It is a sultry August evening in the Florida Keys as the setting sun fires the western sky with colors so stunning that they defy description. As imbibing seekers of the green flash look on, the show soon ends amid hues of red, pink and orange that help define these majestic islands as America’s Caribbean. As the man in the old beer commercial used to say, “It just doesn’t get any better than this.” But beneath the beauty, something ominous lurks.
The breathtaking natural light show isn’t merely the result of the setting sun reflecting through wisps of distant clouds. There’s yet another reason, which, ironically, is more evident at midday. During the long summer season, residents from South Florida to the Caribbean have long endured a visitor from another continent. Its presence is very subtle except on days when the otherwise crystal-clear atmosphere is tainted with a reddish-brown haze. That haze — as incredible as it may seem — is caused by African dust transported all the way from the Sahara desert.
Some find it hard to believe that the dull haze and spectacular sunset are courtesy of such a global event, but the evidence is incontrovertible. Almost a century ago scientists surmised this almost unbelievable journey of African dust to the New World, and modern satellite technology has documented it. However, what the dust brings isn’t just memorable sunsets. Only recently have we begun to uncover the possible harmful environmental consequences of this long-ranging atmospheric phenomena. And in doing so, science is learning yet another lesson about how our planet functions, and how we and everything else are connected.
Off It Goes Into the Wild Blue Yonder
“Imagine approximately 1 billion Volkswagens traversing the atmosphere each year,” says Dale Griffin, a microbiologist with the U.S. Geologic Survey (USGS). “That’s how much dust circulates in the atmosphere.” Every year several hundred million tons of African dust are transported westward over the Atlantic to South Florida, the Caribbean and as far south as the Amazon. Stoked by summer storms, the warm air can lift dust 15,000 feet (4,545 m) or more above the Sahara and out across the Atlantic.
But as incredible as this may sound, to scientists it’s not news. They have long known that dust clouds deposit nutrient-rich soils thousands of miles from their origin. In fact, according to Gene Shinn, senior geologist with the USGS Center for Coastal Geology in St. Petersburg, Florida, African dust supports the robust bromeliad (air plant) ecosystem high in the tree canopy of the Amazon rain forest. And Shinn should know. He was the first person to recognize the possible environmental consequences of the African dust storms on coral reefs. “The ecosystem in the tree canopy [of the Amazon],” says Shinn, “is based on [Africa’s] red soil and includes various bugs and worms.” During severe storm events, even live African grasshoppers have survived the several-thousand-mile flight. Furthermore, the process of dust transport isn’t unique to the Caribbean. Studies have proven that essential nutrients in Hawaiian rain forests are transported as dust from Asia; and dust originating from central China reaches the American Midwest. (For a fascinating look at the phenomenon in the Pacific, see the Web site reference in the sidebar.)
While dust storms have been transporting soil around the Earth since time immemorial, what’s new is their increasing magnitude, especially the African storms. These have been triggered by events of the last 30 years. Starting in the 1970s, long-term droughts, poverty brought on by warfare, increasing population and poor agricultural practices led to an unprecedented desertification of a region called the Sahel. This is a geographic belt that straddles the entire African continent along the southern fringes of the Sahara desert. In essence, the Sahara has been steadily growing, and the resulting dust has fueled larger and larger dust storms. “We know that the variations in dust concentration measured in the Caribbean and Western Atlantic correlate with rainfall deficits in North Africa, especially in the Sahel region,” says Dr. Joe Prospero, an atmosphere chemist at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science (RSMAS) who studies African dust storms. “If the dust is altering the health of ecosystems, then this could be a significant climate-related effect,” he says.
The Puzzle Comes Together
The hypothesis that much of the coral reef decline in the Caribbean could be a result of dust from North Africa was first championed by Gene Shinn. A highly respected member in the coral reef scientific community, Shinn is a renowned expert on marine sediments and groundwater movement. During his 40-year career, he has witnessed dramatic changes in coral reefs. Shinn was particularly puzzled by one question: While it’s easy to understand how coral reefs near population centers are in decline, why is it that even remote reefs, nowhere near any human activity, are declining? The usual response from his colleagues was increased water temperature due to global warming. But the global warming theory didn’t seem to hold water (sorry for the pun) in all cases, or at least, it wasn’t the whole story. There must be another contributor, Shinn thought.
A major piece of the puzzle fell into place when Shinn learned of the work being done by Joe Prospero. Beginning in 1965, Prospero measured African dust levels on the island of Barbados, which started showing dramatic increases around 1970 (See figure 1). Noting, particularly, the peaks occurring in 1983 and 1987, Shinn began to see a pattern. He knew that episodes of coral bleaching began proliferating in Florida and the Caribbean in the late 1980s and 1990s, with a major event occurring in the summer of 1987. Furthermore, in 1983 the black spiny sea urchin (Diadema antillarum) was almost eradicated from the Caribbean by what scientists have attributed to a still-unidentified disease.
