Scientists identify ‘tipping point' that caused clumps of toxic Florida seaweed
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.
'This was a surprising result,' said Frank Muller-Karger, distinguished professor and biological oceanographer at USF's College of Marine Science. His team of international researchers used computer modeling to replicate the transport of blooms on strong currents controlled by negative North Atlantic oscillation (NAO), and how the changing atmospheric conditions helped create the giant sargassum belt.
'We had posed the hypothesis before that it is not the rivers that feed the formation of the sargassum blooms in the tropical Atlantic. This model supports the idea that nutrients from slightly deeper layers in the ocean feed the blooms.'
Muller-Karger continued, saying that 'models showed that some patches of the sargassum were swept up by the wind and currents from the Sargasso Sea toward Europe, then moved southward, and from there were injected into the tropical Atlantic.
'At first, we saw just a few patches being pushed south by the NAO. But these algae patches were met with the right conditions to grow and perpetuate blooms. This population of algae, now separated from the Sargasso Sea, forms new blooms every year thanks to having enough light, nutrients and warmer temperatures,' he said.
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.
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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.
They said plastic debris in ocean pollution mixed with the sargassum to create a proliferation of the bacteria, creating an additional health risk for tourists, municipal crews clearing the seaweed and volunteers conducting beach clean-ups.
'These Vibrio are extremely aggressive and can seek out and stick to plastic within minutes,' Tracy Mincer, assistant professor of biology at FAU's Harbor Branch Oceanographic Institute and Harriet L Wilkes Honors College, said.
'We really want to make the public aware of these associated risks. In particular, caution should be exercised regarding the harvest and processing of sargassum biomass until the risks are explored more thoroughly.'
The USF research, a collaboration with scientists from the University of Toulouse, Sorbonne University and the Center for Scientific Research and Higher Education, was published in Nature Communications.
Their report analyzed decades of wind, currents and 3D nutrient measurements from the Atlantic to model the annual blooms.
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