World’s Largest Seaweed Bloom Stretched From Africa To The Gulf Of Mexico Last Year

Too much Sargassum can present challenges for marine life and particularly becomes a problem when it collects along coastlines and rots, as shown here in Cancun in 2015. Michael Owen

A gargantuan seaweed blob stretching from the west coast of Africa to the Gulf of Mexico has been detected by researchers. It is believed to be the world’s largest seaweed bloom, prompting concern from scientists who say that this “new norm” has the potential to suffocate species and disrupt coastal communities.

Dubbed the Great Atlantic Sargassum Belt (GASB), the pelagic mat of seaweed stretches a total of 8,850 kilometers (5,500 miles) and weighs more than 20 million tonnes (22 million tons). Scientists were able to track the growth of the macroalgae belt using NASA satellite observations from between 2011 and 2018, according to a new study published in Science.

Sargassum floats in patches around the Gulf of Mexico and the central Atlantic’s Sargasso Sea. In appropriate amounts, Sargassum keeps the ocean healthy by providing habitat and refuge for marine mammals and producing oxygen via photosynthesis like plants. But when it booms, large mats of seaweed can make it difficult for species to move and breathe, particularly along the coast. When it dies, the seaweed sinks to the seafloor and can smother vital coral and seagrass. Onshore, dead and dying Sargassum releases hydrogen sulfide gas that smells like rotten eggs and can be harmful to people with respiratory issues. It’s also a nuisance – last year, the government of Barbados declared a national emergency after seven beaches were inundated with the seaweed.

NASA’s satellite data confirms that the record-breaking seaweed belt forms in the summer months (Northern Hemisphere). 2015 and 2018 had the biggest of the massive blooms that started in 2011. USF College of Marine Science

Researchers observed that a shift in Sargassum blooms has occurred since 2011 (with the exception of 2013) in places where the seaweed hadn’t occurred before due to a combination of environmental and human-driven factors. In the spring and summer, the Amazon River discharges nutrients into the ocean that spur the growth of macroalgae. In recent years, an uptick in deforestation and agricultural production in the region has resulted in increases in nitrogen and phosphorus, which was confirmed by the study’s preliminary findings. In the winter, natural upwelling that occurs off the coast of West Africa delivers more nutrients from deep waters to the surface where Sargassum grows. In addition, researchers say that the ocean’s chemistry must have changed in order to provide such favorable conditions that allow for the blooms to become so large.

"This is all ultimately related to climate change because it affects precipitation and ocean circulation and even human activities, but what we've shown is that these blooms do not occur because of increased water temperature," study author Chuanmin Hu in a statement. "They are probably here to stay."

Predictions are difficult to make given the unprecedented nature of the GASB, but the researchers say they hope their work provides a framework for understanding and responding to such phenomena in the future.

"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," said Hu. "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.”

Further validation incorporating other possible explanations is also required in order to consider all conditions that may contribute to the GASB. 

The team used numerical simulations to confirm that the Sargassum belt forms its shape in response to ocean currents. They divided the Atlantic Ocean into several regions for the current study. USF College of Marine Science
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