Seaweed mass ‘disappears,’ stumping local researchers

A floating seaweed mass that was once 5,000 miles long and visible from space has virtually disappeared from the Gulf of Mexico, leaving local scientists searching for answers.

The “Great Atlantic Sargassum Belt” ballooned to a record size in March before slightly decreasing in May. However, that is when a Florida Atlantic University study warned that the “seaweed” mass and accompanying floating plastic could carry Vibrio vulnificus, or “flesh-eating” bacteria, ashore.

As it turns out, fears of stinky brown algae inundating beaches and causing health and economic issues were largely unfounded. Dr. Chuanmin Hu, a College of Marine Science professor at the University of South Florida St. Petersburg campus, struggled to explain the phenomena.

“I know you have a lot of questions,” Hu said. “But unfortunately, I don’t have most answers for them.”

Sargassum masses can trap boats like this one docked in the Cayman Islands. Screengrab, video by Sandy Hill.

Hu and his colleagues at the USFSP’s Optical Oceanography Laboratory track and study sargassum blooms using satellites and fieldwork. NASA and the National Oceanographic and Atmospheric Administration (NOAA) began funding the College of Marine Science’s efforts in 2011 when a few intermittent clumps began forming the trans-Atlantic belt.

The mass has grown exponentially in the past 12 years, setting new records in 2018 and 2022. This year’s bloom was the largest in history for March, and researchers believed it would continue growing until peaking in June or July.

In addition to identifying what is fueling the belt’s exponential growth, they are now working to understand what caused its rapid dissipation.

The 5,000-mile mass confoundingly decreased by 75% in the Gulf last month and is no longer visible by satellites. “We know what happened, but we don’t know much about why,” Hu said.

“That is pretty much true for everything we observe in the ocean,” he added. “Because there are many factors. Until we have some dedicated studies and experiments, all we can do is, put some data together and speculate.

“In any case, it’s good news for Florida because the season is over.”

A USF College of Marine Science satellite-based map from June shows the Gulf of Mexico inexplicably clear from any significant blooms. Image provided.

Hu explained that small amounts of sargassum could continue washing ashore but not enough to affect beachgoers or require extensive cleanups. He noted some coastlines in the Keys “got it pretty bad” through May.

He said beaches began clearing up in June and would mostly remain free from a rotting mess. The Great Atlantic Sargassum Belt is actually the world’s largest brown algae bloom, and it absorbs carbon dioxide and provides a rich marine habitat while drifting between the coast of Africa and the Gulf of Mexico.

However, the seaweed begins degrading water quality as it reaches shorelines. It also emits a noxious “rotting egg” odor, attracts insects and bacteria and repels tourists – impacting local economies.

Hu speculates that offshore tropical storms and above-average winds helped reduce the mass to small clumps or killed the bloom and caused it to sink. While he declined to opine on recent heat waves contributing to the historical decrease, Hu noted that plants can only survive in specific temperature ranges.

While Florida’s beaches are now safe, he said there is still a significant amount of sargassum in the Caribbean Sea and Western Atlantic, which could threaten island nations. He added that when accumulations fail to register on satellite imagery, “it is like they don’t exist.”

Hu also pledged that the mass would return next spring.

“It will not disappear all of a sudden,” he said. “It will come back next year; we just don’t know the magnitude.”

Dr. Chuanmin Hu, a professor at the College of Marine Science, stands in front of a research vessel. Photo by Mark Parker.

Like the harmful algal blooms that cause red tide, Hu said climate changes and nutrients – nitrogen, phosphorus and iron – can fuel growth. Many factors play a role, and much work remains before scientists better understand the causes.

Hu said local researchers will increase efforts to monitor near-shore environments rather than focusing on open seas. The goal is to inform coastal populations, but first, they must increase their understanding of an “abnormal” phenomenon.

“This year is really strange,” Hu added. “It started early, but all of a sudden, it shrank dramatically. It’s never happened in history at this time.”