The Caribbean would usually experience a few small mats of sargassum washing ashore in a given year, until 2011, when the seaweed first began arriving in unexpectedly large waves. Similar pileups have occurred almost every year since; 2015 and 2018 saw especially bad blooms. Some countries have set up nets to block the incoming algae, or hired people to clear affected beaches with rakes and backhoes. And still the sargassum comes.
The seaweed does have one very convenient trait: The chlorophyll pigment within it reflects infrared light more strongly than the surrounding seawater does. To satellites that detect infrared, sargassum blazes like a bonfire. Six years ago, Jim Gower from Fisheries and Oceans Canada used satellite images to show that the 2011 bloom had an unusual origin. In April, sargassum had begun growing off the coast of Brazil and near the mouth of the Amazon River, in an area far south of its normal range. By July, it had spread across the entire Atlantic.
That figure is likely to be an underestimate: With a spatial resolution of one kilometer, the satellite data doesn’t capture small chunks of Sargassum. “It highlights the most aggregated areas rather than describing the entirety of what is present,” says Deb Goodwin, an oceanographer at the Sea Education Association.
The Great Atlantic Sargassum Belt is a loose collection of seaweed scattered over a very large area, not a continuous bridge. It’s also not produced by the Sargasso Sea, which lies further north; Wang’s team confirmed that by simulating how particles of seaweed would move in the Atlantic’s currents. They concluded that the belt likely develops from local patches of sargassum that occur naturally in the tropics. But such patches have always existed. Why have they only recently started to form sprawling blooms?
Wang’s team thinks that the new growth was connected to two factors on opposite sides of the Atlantic: the water discharged by the Amazon and upwelling currents rising off West Africa. These two phenomena pump nutrients into the tropical Atlantic. When they’re unusually strong, as they apparently were in 2009, they effectively flood the ocean with fertilizer, allowing sargassum to run amok.
But why, then, did the seaweed not bloom in 2010? Wang’s team thinks that it was delayed by low salinity (due to the influx of Amazon freshwater) and abnormally high temperatures—conditions that suppress the growth of sargassum. Only in 2011, when temperatures returned to normal, could the seaweed make use of the influx of nutrients from previous years, and go wild.
And the bigger the blooms in the summer, the more likely they are to leave behind patches that survive through the winter. If the conditions are right the following year, these “seed populations” can restart another bloom. “Each successive bloom makes it difficult to imagine an end to this self-reinforcing cycle,” says Amy Siuda, an ecologist and oceanographer at Eckerd College. “This is likely the new normal.”
Chuanmin Hu, who led the study, agrees. “I have to emphasize that we have no direct evidence to prove any of this,” he says. “These are our speculations, some educated and some hand-waving.” They’ve been forced into that because many of the factors they identified aren’t regularly measured. For example, they could only find data on the nutrients in the Amazon for two years: 2010 and 2018. The latter levels were much higher, which might explain why sargassum blooms were so big that year. Or it might not. The river might have more nutrients due to increased fertilizer use, and stronger runoffs due to deforestation. Or it might not. “I don’t think there’s enough data,” Hu says. “It takes a huge amount of money to go there and take measurements.”
Of the four factors that the team identified, only sea surface temperatures are regularly measured. And while many scientists have suggested that hot water could speed the growth of sargassum, “we found the opposite,” Hu says. That’s not to say climate change is irrelevant, he cautions: Changing patterns of rain and wind could, for example, influence the strength of the West African upwelling. Nor should the Caribbean count on rising temperatures to solve its sargassum woes, because the pace of change is likely too slow to make a difference in the near future.
Hu adds that other factors could be behind the rise of the Sargassum Belt, including nutrient-rich dust blowing in from the Sahara and changes in ocean currents. And several aspects of the blooms still don’t make sense. “If I were you, I would ask: If you have so much nitrogen and phosphorus, why do other [algae] in the ocean not grow as fast?” he says. “I can’t answer that.”
Goodwin adds that “scientific understanding of Sargassum growth and bloom dynamics under natural, open ocean conditions is extremely limited,” since scientists have only addressed these questions in lab experiments. And the sargassum itself is changing, too. Siuda says that the recent blooms have included “a previously rare and genetically distinct form of sargassum” that comes from the south, differs from those in the Sargasso Sea, and harbors a slightly different community of organisms.
