World's Largest Seaweed Bloom Discovered from Space, Stretching 5,000 Miles Across the Atlantic

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.

The team also found that ocean circulation plays an important role in 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."

sargassum bloom
It's been a particularly bad year for brown seaweed in the Florida Keys. Brian Lapointe, Ph.D., Florida Atlantic University's Harbor Branch Oceanographic Institute