What was done
The authors recount the general history of coral reef decline throughout the Caribbean over the past quarter century.  They then note that "coincidental with the decline of Caribbean coral reefs over the past 25 years there has been a sharp increase in the transport of African dust to the western Atlantic."  Of particular note is their statement that "Caribbean-wide mortalities of acroporid corals and ? coral bleaching beginning in 1987, correlate with the years of maximum dust flux into the Caribbean."  Hence, the authors put forward the hypothesis that the influx of dust has been partially to blame for much of the declining health of Caribbean corals, including that manifest as coral bleaching, over the past quarter century.

What was learned
In support of their hypothesis, the authors note that atmospheric dust "can serve as a substrate for numerous species of viable spores, especially the soil fungus ? Aspergillus sydowii, the cause of an ongoing Caribbean-wide seafan disease."  They further note that this fungus has been cultured from air samples taken during dustfalls in the Virgin Islands, but that spores of the fungus are absent when the air is clear.  It is their contention that these observations and experiments "provide a reasonable explanation for the near synchronous widespread distribution of [coral disease] outbreaks around remote oceanic islands in the Caribbean."

What it means
"While it may be rightfully argued," in the words of the authors, "that the African dust hypothesis is based on coincidental observations," they go on to correctly note "that the prevailing warm-water theory for coral bleaching was [also] initially based on coincidental evidence."  Hence, they are well within their right to state that "we believe a possible link between zooxanthellae expulsion, elevated water temperature, and dust delivery of pathogens and nutrients needs to be investigated."  Indeed, there is no question that the last word on the important subject of water temperature and coral bleaching has yet to be written; and we must be careful to fully explore all of the many ramifications of the "hundreds of millions of tons/year of soil dust that have been crossing the Atlantic during the last 25 years" before jumping to conclusions on so weighty a matter.