""Diseases were first reported on coral reefs in the Caribbean in the late 1970s, and today disease is considered the primary factor causing mortality in corals," said Ginger Garrison, a USGS ecologist in St. Petersburg. "Caribbean coral reefs were the first ones that were hit and were hit hardest, but the problem today is global and is very serious."
One source of the deterioration of Caribbean reef health by diseases and other factors may be found halfway around the world (the subject of another USGS video podcast. Hundreds of millions of tons of dust are carried each year from the Sahara and Sahel regions of Africa to the Caribbean, the eastern United States, and beyond. At times, these dust air masses cover the tropical Atlantic and the entire Caribbean Sea. Although African dust has been carried out of the Saharan Desert and Sahel region and into the Caribbean and the Americas for hundreds of thousands of years, there have been significant changes in the past 40 years: the quantity of dust has increased and the composition has changed.
|Sahara dust, NASA, 2004|
"Larger amounts of dust began to be carried out of the Sahara in the early 1970s due to a number of factors, including global climate, changes in regional meteorology, and local human activities," said Garrison. "During this same period, the composition of the dust changed. Toxic chemicals produced by the combustion of biomass, fossil fuels, and the burning of garbage and plastics in the source region have been carried along with the dust particles from Africa into the Caribbean. In addition, the source region is using more pesticides on crops as well as to fight mosquitoes (which transmit malaria and other diseases to humans) and crop-eating desert locusts. The pesticides appear to be coming across with the dust as well."
Changes in the quantity and composition of dust correlate with increased mortality from Caribbean coral diseases; however, causation has not been shown. To test the hypothesis that African dust is a factor in the deterioration of Caribbean coral reefs, scientists analyzed air samples from a dust-source area in Mali, West Africa; from a site off the west coast of Africa in Cape Verde; and at downwind sites in Trinidad and Tobago in the southeastern Caribbean and the U.S. Virgin Islands in the northeastern Caribbean.
It has been discovered that viable bacterial and fungal spores are transported long distances across the ocean in African dust events. Scientists have also found very fine particles that can be easily inhaled into the lungs of humans. According to USGS geologist and public-health specialist Suzette Morman, fine particulate matter has been correlated with increased rates of heart attack and stroke and exacerbations of asthma and other respiratory diseases. (For example, see "Cardiovascular mortality and long-term exposure to particulate air pollution…" in Circulation, 2004, v. 109, p. 71-77, http://dx.doi.org/10.1161/01.CIR.0000108927.80044.7F.)
So far, carcinogens, neurotoxins, endocrine disruptors, and suppressors of immune systems have been identified in African dust air masses. (See article by Garrison and others in the International Journal of Tropical Biology and Conservation, 2006, v. 54, suppl. 3, p. 9-21, http://www.ots.ac.cr/tropiweb/intpages/suppl/sup54-3.html.) Some of these chemicals may have long-term effects on ecosystems because they persist in the environment for years, accumulate in organisms, are toxic and (or) carcinogenic, and interfere with physiological processes in low concentrations. Plumes of pollutants originate in industrialized as well as developing areas throughout the world and can have global impacts when transported long distances through the atmosphere.
Scientists are now beginning to test the toxicity of African dust and associated chemical contaminants on the life stages of many kinds of marine organisms, including corals, to see if they harm marine life and, if so, how they do so. And Shinn continues to monitor the health of coral ecosystems in the Keys, while moving into a sixth decade of documenting changing environmental conditions."
Image caption: "
"A very concentrated area of Saharan Dust has traveled all the way from Africa and is now heading toward Florida. The northern Caribbean have already been dealing with Saharan dust with very hazy skies.
Image caption: "A webcam image from the south shore of St. John in the Virgin Islands at 9:15 a.m. on Tuesday, July 17, 2012, (to the left) shows hazy, dusty skies. At this time, they were located on the eastern edge of the dust. The image to the right was the same webcam around 3:00 p.m. on Tuesday. The dust had moved on to the west. Webcam images are from Great Expectations."