Images from space reveal a previously unknown structure stretching almost 5,000 miles across in the Atlantic Ocean

What was once largely confined to the Sargasso Sea has evolved into the Great Atlantic Sargassum Belt. In May alone, scientists detected 37.5 million tons of this brown seaweed drifting between the western coast of Africa and the Gulf of Mexico. According to a study published in Science titled ‘Harmful Algae’, these algae have been developing at varying levels over the past four decades, with many of the nutrients that fueled their early growth originating from the Amazon River.

Researchers at the Harbor Branch Oceanographic Institute at Florida Atlantic University explain that they have spent four decades analyzing satellite data, field observations, and chemical studies to understand how a massive bloom first detected in 2011 evolved into an almost annual event. However, they believe that Portuguese explorers and adventurers may have already described the phenomenon in accounts from their voyages to the Americas during the 15th century.

What is Sargassum and why does it matter?

Sargassum is a genus of brown macroalgae (Sargassum) that drifts freely across the ocean. In the open sea, it provides habitat for a wide variety of marine species. Along coastlines, however, it becomes a significant environmental, economic, and public health problem. It also affects tourism, as visitors expecting pristine, postcard-perfect beaches often arrive to find foul-smelling shorelines covered with seaweed that are difficult to access.

The problems do not stop there. What functions as a valuable ecosystem offshore becomes a nuisance on land. As the seaweed decomposes, it produces unpleasant odors and can release hydrogen sulfide, a toxic gas associated with the decay of organic matter.

Large accumulations washed ashore can smother beaches, disrupt coastal ecosystems, create low-oxygen zones, and damage coral reefs. For economies that depend heavily on tourism, the impact is particularly severe. Local authorities must devote significant labor and resources to continuous beach cleanup efforts, often with little lasting success.

According to Brian Lapointe, Ph.D., lead author of the study and research professor at FAU Harbor Branch, “Our review takes a deep dive into the changing story of sargassum – how it’s growing, what’s fueling that growth, and why we’re seeing such a dramatic increase in biomass across the North Atlantic. By examining shifts in its nutrient composition – particularly nitrogen, phosphorus and carbon – and how those elements vary over time and space, we’re beginning to understand the larger environmental forces at play.”

Recent satellite observations, ocean circulation models, and field studies have helped solve this puzzle by tracing the seasonal movement of sargassum from nutrient-rich coastal waters, especially in the western Gulf of Mexico, into the open ocean through the Loop Current and the Gulf Stream. These findings support early theories suggesting that sargassum originating in the Gulf could help sustain populations in the Sargasso Sea.

“These nutrient-rich waters fueled high biomass events along the Gulf Coast, resulting in mass strandings, costly beach cleanups and even the emergency shutdown of a Florida nuclear power plant in 1991. A major focus of our review is the elemental composition of sargassum tissue and how it has changed over time,” Lapointe said.

Researchers also identified a major shift “away from natural oceanic nutrient sources like upwelling and vertical mixing, and toward land-based inputs such as agricultural runoff, wastewater discharge and atmospheric deposition. Carbon levels in sargassum also rose, contributing to changes in overall stoichiometry and further highlighting the impact of external nutrient loading on marine primary producers,” Lapointe explained.

“The expansion of sargassum isn’t just an ecological curiosity – it has real impacts on coastal communities,” Lapointe said. “The massive blooms can clog beaches, affect fisheries and tourism, and pose health risks. Understanding why sargassum is growing so much is crucial for managing these impacts. Our review helps to connect the dots between land-based nutrient pollution, ocean circulation, and the unprecedented expansion of sargassum across an entire ocean basin.”

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