Coral reefs in the Caribbean Sea and the western Atlantic Ocean have been in a continual state of decline for the last 20 years. Scientists traditionally have looked to human influences as the culprit.
But according to a relatively new scientific study the real problem may be blowin' in the wind.
The demise of coral reefs coincides with large increases in the influx of dust from Africa.
Indeed, the hundreds of tons of soil dust that have crossed the Atlantic yearly for the past 25 years could be a significant contributor to coral reef decline -- as well as a factor in other areas such as human health -- according to Eugene A. Shinn, a research geologist with the U.S. Geological Survey in St Petersburg, Fla.
Shinn, a leader of several popular AAPG field seminars that use the Caribbean as a setting, has long been recognized as an expert in not just the region's geology but also its total environment.
"Atmospheric transport of dust from North Africa may be responsible for a number of environmental hazards, including the demise of Caribbean corals, red tides, amphibian diseases, increased occurrence of asthma in humans, and decrease of oxygen in estuaries," Shinn said.
Shinn, along with several fellow scientists, reported on the problem in a paper titled "African Dust and the Demise of Caribbean Coral Reefs" in the AGU journal, Geophysical Research Letters.
USGS scientists have monitored coral reef vitality for nearly 40 years. Atlantic coral diseases were first reported in Florida and Bermuda in the 1970s, but these early reports received little attention until the late 1980s when the problem could no longer be ignored.
The Problem Emerges
Black band disease on corals first appeared in the Caribbean in 1973, and from 1978 to 1983 several coral species suffered a period of die-off in Florida, Bermuda and the Caribbean.
In 1983 the herbivorous sea urchin Diadema antillarum was virtually killed out throughout the Caribbean. Some unknown pathogen first decimated Diadema populations in Panama in January, and by July had spread to coral reefs in Belize, Mexico and the Florida Keys -- indicating rapid transport of the pathogen in the main Caribbean current.
Shinn and Garriet W. Smith, with the University of South Carolina-Aiken, observed the die-off in Florida and San Salvador in the eastern Bahamas, well away from the major current flow.
"The effect of the Diadema die-off where it occurred was immediate and obvious," Shinn said. "Algae normally grazed from dead coral surfaces proliferated, interfering both with coral recruitment and growth.
"I've been watching coral reefs since the 1950s, photographing the same coral for over 40 years," he continued. "These diseases became apparent to me in the late 1970s, and the problems accelerated dramatically in 1983."
During the summer of 1983 elkhorn and staghorn corals, two major Caribbean reef building species, also experienced mass mortality. Entire forests of these branching reef builders perished, and in the absence of Diadema grazing, dead elkhorn and staghorn branches were quickly overgrown by fleshy algae, which retard the establishment of coral larvae.
In addition, a pathogen affecting Caribbean sea fans was reported at about the same time and that soft coral species also suffered mass mortalities.
"The reefs have never been the same," Shinn said. "Algae began growing everywhere."
A Connection is Made
A second major coral reef event occurred during the warm, quiet, almost hurricane-free summer of 1987.
This major disease struck Caribbean populations of coral and sponges, resulting in bleaching of the coral surfaces. Corals such as brain coral expelled the symbiotic algal cells that give it color, leaving it snow white -- a condition called bleaching.
Episodes of bleaching and black-band diseases proliferated and continued into the late 1990s. Finally, in the mid-1990s a Caribbean-wide pathogen affecting sea fans was reported. Scientists determined that the pathogenic agent was a soil fungus aspergillus.
These major coral disease events coincided with increases in warm water associated with El Niño weather patterns, so many scientists initially believed these die-offs were the result of pollution, sedimentation or warm water associated with the North Atlantic Oscillation, which coincides with the larger Pacific El Niño Southern Oscillation.
"I didn't correlate these coral diseases with the large increases in African dust transported to the western Atlantic and Caribbean Sea until one day I read a little article in Geotimes on work done in the Amazon Rain Forest that showed the rainforest received some of its essential nutrients from African dust," Shinn said.
"I started thinking if this dust could support plant growth half way around the world, it might also explain some of the algae growth on coral reefs."
Day of the Locusts
African dust blowing across the Atlantic is certainly not a new discovery. Mariners have noted the phenomenon in their ships' logs for hundreds of years. The prevailing easterlies that bring the dust clouds are the same trade winds that blow hurricanes toward the United States and Caribbean every year.
Shinn contacted Joe Prospero, who is with the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami and has been monitoring dust in Barbados since 1965. His studies showed significant increases in African dust transport in the 1970s, which correlated with drought conditions in northern Africa.
"One of the first things that jumped out at me while I studied his data was big spikes in dust transport in 1983 and 1987 -- the two years we experienced the massive mortality rates on coral reefs," Shinn said.
It seemed like a reasonable hypothesis that African soil dust resulting from the prolonged drought in the over grazed grasslands of the Sahel and the desiccation of Lake Chad could contain abundant fungal spores.
"In October 1989 large African desert locusts were transported in dust clouds to Trinidad," Shinn said. "If a one-inch long grasshopper can be blown over in the dust, it should come as no surprise that African dust could contain soil fungus spores."
Conventional wisdom holds that ultraviolet light kills all the spores, fungi and bacteria before it can cross the Atlantic -- but Shinn and colleagues have identified many species of fungi, including aspergillus spores, in aerosols collected during dust events in the Virgin Islands.
