Seaweed meets Climate Change

Mats of dead sargassum line the beaches of Cape Canaveral

Sargassum seaweed now regularly piles up on Florida beaches 

The seaweed ‘belt’ extends 

across  the Atlantic Ocean to Africa 

The piles of sargassum just keep getting taller on Florida's beaches, and the smell keeps getting worse. According to a new study led by scientists from the University of South Florida-St. Petersburg, everyone better get used to it. 

By analyzing satellite imagery, Chuanmin Hu of the College of Marine Sciences and his colleagues found that, starting in 2011, the once-spotty growth of sargassum now regularly forms thick, sprawling mat that stretches across the surface of the Atlantic Ocean, a mat that they're calling the Great Atlantic Sargassum Belt. 

Follow Phillip

Sígueme en Instagram

@majikphil

Last year's belt stretched from the west coast of Africa to the Caribbean Sea, which the scientists labeled "the biggest seaweed bloom in the world." "Based on the last 20 years of data, I can say that the belt is likely to be a new normal," Hu said.

Sargassum is a genus of brown (class Phaeophyceae) macroalgae (seaweed) in the order Fucales.  Sargassum muticum may be among the most common.  Historically it was referred to as "Japanese wireweed." 

The weed can grow up to 10 cm per day during peak growth periods.  Wikipedia has an exhaustive list of some 250 species of Sargassum here:  List of Sargassum species.

Look closely at individual floating plants and discover a diverse ecosystem, hanging on.

The great Atlantic Sargassum belt

The study, just published in Science, found that the expansion of the sargassum coincides with two key events in the spring and summer. One, in the Amazon's rain forest, the continued deforestation by farmers has led to an increase in the amount of nutrient pollution flowing out to sea from the Amazon River. Meanwhile (2) a regular upwelling current from the ocean's depths pushes nutrients that had been on the bottom up to the surface to further fuel the seaweed's growth. 

Sargassum is a naturally occurring plant floating in the Atlantic, growing in mats and patches here and there, particularly in the portion known as the Sargasso Sea. 

The two predominant species of sargassum in the Sargasso Sea are the only seaweeds in the world that don't begin life attached to the seafloor.  As a consequence of this life unmoored, the translucent gold- to amber-colored mats exist at the whim of the winds and currents, pulled apart in storms and reassembling in calmer seas, their edges locking together like Velcro.  The weed masses vary in size from several miles long to pieces no bigger than your hand.

Although it doesn't look like much, it provides essential habitat for sea turtles, crabs, fish and ocean-going birds. As an ecosystem, it's so vital to the ocean's health that acclaimed oceanographer Sylvia Earle, who grew up in Dunedin, has compared it to a golden rainforest. Many species have evolved to look just like sargassum and live within the floating mats.

The Sargassum Fish (Histrio histrio) is a frogfish in the family Antennariidae, the only species of its genus.
Photo:  David Liittschwager

Life in the North Atlantic depends on this floating weed

. . .but Sargassum is a nuisance on some beaches while providing shelter and a feast for a complex and amazing variety of marine life.

Christopher Columbus was the first to report the seaweed floating in what became known as the Sargasso Sea. According to National Geographic, the sailors aboard Columbus’ Santa María found it to be less of a marvel and more of an obstruction, noting in their ship's log that in some places it was “so thick that it actually held back the ships.” Later explorers noticed that the air bladders that keep it afloat resembled a type of grape called sargazo — hence the name. 

Sargassum clump drifting near Bermuda
Photo:  David Liittschwager

In recent years sargassum has become a major headache for beachgoers in Mexico and across the Caribbean as well as Florida, with vast mats of the stuff washing ashore and filling the sea breezes with an eye-watering stench reminiscent of rotten eggs as all the organisms living within the seaweed die as it is beached.

Last year 20 million tons of it floated through the Atlantic, and when it died, it sank to the bottom where it smothered corals and sea grass beds. Hu and his colleagues examined satellite images of the sargassum dating back to 2000 to see how the seaweed's growth has changed to produce the belt. Prior to 2011, they saw that it primarily floated in patches around the Gulf of Mexico and the Atlantic. But starting in 2011, the mats became larger and thicker, spreading into areas where it had never been spotted before. 

 "The ocean's chemistry must have changed in order for the blooms to get so out of hand," Hu said. 

When dried the prickly seaweed is uncomfortable on bare feet but creates irreplaceable erosion control aiding in the building new sand dunes.

His team was able to correlate the explosion of sargassum with increases in the Amazon's deforestation and also an increase in the use of fertilizer as farms supplant the rainforest. Major blooms occurred every year between 2011 and 2018, except for one. 

As the heaps of sargassum washed ashore in Florida, they produced headlines like this one from the Palm Beach Daily News in April: "Seaweed siege unusual during tourism season." 

One hotel tried sending workers out every morning to gather up all the sargassumand bury it, but they spent all day working on that only to see more of the stuff float in the next day. 

A diver beneath a mass of sargassum near Cozumel, Mexico
Photo:  David Doubilet

Cancun Has a Nasty Seaweed Problem and Tourists are Staying Away

Sargassum is washing up, turning beaches into a slimy, brown eyesore

Thousands upon thousands of tons of sargassum have been piling up in Cancun and dozens of nearby beaches in the Yucatan Peninsula, such as Tulum, Playa del Carmen, and Holbox.  Not only is sargassum an eyesore for tourists, but as organisms in the weed die it reeks, smelling of rotting eggs, or worse.

A worker trying to clean sargassum from a Tulum, Mexico Beach
Photo:  Justin Sullivan

By calling this the "new normal," Hu said he didn't mean there would be bigger and bigger blooms every year. Instead, what he meant is that there's no going back to the pre-2011 patterns of growth. 

 "In general and on average, future years will be more like 2011-18 rather than before 2011," he explained. There are still aspects of the sargassum's interactions with the ocean that need study, he said. 

For instance, the scientists do not know what, if any, role might be played by ocean acidification, which results from the world's oceans soaking up more carbon dioxide than they can handle, which makes the water more acidic.

Sargassum Seahorse (Hippocampus erectus) aka the "Lined Seahorse," or "Spotted Seahorse," or "Northern Seahorse"
Photo:  Matt Sullivan

Ultimately, though, Hu said climate change is a major factor because it's causing increased rainfall that then washes the excess nutrient pollution out to sea, fueling the growth of the sargassum. 

And what can the beach communities of Florida and other places do about the piles of putrid plants? "There is no way to kill these naturally occurring blooms," Hu said, "so the only way is to learn how to mitigate the impacts and how to adapt."