World's largest seaweed patch heads to Florida. Why we should worry

Beachgoers walk past seaweed that washed ashore on March 16, 2023 in Fort Lauderdale, Florida.

The world’s largest seaweed mass is entering the Gulf of Mexico and expected to float toward beaches along Florida and the Gulf as tourist season starts.

Tracts of sodden seaweed emitting a foul smell have already begun washing up like a bad omen on southern Florida beaches.

In Mexico’s Riviera Maya, hundreds of tons of the algae has piled up along the beaches as the mass moves westward between the Yucatan and Cuba.

The annual Atlantic sargassum belt grew to previously unseen proportions this year, spanning 5,500 miles, twice the width of the United States, and weighing nearly 13 tons.

“The large quantities already in the (Caribbean Sea and to the east) will continue to accumulate and migrate westward, creating beaching hazards along the way,” NASA and University of South Florida Optical Oceanography Lab projected.

WHAT IS IT?

The island-like expanse is carried about the Atlantic by small buoyant bladders, called pneumatocysts, which resemble berries and are filled with oxygen.

When they’re not menacing beaches, the huge rafts of rootless sargassum serve a vital ecological purpose, providing food, refuge and breeding grounds to birds, shrimp, fish and crab in the North Atlantic. The tide of leafy brown seaweed, with its sprawling stems and offshoots, acts as a nursery for commercially farmed fish, such as mahi mahi, jacks and swordfish.

The sargassum belt travels between West Africa and the Gulf of Mexico, where it circles and settles along the shore.

WHY WE SHOULD WORRY

Dr. Andrea Boggild, Medical Director of the Tropical Disease Unit at Toronto General Hospital, said she became aware of the harmful health effects of sargassum in 2018:

“I began to see patients returning from Caribbean travel with marine estuary syndrome-like symptoms including eye irritation, rashes, and respiratory symptoms, which also may have reflected short-term exposure to Sargassum,” she wrote via email.

“It is worth noting that as much of a problem Sargassum is for the tourism industry and travellers, the adverse health effects are borne mostly by residents of affected areas due to passive daily exposure and the more intensive exposures that result from actively working to remove the seaweed from beaches,” she added.

“Chronic exposure to these gasses can lead to conjunctival and neurocognitive symptoms such as memory loss and impaired balance, as well as non-specific symptoms such as headache, nausea and fatigue,” she said. “Rash and itchy feet from walking on Sargassum rafts is also reported.”

Bogglid tells travellers to “avoid touching or going” and beaches “where the smell of rotten eggs is in the air.”

“Upon return from travel, travellers should seek medical attention should any symptoms suggestive of Sargassum toxicity — such as shortness of breath, palpitations, chest pain, skin lesions, headache, memory loss or vertigo — arise,” she added.

The sargassum can also impede boaters, slowing down their vessels and causing damage as it gets sucked into the intake and becomes entangles on propellers.

Waters near a Tulum resort are brown from sargassum, a seaweed-like algae, on June 15, 2019 in Tulum, Mexico.

HOW IT GOT SO BIG

As a frequent traveller herself, Bogglid saw “huge swaths” of sargassum along coastlines, which built up faster than local mitigation teams could remove it, she noted.

Studies tracking sargassum have shown blooms starting off the northern coast of Brazil flourish as it’s fed nutrient-rich runoff from the Amazon and the Congo rivers due to deforestation and fertilizer use.

After reaching the shore, it accumulates insects and bacteria that deoxygenate the water and harm coral. As it decays, the seaweed releases hydrogen sulphide, which can cause cardiovascular, neurologic and sometimes severe respiratory symptoms, Bogglid said.