Anomalously high water temperatures, associated with climate change, are increasing the global prevalence of coral bleaching, coral diseases, and coral-mortality events. Coral bleaching and disease outbreaks are often inter-related phenomena, since many coral diseases are a consequence of opportunistic pathogens that further compromise thermally stressed colonies. Yet, most coral diseases have low prevalence (<5%), and are not considered contagious. By contrast, we document the impact of an extremely high-prevalence outbreak (61%) of white-plague disease at 14 sites off southeastern Florida. White-plague disease was observed near Virginia Key, Florida, in September 2014, and after 12 months had spread 100 km north and 30 km south. The disease outbreak directly followed a high temperature coral-bleaching event and affected at least 13 coral species. Eusmilia fastigiata, Meandrina meandrites, and Dichocoenia stokesi were the most heavily impacted coral species, and were reduced to <3% of their initial population densities. A number of other coral species, including Colpophyllia natans, Pseudodiploria strigosa, Diploria labyrinthiformis, and Orbicella annularis were reduced to <25% of their initial densities. The high prevalence of disease, the number of susceptible species, and the high mortality of corals affected suggests this disease outbreak is arguably one of the most lethal ever recorded on a contemporary coral reef.

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Over recent decades, coral reefs worldwide have experienced severe sea-surface temperature (SST) anomalies. Associated with an El Niño-Southern Oscillation (ENSO) event of 1997-1998, nearly 100% mortality of the space-dominant coral Agaricia tenuifolia was reported at several shelf lagoonal sites of the Belize barrier reef system; a less abundant congener, A. agaricites, had lower mortality rates. We assessed A. agaricites and A. tenuifolia populations at coral reef ridges in the south-central sector of the Belize shelf lagoon and forereef sites to document recovery following the 1998 ENSO event and subsequent passage of Hurricane Mitch. To investigate the difference in heat stress tolerance between the 2 species, heat shock protein (HSP) expression was examined in the laboratory under ambient (28°C) and elevated (+ 6°C) temperatures. Populations of A. agaricites and A. tenuifolia surveyed at forereef sites in 1999 showed after effects from the 2 disturbances (partial colony mortality was ~23 and 30% for A. agaricites and A. tenuifolia, respectively), but partial mortality declined by 2001. At reef ridge sites, A. tenuifolia exhibited 75 to 95% partial colony mortality in 1999 compared to 18% in the less abundant A. agaricites. We measured a significant increase in percentage live cover at ridge sites for both Agaricia species from 1999 to 2001, except at Tunicate Ridge; at this site, which has restricted water flow, live A. tenuifolia cover remained low (~10%) 3.5 yr after the 1998 warming event, due in part to high sponge cover (> 75%). Immunoblotting results indicated that A. agaricites had twice as much HSC 70 (16.9 µg cm -2 ) as A. tenuifolia (8.7 µg cm -2 ) at ambient temperatures and 6 × as much under the + 6°C treatment. In addition to the inducible response by A. agaricites, this species expressed HSP 90, whereas A. tenuifolia did not. The distinctive patterns of population recovery and HSP expression suggest that A. tenuifolia has a lesser ability to produce HSPs for protection against environmental stress than A. agaricites. Such differences in resilience to large-scale environmental disturbances such as intermittent ENSO episodes may drive a dramatic change in coral species abundance patterns.