Little is known about this strain, or how the bloom is affecting the ecology around it, which makes it hard to predict how it will react to future conditions. And since it likely evolved in relative isolation from its northerly relatives, its northward expansion suggests that “environmental conditions and ocean circulation patterns in the central Atlantic may have been shifting, undetected, for longer than the time interval examined by [Wang and her colleagues],” says Goodwin. “A critical larger question emerges: What drove such an ecological transformation at unprecedented scale?”
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Researchers have identified the largest bloom of macroalgae in the world, according to a study published in the journal Science.
Macroalgae is a term used to to refer to seaweeds and other large-celled marine algae which can usually be seen with the naked eye.
The bloom—which has been dubbed the “Great Atlantic Sargassum Belt” (GASB)—is so vast that it stretches for around 5,500 miles from the coast of West Africa to the Gulf of Mexico, an international team of scientists said. Consisting primarily of sargassum—a type of brown seaweed—the GASB weighs a staggering 20 million tons.
The team, led by Chuanmin Hu and Mengqiu Wang from the University of South Florida (USF,) identified the extent of the bloom using satellite observations.
“We started this research because many coastal areas near the Caribbean Sea and West Africa have been experiencing severe Sargassum beaching events since 2011. Our knowledge on many important questions—such as where the Sargassum comes from, how much there is and can we predict the blooms—is extremely limited,” Wang told Newsweek. “That’s why we started this research to investigate the large-scale phenomenon with satellite imagery.”
Sargassum floats on the surface of the ocean in large congregations which attract marine animals, such as fish, birds and turtles, as well as producing oxygen via photosynthesis.
“In the open ocean, Sargassum provides an essential habitat and refuge for all sorts of marine animals,” Wang said.
However, too much Sargassum can cause ecological and economic problems, particularly when large clumps wash ashore—as has been occurring with more frequency around Atlantic and Caribbean coastlines over the past decade or so.
For example, large mats of sargassum can impede the movement or breathing of some marine animals, or even cause beaches to smell like rotten eggs, driving away tourists.
“When too much Sargassum piles up on the beaches, it can be harmful to the local environment, tourism, and artisanal fisheries etc., and could also be a public health concern,” Wang said.
Until about 10 years ago, large Sargassum blooms—which grow and recede over the course of a year—were mostly restricted to the Gulf of Mexico and the Sargasso Sea, a region of the North Atlantic. But in 2011, the amount and geographic extent of the seaweed began to rapidly increase, puzzling scientists—who lacked the data needed to identify a cause for this trend.
To try and understand more about this phenomenon, the authors of the Science study analyzed around 20 years of satellite data, as well as information on fertilizer use and deforestation rates in the Amazon rainforest. Within this period, the wider Atlantic region experienced widespread Sargassum blooms every year from 2011 to 2018—except for 2013.
The analysis revealed the vast extent of the GASB as of June 2018, while also indicating that the phenomenon was likely linked to discharges of nutrients from the Amazon River into the Atlantic.
“Before this study, it has been assumed that Sargassum mainly lives in the Sargasso Sea and Gulf of Mexico,” Wang said. “This is the first study to show the recurrent pattern of the GASB covering such a large spatial area across the tropical Atlantic, the Caribbean Sea, and the Gulf of Mexico.”
When this excess of nutrients enters the ocean, it fuels a rapid increase in the growth of certain types of algae. The scientists say that in recent years, the quantity of nutrients entering the ocean from the Amazon may have risen as a result of increased deforestation and fertilizer use in the rainforest—a potential factor in the rapid growth of Sargassum blooms.
driving the formation of the belt. Furthermore, they suggest in the study that a process called “upwelling”—where deep, cold, water rises to the surface—taking place off the West African coast may be delivering nutrients, which are contributing to the bloom.
“The evidence for nutrient enrichment is preliminary and based on limited field data and other environmental data, and we need more research to confirm this hypothesis,” Hu said. “On the other hand, based on the last 20 years of data, I can say that the belt is very likely to be a new normal.”