Hu, professor of optical oceanography at the University of South Florida College of Marine Science. In a recent paper in the journal Science, he and his co-authors identified what they called the “great Atlantic Sargassum belt”—a vast, recurring swath of seaweed that stretches more than 5,000 miles, from West Africa to the Caribbean Sea and the Gulf of Mexico.

In an interview with the Energy and Climate Partnership of the Americas (ECPA), Hu explained that the seaweed is dispersed unevenly within this belt, with dense stretches in some places and scattered clumps in others. The belt tends to be widest and thickest in the Caribbean Sea, he said, because the circular motion of the currents keeps the seaweed from moving out to the open ocean right away.

Hu and his team detect and track the presence of the floating seaweed using NASA satellite images, along with sophisticated algorithms they have developed to interpret the data. In 2015, with funding from NASA, they set up the Sargassum Watch System to make the information publicly available.


Possible Causes


What accounts for the quantity of sargassum now being seen in tropical waters? It’s not about warming sea-surface temperatures, Hu said, but about the circulation of nutrients.

He posits that beginning in 2011, the cumulative effect of several factors drove up the level of nutrients in the ocean to a “tipping point” that triggered these large-scale sargassum blooms. These factors include stronger upwelling from the ocean floor off West Africa and increases in deforestation, precipitation, and use of fertilizers along the Amazon River.

Such factors are ultimately related to climate change, which affects ocean circulation, patterns of drought and precipitation, and human behavior such as agriculture practices, according to Hu.

Since 2011, there has been only one non-bloom year, 2013. The peak detected so far was in 2018, Hu said, noting that 2019 seems to be comparable to the second-highest year on record, 2015. He and his colleagues speculate that this sargassum belt will continue to recur seasonally, roughly in the summer months of the Northern Hemisphere, and that extensive beaching of the seaweed may be a “new normal.”

Different hypotheses have been put forth about the proliferation of sargassum—for example, some have suggested hurricanes may be a factor—and Hu acknowledges that scientists still have a lot to learn.

“We are perhaps touching just some parts of a gigantic elephant,” he said, alluding to the old parable of blind men trying to describe something based on a limited amount of information. “Maybe there are other parts that we have not even touched yet.”

In the meantime, as with hurricanes, areas that are feeling the biggest impact must learn to adapt—to prepare for the worst, mitigate the negative effects, and in the case of sargassum, look for ways to “turn this nuisance into a profitable product,” Hu said. (See related story).

If someone figured out how to use the seaweed in medicine or as a biofuel—this latter possibility is under study by the U.S. Department of Energy—“you’d never have a sargassum beach problem because people would collect it even before it reached the beach,” Hu said.

“The important thing is, how do we work together collectively, between researchers and industry, environmental groups and individuals, to make a bad thing into a good thing?”

For his part, Hu will continue to work with satellite data to puzzle out the mystery of sargassum from a distance. “I stay away from it,” he said with a chuckle. “I don’t like the smell.”