Why was a floating seaweed taking an entire ocean? Researchers have the answer

Sargasse’s expansion through the Atlantic is linked to nutrient pollution and ocean circulation. Its growth now affects ecosystems and coastal communities.

Researchers from the Harbor Branch Oceanographic Institute of Florida Atlantic University have compiled a complete review covering forty years of data on the pelagic sargasse, the floating brown algae that play a crucial role in the Atlantic Ocean.

For decades, scientists thought that Sargasse was largely limited to poor water in nutrients from the Sargassus Sea. It is now clear that these algae have become a widespread and rapid presence across the Atlantic, with its expansion linked to both natural variability and nutrient -focused contributions.

Published in the journal PestsThe review examines the emergence and persistence of the Great Sargasse belt of the Atlantic, a huge seasonal flowering that extends from West Africa to the Gulf of Mexico.

Since its first observation in 2011, this belt has formed almost every year – except in 2013 – and in May has reached a record biomass of 37.5 million tonnes. This figure excludes long -term background biomass of 7.3 million tonnes that are generally found in the Sargassus Sea.

Link the enrichment of nutrients to the expansion of Sargasse

The analysis incorporates historical oceanographic recordings, modern satellite data and detailed biogeochemical studies to better explain changes in the abundance, distribution and balance of sargasse nutrients. The results emphasize the influence of the nutritional load based on humans on ocean processes and the urgent need of international collaboration to follow and alleviate the impacts of these vast algae flowers.

“Our criticism plunged deeply into the changing history of Sargasse – how it develops, which fuels this growth and why we see such a spectacular increase in biomass across the North Atlantic,” said Brian Lapointe, Ph.D., principal author and research teacher at FAU Harbor Branch. “By examining the changes in its nutritional composition – in particular nitrogen, phosphorus and carbon – and how these elements vary in time and space, we start to understand the larger environmental forces in play.”

At the start of the journal, Brian Lapointe and his colleagues, Deanna F. Webber, research coordinator, and Rachel Brewton, Ph.D., assistant research professor at the Fau Harbor branch, note that the first oceanographers have mapped the Sargasso Sea by following the surface plates of Sargassum. They supposed that the seaweed prospered in its warm, clear but poor in nutrients. This idea later presented a paradox, as researchers in the middle of the 20th century then described the same region as a “biological desert”.

Resolve the paradox with modern studies

However, recent satellite observations, ocean circulation models and field studies have resolved this paradox by drawing seasonal transport of sargasse from coastal areas rich in nutrients, in particular the Western Gulf of America, in the ocean opened via the loop current and the Gulf Stream. These results support the first theories by explorers who proposed that the Sargasse from the Gulf could feed the populations of the Sargassus Sea.

Remote design technology has played a central role in these discoveries. In 2004 and 2005, satellites captured extended sargassum winds – long narrow lines or floating sargasse bands – in the Gulf of West of America, a region experiencing increased nutrient loads from river systems such as Mississippi and Atchafalaya.

“These water -rich waters have fueled high biomass events along the Gulf coast, which causes mass shutters, expensive beach cleanings and even the emergency stop of a Florida nuclear power plant in 1991,” said Lapointe. “A major objective of our review is the elementary composition of the sargassum fabric and how it has changed over time.”

Growth rate and nutrients limit

Laboratory experiences and field research dating from the 1980s have confirmed that Sargasse developed more quickly and is more productive in neritic waters enriched in nutrients than in the oligotrophic waters of the open ocean. Controlled studies have revealed that the two primaries[{” attribute=”” tabindex=”0″ role=”link”>species, sargassum natans and sargassum fluitans, can double their biomass in just 11 days under optimal conditions. These studies also established that phosphorus is often the primary limiting nutrient for growth, although nitrogen also plays a critical role.

From the 1980s to the 2020s, the nitrogen content of sargassum increased by more than 50%, while phosphorus content decreased slightly, leading to a sharp rise in the nitrogen-to-phosphorus (N:P) ratio.

https://www.youtube.com/watch?v=X3F1TXFYVSW
Sargasse’s story over four decades. Credit: Brian Lapointe, Fau Harbor Branch

“These changes reflect a distance from the natural sources of ocean nutrients such as upwelling and vertical mixture, and to land inputs such as agricultural runoff, wastewater discharge and atmospheric deposit,” said Lapointe. “The carbon levels in the Sargasse have also increased, contributing to changes in global stoichiometry and putting more emphasizing the impact of the load of external nutrients on marine primary producers.”

The journal also explores how the recycling of nutrients in the Sargasse Windrows, including the excretion of associated marine organisms and the microbial degradation of organic matter, can support the growth of nutrient -poor environments. This micro-scale recycling is essential to maintain sargassum populations in certain parts of the ocean which, otherwise, would not support high productivity levels.

Influence of the flow of the Amazon river

Sargassum data collected near the mouth of the Amazon river support the hypothesis that the outputs of nutrients in this large river significantly contribute to the development of the Gasb. The variations in the sargasse biomass were linked to the cycles of floods and drought in the Amazon basin, moreover connecting the entrances of land nutrients to the open ocean.

The Gasb formation seems to have been sown by an extreme atmospheric event – the negative phase of the North Atlantic oscillation in 2009 to 2010, which may have helped to move surface waters and the south south south south south south in the tropical Atlantic.

However, researchers warn that there is no direct evidence of this movement. In addition, genetic and morphological data suggest that certain Sargasse populations, in particular the S. Natans dominating Var. Wingei, was already present in the Tropical Atlantic before 2011, indicating that this region may have had a neglected role in the early development of the Gasb.

“The expansion of sargasse is not only an ecological curiosity – it has real impacts on coastal communities. Massive flowers can obstruct beaches, affect peaches and tourism and pose health risks, “said Lapointe. “Understanding why Sargassum develops so much is crucial to manage these impacts. Our examination helps to link the points between pollution of land nutrients, oceanic circulation and the unprecedented expansion of the Sargasse in an entire ocean basin. ”

Reference: “Productivity, growth and biogeochemistry of pelagic sargasse in a changing world” by Brian E. Lapointe, Deanna F. Webber and Rachel A. Brewton, August 8, 2025, Pests.
DOI: 10.1016/J.HAL.2025.102940

This work was funded by Florida Department of Emergency Management, United States Environmental Protection Agency, South Florida Program Project and the Noaa Monitoring and response to the event for the flowering program of harmful algae. Historical studies included in the examination were funded by the Nasa Program of biology and ocean biogeochemistry and ecological forecast program, scientific program of the NOAA Restore, National Foundation of Sciences, specialized registration plate “Save Our Seas” and discretionary funds, granted by the Harbor Branch Oceanographic Institute Foundation, and a Red Wright scholarship of the Bermuda Biological Station.

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