Sargassum on Delray Beach in South Florida Brian Cousin, Florida Atlantic University’s Harbor Branch Oceanographic Institute

Satellites have revealed the biggest ever microalgae bloom, stretching from West Africa to the Gulf of Mexico. Its continuing spread threatens ecosystems and marine life.

The stretch of brown seaweed, known as the Great Atlantic Sargassum Belt, is made up of floating islands of the algae that attract fish, birds and turtles.

But over the last decade, its growth has ballooned dangerously. By the middle of last year, the belt of algae had grown to more than 20 million metric tons and stretched almost 9000 kilometres.


Now, beaches along the Atlantic and Caribbean are seeing knee-high swathes of this seaweed mat washing ashore, posing a threat to the vital tourism industries of some of these communities.

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Ordinarily, these patches of algae can help provide refuge for marine animals in the open ocean and help oxygenate the water through photosynthesis. But rampaging growth, especially around coastal regions, can hamper animals’ ability to move and breath and can choke other corals and plantlife.

Sargassum off Big Pine Key in Florida Brian Lapointe, Ph.D., Florida Atlantic University’s Harbor Branch Oceanographic Institute

To understand what has been causing this spread, Mengqiu Wang at the University of South Florida and colleagues went back and analysed two decades of satellite data, cross referencing it with data that included Brazil’s fertilizer consumption patterns, Amazon deforestation rates and Amazon River discharge.

Before 2011, the amount of Sargassum around the equatorial Atlantic and Caribbean was negligible. But a major shift occurred in 2011, when the blooms spread into these regions for the first time on record.

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Wang found that the annual blooms seen almost every year from 2011 aligned with nutrient-rich discharge flowing out from the Amazon River during summer and spring, and winter upwelling near the West African coast that brings nutrient-rich deeper water into shallower depths.

Increasing deforestation and fertiliser use in Brazil over recent years has enriched the nutrients flowing from the Amazon and helped the algae bloom, wrote Wang.

With few signs that is slowing, these algal blooms are likely to be the new normal, says Wang.

Journal reference: Science, DOI: 10.1126/science.aaw7912