A One-Two Punch
While a correlation seems to exist between dust storms and perturbing events on Caribbean coral reefs, this doesn’t answer the question of how dust might be the cause of the reefs’ woes. As it turns out, the cause appears to come from two different mechanisms. The first you might be familiar with if you take multivitamins. Women, in particular, are often advised to take vitamins containing the micronutrient iron. While plants don’t need iron for the same reasons as humans, it’s nonetheless an important nutrient to them as well. Scientific experiments have, in fact, documented that enriching the ocean with iron can result in enormous plankton blooms, demonstrating that low levels of iron may be a limiting factor in ocean productivity. And guess what’s in African dust? You got it; along with other nutrients such as phosphate, it contains lots of iron.
The working hypothesis is that dust may act as a fertilizer on coral reefs, allowing fleshy algae to grow much faster than normal. This, combined with the demise of herbivorous reef fish populations due to overfishing, results in the overgrowth of algae, eventually leading to what scientists call a “phase shift” from a coral reef to an algal reef. But the problems caused by the dust isn’t just the ability to act as some superfertilizer; there are also organisms within the dust that can cause disease.
Alien Invaders From the Atmosphere
Beginning in the 1980s, several environmental events occurred that puzzled the scientific community. These included the demise of Caribbean coral reefs, an increased frequency of red tides, a mass die-off of frogs throughout Central America, and the accelerated eutrophication (high nutrient input) of estuaries. There’s even been a dramatic rise of asthma in humans attributed to environmental change. For reef scientist Shinn, the decline in Caribbean coral reefs was the most striking. Although a disease called black-band was first reported in Bermuda on brain corals in the 1970s, it didn’t become widespread in other species in the Florida Keys and Caribbean until 1985. Could the African dust theory help explain this? While the jury is still out, an increasing number of scientists are beginning to take the once-scoffed-at theory seriously. But in this case, what might be causing the problem isn’t iron or any other inorganic substance. This time the culprits are living creatures — microbes that cause disease.
Disease is also at the center of another intriguing indication that African dust could be wreaking further environmental havoc. Several years ago divers in Florida began noticing a curious infestation plaguing sea fans. Tiny lesions began growing like cancer, consuming their host, and leaving behind algal-covered skeletons of what once were healthy sea fans. Once rare, this disease is now quite common and has decimated sea fan populations, particularly along the southeast Florida coast. (Most recently, the disease has also been documented in sea whips.) Then, a shocking breakthrough. Garriet Smith, a researcher at the University of South Carolina, identified the cause of sea fan disease as a soil fungus, Aspergillus. The puzzling thing was that Aspergillus cannot reproduce in seawater, which meant there had to be a continuing source of spores to explain the ongoing nature of the disease. Later, Smith and his colleagues isolated from the dust samples a particular species of fungus, Aspergillus sydowii, which they used to inoculate healthy sea fans. The result: Fifty percent of the infected sea fans showed signs of the disease. As Gene Shinn said, “So far, that’s the best smoking gun we have for proof that microbes transported in the dust are having a detrimental ecological effect.”
Another frightening aspect of the Aspergillus discovery is that the fungus isn’t just a problem for sea fans; it’s also a source of significant infection in humans. In fact, it accounts for a high portion of infection-related deaths in hospitals, is a leading cause of death due to lung infection among AIDS patients, and has even been linked to severe illness in healthy individuals. This finding initially led to suspicions that Aspergillus was being introduced to the marine environment by the dumping of biohazardous materials at sea. But the African dust proponents have provided a compelling and much more plausible theory.
While scientists have known for a long time about the wide-ranging transport of dust in the upper-level winds, a more recent discovery is that the African dust plumes carry with them a host of tiny, hitchhiking microbes besides Aspergillus. These include both bacteria and viruses. The potential worldwide implications for not only marine ecosystems, but also human health, are obvious.
Until recently, microbiologists assumed that during the five- to seven-day trip required for African microbes to reach North America and the Caribbean, most would be killed by ultraviolet rays in the upper atmosphere. But, like much of what science originally assumes, this isn’t true. Cultures made from African dust samples taken from the Virgin Islands have been analyzed and have been found to contain almost 130 different kinds of bacteria and fungi. Most of the dust ends up in south Florida, where it has spawned red-tinged sunsets for years. Again, said Dr. Dale Griffin, “We typically isolate about two colonies of fungi from clear air samples, whereas we might recover 20 to 40 isolates of fungi and bacteria from samples taken during dust events.”
Exactly how the microbes survive hasn’t been fully explained, but Griffin thinks that several mechanisms are possible. First, the plumes contain not only dust but also lots of smoke from slash-and-burn agriculture. This might act to filter some of the ultraviolet light. Additionally, some of the dust is transported in the lower atmosphere where microbes may not be subjected to ultraviolet radiation at levels high enough to be lethal. And finally, the microbes may adhere to cracks, crevasses or other shaded areas of dust particles. However they do it, they arrive alive.
