A record-breaking 37.5 million metric tons of seaweed was detected during May, with biologists warning that some Caribbean resorts may be blanketed by the naturally-occurring phenomenon during the coming weeks.
According to a new report from the University of South Florida’s Optical Oceanography Lab, the observed amount of seaweed was the largest detected amount in the Atlantic basin since monitoring began in 2011.
“As predicted last month, while the spatial distribution patterns of Sargassum remained stable, Sargassum amount in each region increased in May considerably,” researchers stated. “In every (area) but the Gulf of Mexico region, Sargassum amount exceeded its historical record. In particular, total Sargassum amount from all regions combined further increased from 31 (million) tons in April to 37.5 (million) tons in May, making a new record.”
Small amounts have already been detected on some beaches along the Gulf Coast and Florida, with experts and beachgoers alike wondering if or when the more substantial blooms will invade beaches across the southern US.
During typical years, the Florida Keys may see widespread reports during the late spring, with more significant impacts across the Florida peninsula during the summer and fall.
Photos and videos from eastern Puerto Rico show several waterways and beaches covered by the seaweed, with likely months to go before their inundation is over.
“Although 2025 is a record year in terms of Sargassum amount, whether a beach or small region receives record-high Sargassum inundation depends not only on the offshore Sargassum amount, but also on local factors that are difficult to predict, including winds and ocean currents,” USF staff stated.
Vacation destinations on alert
The exact causes of the growing blooms are not fully understood, but researchers point to climate change, fertilizer runoff and deforestation in South America as possible significant contributors.
Health experts say that while the seaweed may look and smell unpleasant, it poses little risk to humans, but it can significantly impact the tourism industry.
Popular beaches can turn into foul-smelling destinations, forcing local governments to spend significant resources on cleanup and restoration efforts.
As part of the Mexican government’s response, officials planned to focus on discovery and containment efforts in the waters surrounding nearly four dozen popular beaches along the Yucatán Peninsula and Caribbean coast.
For areas around the Yucatán Peninsula, seaweed typically washes ashore between April and October but, due to climate change, its increased presence has become particularly problematic in recent years.
It remains undetermined if the seaweed will plague Florida like in 2023, when many beaches were covered in the sticky mess.
Looking ahead, the report expects the sargassum to continue to grow in coverage throughout June, particularly in the western Caribbean and into the Gulf.
For some species of marine life, the brown algae is considered to be helpful, with biologists stating that the buildup provides food and refuge for small fish, crabs, shrimp and other organisms.
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