A strange brown presence has been spreading across the Atlantic for years — and scientists say it’s a warning sign

Fifteen years ago, what scientists are now calling the Great Atlantic Sargassum Belt was a rare sight, largely confined to the Sargasso Sea in the mid-Atlantic. Today, it has grown into a colossal oceanic phenomenon.

In May 2025, satellites recorded 37.5 million tons of floating sargassum forming an unbroken strip more than 5,490 miles long—from the coast of West Africa all the way to the Gulf of Mexico. That’s more than twice the width of the continental United States.

What exactly is the Great Atlantic Sargassum Belt?

This massive accumulation of seaweed, described in the journal Harmful Algae, is raising alarms across the scientific community. The study, led by Florida Atlantic University’s Harbor Branch Oceanographic Institute, analyzed 40 years of satellite images, field data, and chemical samples. It found that since 2011, the blooms have been intensifying almost every year.

One of the most striking discoveries is the shift in nutrient sources. Between 1980 and 2020, nitrogen levels in the algae rose by 55 percent, while the nitrogen-to-phosphorus ratio jumped 50 percent. The surge isn’t just due to natural ocean processes—it’s being fueled by agricultural runoff, wastewater, and atmospheric deposits.

The Amazon River plays a critical role. During seasonal floods, the river injects nutrient-rich waters into the Atlantic, supercharging the growth of sargassum. In drought years, the bloom slows.

When seaweed turns from ally to threat

The National Oceanic and Atmospheric Administration (NOAA) recognizes sargassum as a vital habitat, sheltering more than 100 marine species. But too much of it flips the balance. When thick mats wash ashore, they rot, releasing toxic hydrogen sulfide gas, suffocating coral reefs, and creating oxygen-depleted “dead zones.” They also emit methane and other greenhouse gases, compounding the climate crisis.

Communities along the Gulf of Mexico and the Caribbean know the costs all too well. Cleaning up the tons of seaweed that blanket beaches each summer can run into the millions, hitting tourism and local economies hard. In fact, back in 1991, a mass stranding even forced the temporary shutdown of a nuclear power plant in Florida.

Why this matters now

Carried by powerful currents like the Gulf Stream, the Great Atlantic Sargassum Belt has become both an environmental and economic challenge. Researchers warn that without coordinated international action—better predictive models, tighter controls on agricultural runoff, and improved wastewater management—the problem will only grow.

For now, scientists are still assessing the long-term impact. But one thing is clear: this unprecedented seaweed belt is reshaping the Atlantic and stands to become one of the defining ocean management challenges of the coming decades.

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