African Dust Clouds Feed Toxic Algae Blooms

TAMPA, Florida, August 29, 2001 (ENS) - Saharan dust clouds travel thousands of miles and fertilize the water off the West Florida coast with iron, which kicks off blooms of toxic algae, shows a new study funded by the National Aeronautics and Space Administration. The researchers say the international dust contributes to deadly red tides that can kill millions of fish and other sea creatures.


This satellite image, taken in February 2000, shows one of the largest Saharan dust storms ever observed (Photo courtesy U.S. Geological Survey)
Storm activity in the Sahara Desert region generates clouds of dust that originate from fine particles in the arid topsoil. Easterly trade winds carry the dust across the Atlantic Ocean and into the Gulf of Mexico.

"Because iron is one of the most common elements in most soils, a certain percentage of the dust contains iron," said Lenes.

Each year, iron from Saharan dust clouds is deposited in the waters off the West Florida coast, where plant like bacteria use the iron to set the stage for red tides. When iron levels go up, the bacteria, called Trichodesmium, converts nitrogen in the water into a form usable by other marine life.

The addition of biologically usable nitrogen in the water makes the Gulf of Mexico a more likely environment for toxic algae to bloom. Toxic algal blooms, sometimes called red tides, have in the past 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.


An image from NASA's satellite based Total Ozone Mapping Spectrometer (TOMS) shows dust coming off regional land sources in Africa and blowing over Florida (Photo courtesy Laboratory for Atmospheres TOMS Project, NASA Goddard Space Flight Center)
Lenes works under John Walsh, one of the principal investigators for ECOHAB: Florida (Ecology and Oceanography of Harmful Algal Blooms), a multi-disciplinary research project designed to study harmful algae. Walsh coauthored a paper on the Saharan dust research, which appears in the September issue of the scientific journal "Limnology and Oceanography."

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 National Oceanic and Atmospheric Administration's (NOAA) Polar Orbiting Environmental Satellites (POES).

The Saharan dust reached the West Florida shelf around July 1, increasing iron concentrations in the surface waters by 300 percent. As a result, Trichodesmium counts shot up 10 times what they had been prior to this event.

Using an 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 dissolved organic 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.


A massive algae bloom off the coast of California. The white object in the photo is a ship (Photo by Peter Franks, courtesy Scripps Institution of Oceanography)
Scientists have labored 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 from breathing airborne toxins produced by Karenia brevis. 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.


A satellite captured this image of a Saharan dust storm in February 2000 (Photo courtesy National Aeronautics and Space Administration)
The study adds to the list of environmental and health problems now linked to Saharan dust storms. Droughts in Africa are causing larger, more frequent dust storms, some of which produce clouds of fine dust covering thousands of square miles. Trans-Atlantic winds can carry the dust to North America and the Caribbean.

One recent study says the sandy particles kicked up in the dust storms may further reduce rainfall, exacerbating the drought conditions that caused the storms and fueling a vicious cycle. Marine biologists have blamed the rapid demise of Caribbean corals on attacks by a fungus carried in Saharan dust.

Last fall, samples of dust carried from the Sahara to the U.S. Virgin Islands were shown to harbor heavy metals, bacteria, fungi and what appeared to be viruses. Some researchers suspect that pathogens in the dust clouds are responsible for the high rates of asthma in the United States and the Caribbean.