Sargassum in the Atlantic Ocean Reaches a New Spring High

Caitlin Dempsey


Sargassum are free-floating seaweed found in the open ocean of the Atlantic. Fueled by nutrient runoff from deforestation and increased fertilizer use in the Amazon, researchers have been tracking the formation of the Great Sargassum Belt each year since 2011.

What is sargassum?

Sargassum is a type of brown algae that is found in warm, nutrient-rich waters around the world. It is a key component of many marine ecosystems, providing food and habitat for a wide range of organisms, from small fish to whales.

The brown seaweed utilizes air-filled sacks resembling berries to create floating island-like clusters in the ocean, which can at times cover vast stretches of the Atlantic.

What is the Great Sargassum Belt?

The Great Sargassum Belt is a vast stretch of seaweed, known as sargassum, that forms in the middle of the North Atlantic Ocean. The Sargassum belt contains randomly dispersed clumps and mats of Sargassum across its 5000-mile span.

A clump of brown algae in the ocean.
A clump of Sargassum floating on the surface of the Atlantic Ocean in the Caribbean. Photo: © Nathie/

Benefits of sargassum

Many species of fish, such as mahi-mahi and tuna, use sargassum as a nursery habitat for their young, while sea turtles feed on the algae and use it as a place to rest and bask in the sun. The sargassum also provides important habitat for a variety of invertebrates, such as crabs, shrimp, and jellyfish.

Sargassum is also important for regulating the ocean’s carbon and nitrogen cycles and for producing oxygen through photosynthesis.

Negative effects of too much sargassum

Overgrowth of this algae, known as massive blooms of sargassum, have a negative impact by washing up on beaches and impacting local economies and ecosystems.

A man in a blue shirt shoveling seaweed that is clogging a beach with two small boats on a bright sunny day with a bright blue sky and the ocean in the background.
Sargussum can overwhelm beaches and impact local fishing industries. Sargassum on a beach in the Riviera Maya, Mexico. Photo: © lduarte/

When sargassum blankets a large area of open ocean, the presence of the algae can crowd out some marine species that require open ocean for movement and oxygen.

Mats of Sargassum have the potential to smother corals and seagrass, while the excessively thick layers of decaying algae can create on beaches located in the Caribbean and the Gulf of Mexico.

Where does the Sargassum Belt occur?

This overgrowth of algae has occurred most years since 2011 due to increase sediments running off from the Amazon River each spring and summer.

This super bloom forms a massive belt of sargassum which floats on the surface of the North Atlantic Ocean, reaching from the west coast of Africa to the Gulf of Mexico

Highest March presence of sargassum in the North Atlantic Ocean

March 2023 has turned out the highest density of sargassum since 2011 driven by the presence of large amounts of the brown algae in the central East Atlantic Ocean region.

A gray shaded relief map with a rainbow categorization showing the densities of sargassum in the Atlantic Ocean.
Map showing the density of sargassum in the North Atlantic Ocean in March of 2023 from NASA’s MODIS satellite instrument. Map: NASA, public domain.

Researchers from the University of Florida’s College of Marine Science measured the density of sargassum in the North Atlantic Ocean using remote sensing data from NASA’s Terra and Aqua satellites.

Using NASA’s MODIS instruments, the ocean is recorded in visible and infrared wavelengths. Despite their brownish appearance, Sargassum algae contain significant amounts of chlorophyll due to their photosynthetic nature. As a result, chlorophyll appears as an intense bright spot in infrared wavelengths, creating a stark contrast to the darker water it is found in.

By analyzing satellite data, the researchers estimate that the accumulation of sargassum in the North Atlantic Ocean in March of 2023 totaled 13 million tons. This represents the highest measured amount of sargassum in March.

The study estimates that the amount of Sargassum is expected to keep rising between April and June, causing the accumulation and westward migration of Sargassum in the eastern Caribbean, some of which may end up in the Gulf of Mexico and affect Florida. The peak month for sargassum abundance is estimate to be June of 2023.

Highest ever abundance of sargassum

The highest ever measurement of sargassum was happened in June of 2018. In a 2019 paper published in the journal Science, scientists found that the Great Atlantic Sargassum Belt held over 20 million metric tons of Sargassum biomass during that month.


Cassidy, E. (2023, April 7). A massive seaweed bloom in the Atlantic. NASA Earth Observatory.

Hu, C., & Xie, Y. (2023, March 31). Outlook of 2023 Sargassum blooms in the Caribbean Sea and Gulf of Mexico. Optical Oceanography Laboratory — College of Marine Science — University of South Florida.

Wang, M., Hu, C., Barnes, B. B., Mitchum, G., Lapointe, B., & Montoya, J. P. (2019). The great Atlantic sargassum belt. Science365(6448), 83-87. DOI: 10.1126/science.aaw7912


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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.

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