The (Red) Tide Turns
Hard corals and sea fans aren’t the only possible victims of the nutrients and microbes carried in African dust. There’s some pretty clear evidence that the very base of the oceanic food chain can be affected.
Algal blooms called “red tides” have plagued coastal areas since biblical times, and one area where the phenomenon is being studied extensively is in the Gulf of Mexico off Florida’s southwest coast. In a study partially funded by NASA, it has been shown that the African dust clouds may have a connection with red tides. Using the latest space-based technology, NOAA and NASA satellites can spot dust clouds en route from Africa to the Americas. As technology improves, many believe that we may someday be able to not only understand red tides, but predict their occurrence.
Jason Lenes, a graduate student at the University of South Florida’s College of Marine Science, has provided some intriguing evidence to support the dust-cloud theory. In a recent study, Lenes and his colleagues have shown that African dust clouds can actually fertilize with iron the water off Florida’s west coast. But this was already surmised. More importantly, what Lenes’ study demonstrates is how some plantlike bacteria use the iron to initiate a red tide event. Here’s how it works: When iron levels go up, these bacteria, (Trichodesmium) convert free-form nitrogen in the water into a form usable by other marine life. The addition of this usable nitrogen in the water makes a much better caldron for toxic algae such as the most notorious red tide organism in Florida, a dinoflagellate called Karenia breve. “This is one of the first studies that quantitatively measured iron from the dust and (linked) it to red tides through Trichodesmium,” said Lenes in a recent interview.
Lenes’ study examined a dust storm that left Africa on June 17, 1999. Reaching Florida’s west coast around July 1, it increased iron concentrations in the surface water by nearly 300 percent. In turn, Trichodesmium counts skyrocketed to 10 times their normal concentration. By October, a huge bloom of toxic red algae (Karenia brevis) formed within the study area (an 8,100-square-mile [21,060-sq-km] region between Tampa Bay and Fort Myers, Florida).
It would be a mistake to assume that the implications of the African dust phenomenon are limited to coral reefs or even the ocean. In a clear and frightening example of how all residents of this planet share a common fate, we humans have as much to be concerned with as any creature of the sea. The evidence from the Caribbean for dust-born disease is very compelling. For example, levels of asthma on the islands of Barbados and Trinidad are among the highest in the world. On these two islands alone, the Caribbean Allergy and Respiratory Association (CARA) has documented a seventeenfold increase in asthma since 1973. Coincidentally, that was also the first year in which Dr. Prospero saw a spike in his data from the dust records on Barbados.
Floridians may also be affected. Since 1980, the number of Americans with asthma has increased 154 percent. The Tampa Bay region, near Jason Lenes’ study site on red tide, has one of the state’s highest asthma rates, with 7.1 percent of students now reporting asthma symptoms, compared with 2.7 percent just four years ago. “According to Prospero’s work, about half the particles breathed in South Florida during summer months originate in Africa,” says Gene Shinn. “The asthma epidemic in areas that are relatively free of industry correlates with the increased flux of African dust that has been continuously monitored in Barbados since 1965.”
But we should be careful about jumping to conclusions. “Not a lot is known about wind-transported disease,” says Prospero. “At this point, it’s a hypothesis that has some supporting evidence. But it’s a complicated subject.”
Like some heavenly judgment for the hundreds of years she’s been plagued by slavery, colonialism and countless forms of exploitation by the West, Africa may wreak its vengeance before the rest of the world even realizes something’s wrong. What the future holds, no one knows. As the song says, the answer is blowin’ in the wind.
Learn More About Dust Plumes and Their Environmental Effect
“African Dust Causes Widespread Environmental Distress,” U.S. Geological Survey Information Sheet, produced April 2000; Website: http://coastal.er.usgs.gov/african_dust/
“Desert Dust Kills Florida Fish,” NASA Science News;
“The Pacific Dust Express,” NASA Science News;
“Emerging Marine Diseases — Climate Links and Anthropogenic Factors,” Science, 1999, Volume 285, No. 5433, pages 1505-1510, authors Harvell, C.D., K. Kim, J.M. Burkholder, R.R. Colwell, P.R. Epstein, D.J. Grimes, E.E. Hofman, E.K. Lipp, A.D.M.E. Osterhaus, R.M. Overstreet, J.W. Porter, G.W. Smith and G.R. Vasta.
“African Dust and the Demise of Caribbean Coral Reefs,” Geophysical Research Letters, 2000, Volume 27, No. 19, pages 3029-3032, authors Shinn, E.A., G.W. Smith, J.M. Prospero, P. Betzer, M.L. Hayes, V. Garrison and R.T. Barber.
by Alex Brylske