The total amount of sargassum, the green and yellow-ish seaweed that washes up on South Florida beaches in spring and summer, is at record-breaking levels, said researchers in a monthly report.
The seaweed, which often stinks of rotten eggs once it washes ashore, has not reached South Florida yet, but it’s currently drifting this way.
A mapping system devised by the University of South Florida Optical Oceanography Lab shows that sargassum levels for April in the eastern Caribbean and the adjacent Atlantic Ocean were 200% higher than their historical records for April.
The total amount when combining all regions, including the Gulf, was 150% higher than the historical record in April. All told, the coverage was 40% higher than the all-time high in June 2022.
Experts expect the floating seaweed mats to enter the Gulf of Mexico, and then drift on Gulf Stream currents to end up along South Florida beaches.
USF has been mapping the seaweed abundance since 2011.
Sargassum usually blooms from May to August, both locally and across the Atlantic. Ocean currents transport it closer to Florida as the spring progresses.
It’s unclear when the sargassum might reach South Florida en masse. Once it does, Barnes said the wind still has to push it ashore.
Though sargassum s a food chain of marine life in the open ocean, once it decomposes on shore, it can release hydrogen sulfide, a gas that has an odor reminiscent of rotten eggs, and can cause respiratory problems.
The amount of the often-stinky seaweed that washes up on beaches has spiked tremendously in the last dozen years, bringing frustration to South Florida beachgoers and causing real economic damage in the Caribbean.
Sargarssum historically grew within the Sargasso Sea, an area surrounding Bermuda and reaching about halfway across the Atlantic. But around 2011, the blooms started to shift south, closer to the equator. This not only increased their size, but put them on a trajectory that swept them into the Gulf, and thus eventually onto South Florida beaches.
Barnes said researchers are looking into why sargassum abundance fluctuates. He said sea-surface temperatures can be too warm, so it’s not as simple as being closer to the equator. Upwellings from deep currents can fuel sargassum with nutrients, he said. Other researchers have suggested that heavy rain years in the Amazon Basin cause more nutrients to flow into the equatorial Atlantic, thus fueling the seaweed.
Parts of the Amazon region faced severe drought in 2024, and some areas are now recovering. But 2025 is expected to remain below normal.
Looking ahead: Researchers said May should see continued seaweed increases in most regions. “Sargassum inundation will continue to occur in most of the Caribbean nations and islands as well as along the southeast coast of Florida,” said the report.
