Extensive coral mortality in the US Virgin Islands in 2005/2006: A review of the evidence for synergy among thermal stress, coral bleaching and disease
In the summer/fall of 2005, extensive coral bleaching on reefs in the US Virgin Islands (USVI) was associated with sea water temperatures exceeding 30°C. Almost all coral species bleached, including Acropora palmata , which bleached for the first time on record in the USVI. As water temperatures cooled, corals began to regain their normal coloration. However, a severe disease outbreak then occurred on deeper, non-acroporid reefs. The disease demonstrated signs consistent with white plague. Monitoring of coral cover along previously established long-term transects on several reefs in St. John and St. Croix was intensified. Data on bleaching and disease were collected before, during and after this bleaching/disease episode. Average coral cover declined by over 50%, from 21.4% to 10.3% at the long-term study sites, within one year of the onset of bleaching, declining further to 8.3% after two years. This loss of coral cover was greater than from all other stressors affecting the USVI reefs in preceding years, and no significant recovery is evident. Disease prevalence increased on bleached A. palmata colonies that were being monitored as well as on the colonies of other species on the deeper reefs. Bleached A. palmata colonies had more disease (primarily white pox and other un-described diseases) than unbleached colonies. The non-acroporid corals that bleached most severely suffered the highest mortality from disease. Although the research summarized in this paper is not conclusive, the results suggest that high water temperatures lead to bleaching, which weakens corals and makes them more vulnerable to diseases.