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Caribbean Coral-Reef Ecologist Studies Dust from the African Sahel

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photograph of narrow street in Djenne, Mali
Mali: Street in Djenne, Mali, showing raw sewage in gutter.
Beagles. Most coral-reef ecologists are not confronted with the possibility of losing weeks of work, travel funds, and hard-won field samples to a beagle. Yet there I was, in Charles DeGaulle Airport, 5 days before Christmas, trying to board a Paris-to-Miami flight, and a beagle was deciding the fate of my air samples. My official passport, official-travel papers, and sheaves of USGS analytical request forms convinced the humans responsible for airport security of my innocence but did not sway the beagle. At long last, she looked up, wagged her tail, and trotted off, with human in tow.

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.

map of central Atlantic Ocean, showing paths from western Africa of northerly summer dust transport to the Caribbean and North America, and southerly winter dust transport to northeastern South America
  Transport of African dust to the Americas: Upper map from Perry, K.D., Cahill, T.A., Eldred, R.A., and Dutcher, D.D., 1997, Long-range transport of North African dust to the eastern United States: Journal of Geophysical Research, v. 102, no. D10, p. 11225-11238.

Lower map (at right) modified from original produced by U.S. Central Intelligence Agency.
map of the west-African nation of Mali, showing the location of the capital, Bamako, in the southwestern part of the country

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 Ám) 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.

photograph taken during clear conditions
Clear day: View from St. John, U.S. Virgin Islands, during clear air conditions in September 2000.
  photograph taken during African dust event conditions
Dust event: Same view from St. John during African dust event in June 2000.

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.

Related Sound Waves Stories
Radio Interview Explores African Dust, Human Health, and Mystery Novels
March 2002

Related Web Sites
Coral Mortality & African Dust Project
U.S. Geological Survey (USGS)
Center for Coastal & Regional Marine Studies
U.S. Geological Survey (USGS), St. Petersburg, Florida

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in this issue: Fieldwork cover story:
African Sahel Dust

Outreach Aleutians Documentary

Radio Interview Explores African Dust

Florida Coastal Storm Defenses

Crystal Demonstration

Oceans Day 2002

Home-Schooled Tour

Black History Month

Environmental Academy Web Site

Regional Science Fair

Meetings Fishing Symposium

Congressional Briefing—Sea Otter Research

Contracting Meeting

Lake Mead

SEABED Technology

Law of the Sea

Awards Recycling Program

Staff & Center News Bill Dillon Retires

John Hughes Clarke—"Imaging Water Mass Variability"

Deltas Seminar

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