For centuries, sailors crossing the Atlantic Ocean have occasionally encountered floating mats of brown seaweed drifting away from land. These spots were largely associated with the Sargasso Sea, an area of relatively calm waters in the North Atlantic Ocean, where floating sargassum has long formed part of a unique marine ecosystem.
This picture has changed dramatically over the past decade.Satellite observations now reveal a huge population of Sargassum extending across tropical Atlantic waters between West Africa and the Gulf of Mexico. What was once considered a more local phenomenon has expanded to become a recurring ocean-wide feature, covering thousands of kilometers and carrying tens of millions of tons of biomass. Scientists are increasingly trying to understand why this shift is occurring, where the nutrients that support it come from, and what it could mean for coastal areas on both sides of the Atlantic.
Atlantic seaweed The belt reaches a record level of 37.5 million tons
The modern story began in 2011 when satellites detected unusually large concentrations of sargassum outside its traditional strongholds. Since then, the floating algae have resurfaced almost every year, forming what scientists call the Great Atlantic Sargassum Belt.A landmark study published in the National Library of Medicine, “The Great Atlantic Sargassum Belt,” described a continuous belt extending from the coast of West Africa across the Atlantic Basin toward the Caribbean and the Gulf of Mexico.
The researchers estimated that during a particularly intense event in 2018, the belt extended about 8,850 kilometers and contained more than 20 million metric tons of Sargassum biomass.Recent assessments indicate that this phenomenon has continued to grow. According to a study published in ScienceDirect titled “Productivity, Growth, and Biogeochemistry of Pelagic Sargassum in a Changing World,” the belt reached a record biomass of about 37.5 million tons, making it the largest event observed since satellite monitoring began.
Why does the Atlantic Ocean’s giant seagrass belt keep growing?
Scientists do not point to a single reason. Instead, they describe a combination of ocean circulation patterns and increased nutrient availability.Ocean currents play an important role in transporting sargassum over vast distances. Water flowing through the Gulf of Mexico, the Loop Current, and the Gulf Stream can transport seaweed to different parts of the Atlantic Ocean. Meanwhile, currents and winds near the equator help distribute floating mats across tropical waters.Nutrients appear to be equally important. Evidence collected over several decades suggests that phosphorus and nitrogen help fuel sargassum growth. While natural sources such as bottom water upwelling to the coastal surface remain significant, researchers have identified increasing contributions from river discharge, agricultural runoff, sewage inputs, and atmospheric deposition.The Amazon River has received special attention. Several studies have linked changes in sargassum abundance to changes in nutrient-rich freshwater entering the Atlantic Ocean from the Amazon Basin.
Years with exceptionally high river discharge often coincided with stronger blooms offshore.
Atlantic seagrasses are showing signs of rising nitrogen levels
One of the most surprising findings from long-term monitoring is that the chemistry of the seaweed itself has changed.According to the study, analysis of hundreds of Sargassum samples collected over four decades indicates that nitrogen concentrations within the algae have risen dramatically. The researchers reported that nitrogen levels in tissues increased by more than 50% compared to 1980s measurements, while nitrogen-to-phosphorus ratios rose sharply.
These changes indicate increased nutrient enrichment across parts of the Atlantic Basin.This pattern suggests that modern sargassum grows in an environment more influenced by nutrient inputs from the ground than was previously the case. Scientists view this as evidence that human activities may now influence ecological processes across a much larger section of the ocean than previously assumed.
How does the Atlantic Sargassum Belt affect marine and coastal life?
Offshore, floating sargassum serves as important habitat.
Fish, sea turtles, invertebrates, and many other marine organisms use drifting algae for shelter, feeding, and reproduction. Some species spend a significant portion of their lives within these floating ecosystems. The situation changes when huge quantities reach the coasts. According to the study, large strandings can cover beaches, change local habitats and create low-oxygen conditions as seagrass decomposes.
This process may release hydrogen sulfide gas, producing a characteristic odor that is often reported in affected coastal communities. Tourism-dependent areas throughout the Caribbean and Gulf of Mexico have spent significant resources clearing beaches during large flows.Environmental impacts can extend beyond tourism. Heavy accumulations have been associated with pressure on seagrass beds, coral reef systems and nearshore marine habitats, raising concern among researchers and coastal managers alike.
