Unprecedented Disease-Related Coral Mortality in Southeastern Florida

Abstract

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.

Figure 1. Location map of the 10 timed-swim sites (green dots) and four permanent coral monitoring sites (red triangles) off Miami-Dade County in southeast Florida.

Location of the NOAA National Data Buoy Center sea surface temperature monitoring stations, VAKF1 (Bear Cut) and FWYF1 (Fowey Rocks) are denoted by blue squares.

Figure 2. Graph showing the relationship between Sea Surface Temperature and prevalence of coral bleaching and white-plague disease at our four long-term monitoring stations.

Temperature was downloaded from NOAA National Data Buoy Center, NOAA National Data Buoy Center, Virginia Key Station (VAKF1), located off the southern tip of Virginia Key in Bear Cut, and Fowey Rock Station (FWYF1), located due east of Soldier Key within Biscayne National Park. Fowey Rock data is depicted by solid purple line and Virginia Key “Bear Cut” Station by solid green line. Note the dampened temperature profile of the offshore Fowey Rock Station as compared with the inshore site from Bear Cut. Horizontal line across the graph at 30.4 °C represents regional coral bleaching threshold. Coral bleaching prevalence, shown as blue histograms, is the proportion of paled and bleached tagged coral colonies recorded at the four long-term monitoring sites during weekly surveys. Similarly, white-plague disease prevalence, shown as red histograms, is the proportion of actively diseased tagged corals at these same sites.

Figure 3. Time-series photographs of tagged Meandrina meandrites colony showing rapid impacts from bleaching and white-plague disease at one of the permanent monitoring sites off Virginia Key.

Photo of unbleached living colony on August 20, 2014 (A); photo of same colony four weeks later during peak of coral bleaching event, September 23, 2014 (B); photo showing colony-wide recovery of zooxanthellae associated with seasonal drop in water temperatures, note residual “paled” coloration of coral tissue and first outward signs of white plague-like symptoms appearing along colony edge on November 11, 2014 (C); photo of same colony, totally dead from white-plague disease only four weeks later now covered with sediment laden algal turf, December 9, 2014 (D). Photos taken by RF.

Figure 4. Photographs of the eight most affected corals to the recent white-plague epizootic off Miami-Dade County, FL, showing active disease on each species.

Disease prevalence recorded is calculated from time-swim survey data (Fig. 1) and graphically represented as proportions, where red represents disease prevalence, and blue represents the proportion of colonies with no signs of disease. Disease prevalence includes colonies with both active signs of white-plague disease and those that were identified as recently killed as a direct result of the disease. The strong coupling between disease prevalence and total coral mortality suggests that disease prevalence was a useful proxy of mortality. Sample size (n=) is the total number of colonies observed at the ten (10) timed-swim survey sites. The combined coral disease prevalence for these eight species is 90%. Coral species abbreviations are as follows: Eusmilia fastigiata (EFAS), Meandrina meandrites (MMEA), Dichocoenia stokesi (DSTO), Colpophyllia natans (CNAT), Pseudodiploria strigosa (PSTR), Diploria labyrinthiformis (DLAB), Orbicella annularis (OANN), and Solenastrea bournoni (SBOU). Photographs taken by WFP.

Figure 5. The best-fit function that described the relationship between the duration of time it took the disease to spread across reefs (in meters) is Distance (m) = 0.22.time in days^2.1399 to spread north, and Distance (m) = 1.28.time in days^1.714 to spread south.

The circles in blue and red are the data indicating the first known arrival of the disease (see Table 3), with red depicting data south of Virginia Key, and blue circles depicting data north of Virginia Key.