Desert Dust Kills
Florida Fish

New research links huge African dust clouds with the "red tides" that kill millions of fish along the Florida coast each year.

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August 30, 2001: It sounds like a story from the Old Testament: Without warning, the sea turns a shade of reddish brown, killing scores of fish and other marine life -- and making the water an unwelcome place for humans.

Such "red tides" have, from time to time, plagued coastal communities for centuries. Now a new study, partially funded by NASA, has revealed a surprising connection between red tides in the Gulf of Mexico and giant dust clouds that blow across the Atlantic Ocean from the distant Sahara Desert. NOAA and NASA satellites can spot such dust clouds en route from Africa to the Americas, raising hopes that space-based data could help scientists predict when red tides will strike the Gulf coast.

Right: Scientists sample Gulf waters for iron, nitrogen, and algae concentrations. African dust not only encourages algae blooms, but also beautiful red Florida sunsets. Image courtesy Florida Marine Institute.

"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 Jason Lenes, a graduate student at University of South Florida's College of Marine Science, and the lead author in the study.

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Red tides, which are actually blooms of toxic algae, have in the past killed huge numbers of fish, shellfish, marine mammals, and birds. They can also trigger skin and respiratory problems in humans.

Storm activity in the Sahara Desert region kicks up fine particles from the arid topsoil there, generating vast clouds of dust. Easterly trade winds carry the dust across the Atlantic Ocean and into the Gulf of Mexico.

The new study shows that these clouds fertilize the water off the West Florida coast with iron. Plant-like bacteria use that iron to set the stage for red tides. When iron levels go up, these bacteria, called Trichodesmium, fix nitrogen in the water, converting it to a form usable by other marine life. The addition of biologically usable nitrogen in the water makes the Gulf of Mexico a friendlier environment for toxic algae.

"This is one of the first studies that quantitatively measured iron from the dust and [linked] it to red tides through Trichodesmium," said Lenes.

The research was partially funded by a NASA grant as part of ECOHAB: Florida (Ecology and Oceanography of Harmful Algal Blooms), a multi-disciplinary research project designed to study harmful algae.

Left: Satellites can track African dust clouds as they migrate across the Atlantic Ocean. This NASA TOMS aerosol movie, which spans the interval June 13 through 21, 2001, shows such a cloud raining bits of the Sahara Desert over the Caribbean and the Gulf of Mexico. [more information]

The study used satellite and ground based measurements to track large dust clouds leaving Africa on June 17, 1999. Lenes and his colleagues followed the clouds using data from the Advanced Very-High-Resolution Radiometer (AVHRR), an imager aboard the NOAA's Polar Orbiting Environmental Satellites (POES).

The Saharan dust reached the West Florida shelf around July 1st, increasing iron concentrations in the surface waters by 300 percent. As a result, Trichodesmium counts shot up to 10 times what they had been prior to this event. Through a complex process involving a special enzyme called nitrogenase, the Trichodesmium used the iron to convert nitrogen in the water to a form more usable for other marine life.

In October, after a 300 percent increase of this biologically-accessible nitrogen, a huge bloom of toxic red algae (Karenia brevis) had formed within the study area, an 8,100 square mile region between Tampa Bay and Fort Myers, Florida.

Right: Satellites are capable of spotting not only globetrotting dust clouds, which can trigger red tides, but also the red tides themselves. This true-color image of the Texas Gulf Coast was captured last year by NASA's Terra spacecraft. The red tide is the dark reddish discoloration in the ocean running southwest to northeast along the coast. [more information]

Scientists have labored for several years 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.

Humans who swim in the Gulf during a red tide can experience respiratory problems by breathing toxins from K. brevis that get in the air. Also, eating shellfish poisoned by red tides 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," Lenes said.

Note: The research reported here appears in the September issue of the scientific journal Limnology and Oceanography.The paper was funded through a number of grants connected with the ECOHAB program. Other funding for this study included grants from the National Oceanic and Atmospheric Administration, the Office of Naval Research and the Environmental Protection Agency. The ECOHAB: Florida Mission is to better understand the factors involved in the occurrence and dispersal of Florida's predominant red tide algae, K. brevis, and to predict and manage harmful algal bloom events.

Information on red tides -- from the Florida Marine Research Institute

Streaming video about red tides -- from the University of South Florida College of Marine Science. Requires Windows Media Player .

ECOHAB: Florida -- information from the Florida Marine Research Institute

All the World's A Stage ... For Dust -- Science@NASA article: Tune in to a NASA website and watch giant dust clouds as they ride global rivers of air, cross-pollinating continents with topsoil and microbes.

The Pacific Dust Express -- Science@NASA article: North America has been sprinkled with a dash of Asia! A dust cloud from China crossed the Pacific Ocean recently and rained Asian dust from Alaska to Florida.

Mobile Homes for Microbes -- Science@NASA article: African dust that crosses the Atlantic and brings beautiful sunsets to Florida also carries potentially harmful bacteria and fungi, a new study shows.

Dust Begets Dust -- Science@NASA article: Everyone knows that dry weather leads to dusty soils, but new research suggests that dust might in turn lead to dry weather.