Study: Fertilizer Runoff Part Of Toxic Sargassum Blooms, But Change In Ocean Current Key To Explosive Growth

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Scientists in Florida believe they have identified a “tipping point” in atmospheric conditions in the Atlantic Ocean they say caused giant clumps of toxic seaweed to inundate beaches around the Caribbean in recent summers. Previous theories for the Great Atlantic Sargassum Belt that has killed marine animals, harmed human health and plagued the tourism industry in several countries include a surfeit of nutrients in the water, such as nitrogen and phosphorus in runoff from intensive farming and carried into the ocean in the Congo, Amazon and Mississippi rivers.

While marine scientists at the University of South Florida (USF) acknowledge that as a contributory factor, they say the primary source of nutrients is a seasonal phenomenon known as vertical mixing in which shifting winds churn up the ocean and deepwater nutrient concentrations are brought to the surface.

They identified atmospheric pressure changes over the Atlantic beginning around 2009 as the tipping point, with variations in circulation and wind patterns pushing more sargassum into the warmer waters of the tropics, where it grew through photosynthesis into the massive blooms that eventually ended up on the beaches of the Caribbean and the US Gulf coast.

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Almost annually for more than a decade, giant clumps of goop from the 5,000-mile-wide sargassum belt have washed ashore in the Caribbean, and from Florida to Mexico, creating problems for local authorities and businesses that rely on tourism. There are also significant health concerns. The rotting sargassum releases large quantities of hydrogen sulfide, which fills the air with an acrid odor similar to rotting eggs, and can provoke asthma and other respiratory issues. In 2023, scientists at Florida Atlantic University (FAU) warned of a “perfect pathogen storm [with] implications for both marine life and public health”, having found high levels of the flesh-eating Vibrio bacteria lurking in the decomposing vegetation.

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https://www.theguardian.com/us-news/2025/mar/22/cause-toxic-seaweed-florida-sargassum

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1. K&R