The Golden
Tide
Since 2011 a band of floating brown seaweed has grown across the tropical Atlantic every year except 2013. It is now the largest seaweed bloom on Earth, and it keeps setting records.
tonnes of floating Sargassum measured by satellite across the Atlantic in May 2025, the all-time record and about five times the 7.3 million tonnes once estimated for the entire Sargasso Sea.
From a quiet sea to a record-setting belt
On one scale in million tonnes: the historic Sargasso Sea estimate is at the left, and the recent June totals and the May 2025 record sit far to its right.
8,850 kilometres of floating forest
The belt runs from West Africa to the Gulf of America, fed by Amazon outflow and by upwelling off Africa (Wang et al., 2019).
Even the densest patches on USF’s satellite maps cover only about 0.4% of the sea surface. The belt is loose clumps and mats scattered over millions of square kilometres. From a ship it is easy to miss. From orbit you can’t miss it.
It first appeared in 2011, after an unusual 2009–10 phase of the North Atlantic Oscillation pushed Sargassum south. According to NASA, the total has more than doubled every five years since then.
A June that keeps breaking records
Total Atlantic Sargassum each June, in million tonnes (USF satellite estimates). The 2025 point is the May record.
The Caribbean takes the heaviest load
Sargassum by region, June 2026 (million tonnes). In the Gulf, 2026 nearly doubled the record set a year earlier.
That is roughly the share of the Sargassum floating off the Mexican Caribbean that actually reaches shore, according to Mexico’s government. Even so, 92.5% of Quintana Roo’s beaches were rated “excessive” by late June 2026.
A richer diet and a fast metabolism
Since the 1980s, nitrogen in Sargassum tissue has risen by more than 50% while phosphorus fell slightly (Lapointe et al., 2025). Under ideal conditions the weed doubles in 11 days.
What it costs when it comes ashore
Beached Sargassum rots within about 48 hours and gives off hydrogen sulfide and ammonia. It also chokes lagoons, drives away tourists and has to be hauled away by the truckload.
Why it matters, and what helps
The belt looks set to return every year. Living with it comes down to forecasting, keeping people away from the gas, and cutting the nutrients that feed it.