Since 2011, a monstrous structure has taken shape in the Atlantic Ocean almost every year, sprawling from the West African coast to the Gulf of Mexico. It’s the Great Atlantic Sargassum Belt—a gargantuan bloom of a brown free-floating seaweed. In May, the seaweed belt hit a record biomass of 37.5 million tons.
In a study published last month in the journal Harmful Algae, researchers from Florida Atlantic University’s (FAU) Harbor Branch Oceanographic Institute outline the rapidly growing seaweed’s development during the last four decades.
“The expansion of sargassum isn’t just an ecological curiosity—it has real impacts on coastal communities. The massive blooms can clog beaches, affect fisheries and tourism, and pose health risks,” Brian Lapointe, lead author of the study and a marine scientist at FAU Harbor Branch, said in a university statement.
“Understanding why sargassum is growing so much is crucial for managing these impacts,” he added. “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.”
Something massive this way comes
Scientists previously believed that sargassum was mostly limited to the Sargasso Sea’s nutrient-poor waters. More recent research, however, has revealed the organism to be quite the traveler, tracing sargassum’s movement from nutrient-rich coastal areas, such as the western Gulf of Mexico, to the open ocean, hitching a ride on the Loop Current (one of the fastest currents in the Atlantic) and the Gulf Stream. In the open ocean, nutrients are usually concentrated at great depth.
In 2004 and 2005, satellite imagery revealed massive sargassum windrows—long bands of floating sargassum—in the western Gulf of Mexico, a region where rivers, including the Mississippi and Atchafalaya, are increasingly dumping nutrients. “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,” Lapointe explained.
In fact, research since the 1980s revealed that the seaweed grows faster and is more productive in shallow nutrient-rich waters than nutrient-poor open ocean waters. In other words, more nutrients mean more sargassum. In certain conditions, the biomass of Sargassum natans and Sargassum fluitans can increase twofold in just 11 days.
Human-driven changes in nutrients
Phosphorus and nitrogen are crucial nutrients for sargassum. From the 1980s to the 2020s, while the seaweed’s nitrogen content rose by over 50%, its phosphorus declined.
“These changes reflect a 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,” Lapointe explained. In other words, human activity. Carbon levels in sargassum are creeping upwards, demonstrating how outside nutrients are changing its makeup and affecting ocean plant life, he added.
The researchers suggest that nutrients from the Amazon River play a huge role in the Great Atlantic Sargassum Belt, too, with floods and droughts in the Amazon basin seemingly associated with changes in the sargassum’s biomass. The team also highlights, however, that sargassum windrows are able to also grow in nutrient-poor waters by recycling nutrients in marine animal poop, among other methods.
“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,” Lapointe explained. “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.”
The study is just one more example of how human activity is driving deeply rooted ecological changes, with the extent of its farthest-reaching consequences still terrifyingly unknown.
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