A USF-led investigation reveals a sharp drop in certain historic sargassum populations, even as larger algal blooms accumulate in tropical Atlantic waters and push thick patches ashore along Caribbean and Gulf coasts.
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TAMPA, Fla. (Dec. 2, 2025) – New findings from the University of South Florida’s College of Marine Science show that specific sargassum populations have declined substantially over the last ten years, while the tropical Atlantic continues to host vast mats of seaweed that inundate beaches in the Caribbean and Gulf regions.
The study notes a steep decline in sargassum abundance in the Atlantic’s north Sargasso Sea since 2015, as reported in a Nature Geoscience paper released this week. Meanwhile, Gulf-sourced sargassum—which traditionally replenishes the Sargasso Sea each year—has also fallen markedly.
embargoed for public release until Thursday, Dec. 4, at 5 a.m. ET, the findings point to ocean warming as a potential driver behind the decline and suggest a major shift in where sargassum can thrive, with possible consequences for marine ecosystems.
“Two opposing trends are unfolding in the Atlantic,” said Chuanmin Hu, professor of oceanography at USF’s College of Marine Science and the study’s senior author. “The tropical Atlantic has seen rising sargassum levels over the past decade, yet the Sargasso Sea now harbors far less sargassum than before.”
Like rainforests, large floating mats of sargassum support rich biodiversity, acting as nurseries for turtles, crabs, shrimp, various fish species, and seabirds, some of which are specially adapted to rely on these seaweed habitats.
But when such mats wash ashore and decay, they can emit foul odors, harm marine life, and disrupt coastal communities. Cleanup efforts in the United States alone cost businesses and governments millions annually.
Hu has tracked sargassum from space since 2006 and helped identify the Great Atlantic Sargassum Belt in 2019. His Optical Oceanography Lab provides satellite imagery and monthly bulletins that keep stakeholders informed about sargassum biomass from orbital views.
For this study, Yingjun Zhang, then a postdoctoral researcher, and Brian Barnes, assistant research professor at USF, analyzed NASA satellite data to gauge changes in sargassum biomass. The team collaborated with researchers at Sea Education Association and Eckerd College, who contributed field data that confirmed declines seen from space and supplied historical records, temperature tolerance information, and diversity-level distribution insights that remote sensing alone could not reveal.
In the Gulf, a spring bloom of sargassum typically travels northward and reaches the Sargasso Sea with peak activity in late fall or winter. The current dip in Gulf sargassum has also reduced the amount of healthy seaweed arriving in the Sargasso Sea.
“These results may indicate the early stages of a basin-scale regime shift in sargassum distribution,” commented Zhang, now a postdoctoral researcher at the Scripps Institution of Oceanography. “Since many marine organisms rely on pelagic sargassum ecosystems, the changes could have meaningful ecological implications.”
Barnes added, “The regime shift also changes seasonality; what used to be fall and winter peaks are increasingly replaced by summer peaks in the north Sargasso Sea.”
By examining the trio of essential factors for plant growth—light, nutrients, and temperature—researchers propose that record Gulf temperatures, more frequent marine heat waves, and possible nutrient competition from Caribbean-sourced sargassum may have hindered Gulf populations. The result is weakened sargassum that struggles to survive upon reaching the Sargasso Sea.
Long-term data show Gulf waters warming at about 0.19°C (0.34°F) per decade from 1970 to 2020, roughly twice the rate of the global ocean. Sargassum in the Gulf favors temperatures between 20–28°C (68–82°F); recent summer temperatures in the Gulf have surpassed 30°C (86°F).
Even the extensive 5,000-mile Great Atlantic Sargassum Belt hasn’t offset the Gulf’s decline. Sargassum arriving from the belt may be acclimated to warmer conditions or arrive in poorer health and struggle in the cooler Sargasso Sea.
“It’s a complex narrative, challenging to decipher due to the vast spatial scale and the fact that each sargassum variety responds differently to environmental cues,” said Deb Goodwin, chief scientist at Sea Education Association and a co-author. “Long-term data on sargassum varieties provided essential context to the satellite observations.”
Looking ahead, the team aims to deepen understanding of how shifting sargassum populations could affect marine ecosystems, including whether competition from the Great Atlantic Belt might drive further declines in the Gulf.
Read more: New sargassum system targets troublesome seaweed
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Journal: Nature Geoscience
Method: Data/statistical analysis
Topic: Not applicable
Article title: Dramatic decline of Sargassum in the north Sargasso Sea since 2015
Publication date: 4-Dec-2025
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