Researchers at Florida Atlantic University’s Harbor Branch Oceanographic Institute have published a comprehensive review examining four decades of changes in pelagic sargassum, a type of free-floating brown seaweed. The study, featured in the journal Harmful Algae, analyzes the growth and distribution of sargassum across the Atlantic Ocean and highlights its increasing prevalence.
Historically believed to be mostly confined to the Sargasso Sea, sargassum is now found widely throughout the Atlantic. Its expansion has been linked to both natural ocean processes and increased nutrient runoff from human activities. The review traces the development of the Great Atlantic Sargassum Belt (GASB), a recurring bloom stretching from West Africa to the Gulf of America that first appeared in 2011 and has recurred nearly every year since, except for 2013. In May, this belt reached a record biomass of 37.5 million tons, not including an estimated baseline of 7.3 million tons in the Sargasso Sea.
Lead author Brian Lapointe, Ph.D., research professor at FAU Harbor Branch, explained: “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.”
Lapointe worked with Deanna F. Webber and Rachel Brewton, Ph.D., both also with FAU Harbor Branch. They note that early oceanographers described sargassum as thriving in warm but nutrient-poor waters; however, later research revealed paradoxes when these same regions were called “biological deserts.” Newer satellite imagery and ocean models have shown that sargassum can travel from nutrient-rich coastal areas—especially those affected by river runoff like the Mississippi—to open ocean via currents such as the Loop Current and Gulf Stream.
Remote sensing was key to these findings. In 2004-2005, satellites detected large windrows—bands of floating sargassum—in areas experiencing high nutrient loads from rivers along the Gulf Coast. Lapointe stated: “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.”
Laboratory experiments since the 1980s have confirmed that sargassum grows faster in water enriched with nutrients compared to open-ocean conditions. Studies show its two main species can double their biomass within about eleven days under optimal circumstances; phosphorus is often identified as a limiting factor for growth.
Over recent decades, researchers observed more than a 50% increase in nitrogen content within sargassum while phosphorus levels declined slightly—resulting in higher nitrogen-to-phosphorus ratios overall.
“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,” said Lapointe. “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.”
The study also discusses how recycling nutrients within dense mats—through excretion by marine organisms or microbial breakdown—helps sustain growth even where nutrients are otherwise scarce.
Data collected near where the Amazon River meets the sea support theories that outflows from large rivers play an important role in developing massive blooms like GASB; fluctuations there have been tied to flood or drought cycles upstream.
The formation of GASB may have been triggered by unusual atmospheric conditions during 2009-2010 but researchers caution there is no direct evidence for this hypothesis yet; genetic data suggest some populations already existed before major blooms began appearing after 2011.
“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,” said Lapointe. “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.”
Funding for this work came from several agencies including state departments as well as federal programs focused on environmental monitoring.
