Brian Handwerk, National Geographic News 14 Jul 08;
Coral reefs in the United States and the Caribbean may be under siege—from a surprising source half a world away.
Scientists say tons of dust from Africa's arid Sahara and Sahel regions could be polluting oceans in the Caribbean and southeastern U.S.
The dusty clouds carry contaminants like metals, pesticides and microorganisms—potentially disastrous news for coral reefs and other marine animals already stressed by warming waters.
"We're trying to actually look at what is in these African dust air masses when the get over to the Caribbean," said Virginia "Ginger" Garrison, an ecologist with the U.S. Geological Survey in St. Petersburg, Florida, who studies how the dust travels.
"We're at the baby-step stages of trying to see how this dust and this stew of things may be affecting organisms—including humans—in downwind areas."
Something in the Air
Air-quality data from a network of sampling sites have revealed intriguing results, Garrison and colleagues said recently at the International Coral Reef Symposium in Fort Lauderdale, Florida.
For instance, Caribbean air samples during African dust events may hold two to three times as many microorganisms, such as bacteria and fungi, as samples taken from the same spot during other periods.
In Florida the Africa-influenced air conditions sometimes deteriorate below U.S. air-quality standards.
Air-quality testing in Mali, the U.S. Virgin Islands, and Trinidad and Tobago has also revealed traces of pesticides, including DDE—a breakdown product of DDT, which is still used as an insecticide in some African countries.
Pesticides are of particular concern to coral reefs because they can interfere with the tiny animals' reproduction, fertilization, or immune function.
These contaminants were highest in Mali and lower in the downwind areas of the Americas. Six pesticides were found at each one of the test sites, Garrison said.
"And there's been very, very little work that has been done on the concentrations of any of these pesticides or PCBs [and how] that would impair coral or coral reef organisms," she added.
Andrew Negri, of the Australian Institute of Marine Science in Townsville, co-authored one of the few previous studies on pesticides and coral.
"The pesticides associated with African dust are primarily insecticides. These can affect the coral host directly," Negri said.
"We have found that two of the identified insecticides, chlorpyrifos and endosulfan, can reduce the settlement and attachment of coral larvae to the ocean floor at very low concentrations.
"I would be particularly concerned if storms containing insecticide-contaminated dust were to occur upwind of coral reefs around spawning time," he said.
Winds of Change
Atmospheric systems such as the Africa-Americas pathway have functioned for thousands of years.
A similar system delivers Asian dust to the western United States, where it accounts for up to an estimated 40 percent of local air particles.
In recent times, however, humans have caused some significant changes.
Desertification and changing land-use patterns can put more dust into the air. Industrialization, pesticide use, waste burning, and other practices have produced air pollutants that ride with that dust to far-flung locales.
But the question of just where air "originates" is tricky, experts say. Traceable substances such as pollen can provide clues to where contaminants come from.
But in Garrison's view, mass mixing around the globe means that we all experience a single, large air system.
"I can watch a dust air mass [via] satellite coming out of Africa, across the Caribbean, and into the eastern U.S., and all of a sudden it peters out," she explained.
"But the air mass is still moving with some dust into the northeast[ern] U.S., where it mixes with a pollution cloud. Then it goes over the North Atlantic to Europe, picks up a bit of their pollution cloud, and then goes back to Africa. So where did this stuff come from?"
Garrison believes that while certain contaminants may be linked to specific areas, such as the African pesticides, no one region is entirely to blame for air-quality issues.
"We're all responsible," she said. "We all have to watch what we're putting into the air."
