Dust from Sahara causes toxic blooms in US

Saharan dust clouds have travelled thousands of miles and fertilised the water off the West Florida coast with iron, setting off blooms of toxic algae, according to a new study.


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Every year iron from Saharan dust clouds is deposited in the waters off the West Florida coast, the NASA-funded study found. Once there, plant-like bacteria use the iron to set the stage for toxic algal blooms, or ‘red tides’. When iron levels go up, this bacteria, called Trichodesmium, fixes nitrogen in the water, converting it to a form usable by other marine life and the addition of biologically-usable nitrogen in the water makes the Gulf of Mexico a more likely environment for toxic algae to bloom. In the past, these red tides have killed huge numbers of fish, shellfish, marine mammals, birds, and can cause skin and respiratory problems in humans.

“This is one of the first studies that quantitatively measured iron from the dust and tied it to red tides through Trichodesmium,” said Jason Lenes, a graduate student at University of South Florida’s College of Marine Science, and the lead author in the study. Storm activity in the Sahara Desert region generates clouds of dust that originate from fine particles in the arid topsoil, which contains iron. Easterly trade winds carry the dust across the Atlantic Ocean and into the Gulf of Mexico.

The study used data from an imager aboard the National Oceanic and Atmospheric Administration’s (NOAA’s) Polar Orbiting Environmental Satellites (POES) and ground based measurements to track large dust clouds leaving Africa. The researchers discovered that when the Saharan dust reached the West Florida shelf it increased iron concentrations in the surface waters by 300%. As a result, Trichodesmium counts shot up 10 times what they had been prior to this event, causing the bacterium to use the iron to convert nitrogen in the water to a more organic form. Soon after, when levels of dissolved organic nitrogen had also increased by 300%, a huge bloom of toxic red algae had formed within the study area, an 8,100 square mile region between Tampa Bay and Fort Myers, Florida.

Scientists have worked for several years in an effort to develop a reliable method to predict red tides, particularly because the results of these blooms can be both physically and economically devastating to a region. “The West Florida shelf is a hot spot for fishing, aquaculture and tourism, all of which can be drastically affected by a surprise visit from a red tide,” said Lenes. Humans who swim in the Gulf can experience respiratory problems by breathing toxins from the red tide, whilst eating shellfish poisoned by the algae can lead to paralysis and memory problems. Around the Gulf of Mexico, scientists and others have recorded fish kills totaling in the millions and manatee deaths in the hundreds resulting from a single red tide bloom.

By using satellites to monitor dust arrivals and Trichodesmium blooms, Lenes said this research could lead to forecasting of red tides. “If you could predict when a red tide is coming, you could close beaches and fisheries ahead of time,” he said.

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