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Caribbean Coral-Reef Ecologist Studies Dust from the African Sahel
I traveled in December 2001 from St. Petersburg, FL, to Bamako, Mali, to set up a chemical-contaminants-sampling station to complement the microbial-sampling station I had set up there the previous year. The trip was exceptionally productive: the samplers were installed, local scientists trained, and samples successfully collected. What was a coral-reef ecologist doing sampling air in the African Sahel? I was collecting data to test our hypothesis that microbial and chemical contaminants carried in African dust may play a role in the decline of Caribbean coral reefs and may pose a risk to human health. Having lived in the Caribbean region for 18 years, I had spent the past decade documenting the continuing decline of coral reefs and was intimately aware of the influx of African dust every summer.
Every year, hundreds of millions of tons of African dust are carried from the Sahara and Sahel across the Atlantic to the Caribbean and the southeastern United States. In the Caribbean, the sky becomes hazy, visibility decreases to a few kilometers, a fine red dust settles on surfaces, and residents complain of sinus problems, coughs, and other ailments said to be caused by the dust. Although the dust has been carried to the Caribbean for thousands of years, the amount transported varies from year to year and has increased drastically since the early 1970s with the beginning of the drought in the Sahel. Composed primarily of soil particles so small (less than 2.5 µm) that our lungs cannot expel them, the dust may transport various microorganisms and chemicals that hitchhike on the small particles. Charles Darwin, on his 1845 voyage aboard the surveying ship H.M.S. Beagle, collected African dust in the Atlantic and, using a microscope, saw live microorganisms on the soil particles. Even larger organisms, African desert locusts (Schistocerca gregaria, as much as 3 inches long or longer), arrived alive in Antigua, Barbados, and Trinidad during a large dust event in 1988. In 1997, Gene Shinn, Garriet Smith, (University of South Carolina, Aiken), and I hypothesized that living microbes carried with the dust may be significant factors in coral-reef decline. In December 1997, Garriet isolated and identified the known seafan disease pathogen (Aspergillis sydowii) in its active, pathogenic form from air samples taken during a dust event in the Virgin Islands. Since that time, A. sydowii has been isolated only from samples taken during dust events in the Virgin Islands (but not during nondust periods), from diseased sea fans, and from air samples from Bamako, Mali. To date, Dale Griffin and Christina Kellogg (St. Petersburg, FL) have isolated more than 150 species of viable bacteria and fungi from Virgin Islands air samples taken during dust events; samples collected during nondust periods contain few microorganisms.
Sampling in a dust-source area was the next step. In December 2000, I installed a sampling station in Bamako, Mali, to collect air samples to be analyzed for microorganisms. While there, I realized that microorganisms transported with the dust might not be the only concern. In Mali, all forms of waste are burned for fuel and to fertilize the thin ribbons of arable land along the flood plain of the Niger River. Until 15 years ago, garbage was predominately animal and plant waste; now, plastic bags and various plastic products are a major component. Garbage burning today severely degrades air quality during periods of clear weather and dust storms (the Harmattan) and may release dioxin and concentrate heavy metals. Anecdotal information (including conversations with local residents and my personal experience of having to seek medical attention for respiratory problems in Mali) suggests that respiratory complaints are common. The third largest river in Africa, the Niger, begins in the highlands of tropical Guinea and flows northward and eastward through Mali. The river is the depository for sewage, pesticides used on croplands, and excreted pharmaceuticals and antibiotics (used against a host of diseases, including malaria and respiratory infections). Mali receives less than 2 cm of precipitation a year and depends on the annual flooding of the Niger to deposit fertile soil on the flood plain. The fine soil particles readily adsorb many of the chemical contaminants carried by the river (pesticides, plasticizers, pharmaceuticals, and combustion products). Strong convective storms can advect these small particles, along with their chemical and microbial hitchhikers, into the atmosphere, where they can be transported thousands of kilometers to the west. Little is known about the movement of living microbes, organic chemicals, heavy metals, or radioisotopes from West Africa into the Caribbean and the southeastern United States. Thanks to an honest beagle, the air samples from Mali made the trip to the laboratory in St. Petersburg, where they will be analyzed for chemical contaminants (a suite of pesticides, polyaromatic hydrocarbons, dioxin, plasticizers, pharmaceuticals, antibiotics, and trace metals) and viable microorganisms. That conscientious beagle must have been off duty 2 days later, when someone boarded the Paris-to-Miami flight and tried to light his running shoe.
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in this issue:
Radio Interview Explores African Dust Florida Coastal Storm Defenses Environmental Academy Web Site Congressional Briefing—Sea Otter Research John Hughes Clarke—"Imaging Water Mass Variability" |
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U. S. Department of the Interior | U.S. Geological Survey
cover story:
