Coral Mortality and African Dust

Online Mini-Documentary Movie Watch USGS scientists Ginger Garrison, Gene Shinn, Chuck Holmes, and Dale Griffin in the online documentary "The Effects of Globally Transported African and Asian Dust on Coral Reef and Human Health"

Introduction - The Dust Hypothesis

Question:

Satellite Images NASA and NOAA satellites have recorded Saharan dust crossing the Atlantic Ocean See the Images

Why have coral reefs that are bathed in clear oceanic waters throughout much of the Caribbean suffered algal infestation, coral diseases, and near extinction of herbivorous sea urchins almost simultaneously during the 1970s, 1980s, and early 1990s? The best known factors detrimental to coral reefs include storms, sewage, sediment run-off from disturbed land, dredging, abnormally high water temperatures, high intensity UV light, overfishing, etc. These factors do not explain the widespread geographic occurrence of disease on reefs or the continuing lack of recovery on damaged reefs. More perplexing, these declines are occurring on reefs remote from as well as near to human activities. What would be a large scale process that could explain this pattern of continuing coral reef decline?

Observations

Photo Gallery see the documented change in six different coral heads over a span of about 40 years Enter the Gallery

Changes in global climate, regional meteorological conditions, and land use in northern and western Africa resulted in severe droughts in the Sahara and Sahel of Africa starting in the 1970s. Hundreds of millions of tons of African dust are transported annually from the Sahara and Sahel to the Caribbean and southeastern U.S. A similar dust system in Asia carries dust from the Gobi and Takli Makan deserts across Korea, Japan, and the northern Pacific to the Hawaiian Islands, the western U.S., and as far eastward as Europe. Although these global atmospheric systems have been transporting fine soil particles for hundreds of thousands of years, the quantities of dust vary annually as a result of global climate, local meteorology, geomorphology of source areas, and human activities. It is thought that the quality of the dust has changed as a result of human-related changes in the source regions and areas over which the dust travels: burning of biomass and waste; use of antibiotics, pharmaceuticals, and pesticides; increased industrialization. Is it coincidental that coral reef declines began at the same time that the quantities of dust transported from Africa increased and the composition of the dust changed? Various peaks in the dust record, at Barbados and elsewhere in the western Atlantic (Prospero, J.M., R.T. Nees. 1986. Impact of the North African drought and El Niño on mineral dust in the Barbados trade winds. Nature 320: 735-738), coincide with benchmark events on reefs throughout the Caribbean (see graph of annual dust flux, below).

Barbados Mineral Dust Annual Average and Benchmark Caribbean Events: The graph Above, courtesy of Dr. Joe Prospero, University of Miami, shows the overall increase in African dust reaching the Caribbean island of Barbados since 1965. Notice the peak years for the dust deposition were 1983 and 1987. These were also years of extensive environmental change on Caribbean coral reefs. [larger version]

Hypothesis

We hypothesize that African and Asian dust air masses transport nutrients (iron, nitrates, other nutrients), persistent organic pollutants (pesticides, polyaromatic hydrocarbons [PAHs], polychlorinated biphenyls [PCBs], dioxins and furans) and viable microorganisms that may adversely affect human health and downwind ecosystems such as coral reefs.

Mortality Events see the events that have affected Caribbean corals Significant Mortality Events

That Saharan dust is deposited in the western Atlantic is demonstrated by the following:

1. The African dust record from Barbados and elsewhere
   
   
2. Red iron- and clay-rich soils on carbonate islands throughout the Caribbean that are known to be of Saharan dust origin (Muhs D.R., Bush C.A., Stewart K.C., Rowland T.R., Crittenden R.C. 1990. Geochemical evidence of Saharan dust parent material for soils developed on Quaternary limestones of Caribbean and Western Atlantic islands. Quaternary Research 33: 157-177).
   
   
3. The Amazon rain forest in South America derives essential nutrients (mainly phosphate) from Saharan dust (Swap R., Garstang S., Greco S., Talbot R., Kallberg P. 1992. Saharan dust in the Amazon basin. Tellus 44: 133-149).

The mechanisms by which dust may affect reefs include deposition of nutrients or pollutants that may:

• interfere with a coral's immune system, making it more susceptible to disease pathogens.
  
  
• interfere with some stage in reproduction (gamete production, fertilization, settling, larval survival).
  
  
• induce pathogenicity in a microorganism in the reef environment.
  
  
• trigger a rapid increase in the number of pathogenic microorganisms.
  
  
• fuel macroalgae or phytoplankton growth, as has been shown for Red tides in the Gulf of Mexico (e.g., Lenes J.M., Darrow B.P., Cattral C., Heil C., Vargo G.A., Callahan M., Byrne R.H., Prospero J.M., Bates D.E., Fanning K.A., Walsh J.J. 2001. Iron fertilization and the Trichodesmium response on the West Florida shelf. Limnology and Oceanography 46: 1261-1277; Walsh, J.J., K.A. Steidinger. 2001. Saharan dust and Florida red tides: The cyanophyte connection. Journal of Geophysical Research 106: 11597-11612);
  
  
• directly deposit pathogenic microorganisms (e.g., Aspergillus sydowii, the fungus that causes sea fan disease throughout the Western Atlantic; Smith G.W., Ives L.D., Nagelkerken I.A., Ritchie K.Bs. 1996. Caribbean sea-fan mortalities. Nature 383: 487; Weir-Brush, J.R., V.H. Garrison, G.W. Smith and E.A. Shinn. 2004. The relationship between Gorgonian coral (Cnidaria: Gorgonacea) diseases and African dust storms. Aerobiologia 20: 119-126).

Sampling Sites >>

African Dust Carries Microbes Across the Ocean: Are They Affecting Human and Ecosystem Health? - USGS OFR 03-028

Synthetic Organic Chemicals, Microorganisms, African and Asian Dust and Coral Reefs - USGS Poster