These nutrients in the African dust may stimulate phytoplankton and benthic algal growth in the normally oligotrophic waters of the Caribbean coral reefs. A USGS microbiologist has 110 different species of bacteria and fungi in culture from four air samples taken in the Virgin Islands.
About 80 percent of those are bacteria and 20 percent is fungi.
Now That's a Cloud
Originally, some researchers speculated that the Caribbean-wide epidemic of aspergillosis in sea fans was related to increased runoff caused by deforestation in the Caribbean. However, outbreaks around isolated Caribbean islands such as San Salvador indicate this cause is unlikely.
Also, identification and culturing of aspergillus from those air samples taken during dust storms in the Virgin Islands show that African dust is an efficient substrate for delivering aspergillus spores.
This year Shinn's colleague and co-author Smith successfully inoculated healthy sea fans with aspergillus cultured from spores in dust sampled from the air during African dust outbreaks in the Virgin Islands. Dust often reduces visibility in the Virgin Islands, even causing temporary airport closings.
This dust has been tracked from North Africa across the Atlantic with NASA Seawifs and TOMS satellite data, which is readily available on the Internet.
This past February, in fact, a NASA satellite photographed one of the largest dust storms ever observed. The brown cloud, approximately the size of Spain, was seen leaving Africa and blowing west across the Atlantic toward the Caribbean and the United States.
Also, satellite images show that African dust transported across the Atlantic goes mainly toward the southern Caribbean and equatorial regions of South America during North American winters and that transport direction shifts northward to impact Florida and the southeastern United States during summer months. This pattern of seasonal change suggests that a dust-born pathogen could impact the southernmost part of the Caribbean around Panama during January and a few months later the entire Caribbean.
The sea urchin Diadema mortality was first reported in Panama in January 1983, and within just a few months spread northward throughout the Caribbean.
"We think these observations and experiments provide a reasonable explanation for the near synchronous widespread distribution of outbreaks around remote oceanic islands in the Caribbean. The Iron-Ex Experiment was done to demonstrate a way to remove CO2 from the atmosphere if ever needed," Shinn said.
The Iron Curtain
In addition to hosting spores, African dust itself is composed of chemical elements such as iron, phosphorous and sulfates, which have been proven to stimulate phytoplankton in tropical waters.
"Richard Barber with Duke University's Marine Laboratory worked on an experiment in the Pacific Ocean, where they basically seeded 75-square kilometers with an iron solution," Shinn said.
This experiment -- done to demonstrate a way to remove CO2 from the atmosphere if ever needed -- was conducted in an area where oceanographers have known there is enough nitrogen and phosphorous in the water to support plankton, but there wasn't any present.
"Scientists believed iron was the missing micronutrient," Shinn said. "The water turned to pea soup when the iron solution was introduced, proving that iron was indeed the missing link."
Shinn said he attended a series of lectures Barber gave concerning his work, and he "realized that iron likely plays an important role in the coral die-off in the Caribbean and western Atlantic. The one ingredient that is always present in African dust is iron -- about 6 percent of the dust is iron oxide."
Human Health Hazards
While Shinn's studies focus on the detrimental affects of African dust on Caribbean and western Atlantic coral reefs, another serious issue is the impact of the dust on human health.
As Shinn observed, the occurrence of asthma has increased worldwide -- and Caribbean island nations are particularly afflicted with the illness.
"I was surprised to learn that about 50 percent of children in Puerto Rico have asthma," Shinn said. "So do the children of Trinidad, which is ground zero for African dust."
He related the story of a Texas woman who moved to a poured concrete, sterile house in St. John, Virgin Islands, because of her allergies to many chemical and petroleum-based products. Her new life was a success until the first dust storm when she became violently ill.
Today she and her husband collect samples for Shinn and his fellow scientists.
"A large number of people have been studying the transport of dust for years," Shinn added. "Much of the research was stimulated by the military during the Cold War era, studying potential fallout patterns.
"I was surprised no one had suggested this influx of dust might have an impact in other areas."
'A Difficult Situation'
So what do we do about the potentially harmful effects of African dust?
Shinn said the situation should be addressed at its root.
"The United States and other countries don't expend much effort educating and teaching African nations about the effects of primitive agriculture and overgrazing in the Sahel," he noted. "It's very similar to what happened in this country in the 1930s, when the government stimulated farmers to go west and plow up the grasslands. Then a drought set in and we endured the devastating Dust Bowl years.
"It is in our own interests to help teach better farming practices, and provide aid for irrigation in North Africa."
However, not all branches of science believe that eliminating African dust from the Caribbean and western Atlantic is the best course of action.
For example, the dust is highly beneficial to rainforests.
"There is geological evidence that the rainforests wax and wan with the influx of African dust," Shinn said. "So scientists are at odds about the beneficial or detrimental effects of the dust. It's a difficult situation to resolve."
Shinn's hypothesis has met with mixed reviews in the scientific community.
"I started looking at the correlation between African dust and the coral reef die-offs in 1996 and it's been an uphill battle," he said. "However, today the theory is beginning to catch on.
"I always say there are three stages of discovery," he continued. "First is, 'You are wrong and I can prove it.' Second is 'You're right, but is it important?' And third is 'Didn't we know this all along?'
"We are somewhere between stage two and three with this study," he laughed.
At least, research funds are finally beginning to trickle in to study the issue of African dust and its impact in the Caribbean and western Atlantic.
"This is a classic story about how science is supposed to work," Shinn said -- "collaboration and a great many little things coming together to push our knowledge forward."