Coral Reefs

, Volume 38, Issue 4, pp 831–836 | Cite as

The extent of coral bleaching, disease and mortality for data-deficient reefs in Eleuthera, The Bahamas after the 2014–2017 global bleaching event

  • Bradley A. WeilerEmail author
  • Travis E. Van Leeuwen
  • Kristine L. Stump


Given the rapid change in coral reef assemblages globally, quantification of coral bleaching events, disease prevalence and mortality is critical. Here we discuss observations on the status of coral reefs in Southern Eleuthera, The Bahamas following the 2014–2017 global bleaching event. A total of 37 unique hard coral species were observed from 1232 surveyed corals across five sites between 2016 and 2017. Overall (± SE), live coral cover was 28.2 ± 11.0% with 11.0 ± 1.6% of these corals showing signs of bleaching, and 2.0 ± 0.6% showing signs of disease. Results suggest levels of coral bleaching observed in Eleuthera are currently lower than some regions globally following the bleaching event. There was no significant difference among sites for new and old mortality types, suggesting regional and/or global scale drivers. More local scale studies, especially for data-deficient regions are needed for the development of regional and local baselines to support future management of local reefs and their fisheries.


Coral reef Caribbean Coral bleaching Coral mortality Coral disease Diversity indices 


Global declines in tropical and subtropical live coral cover have accelerated over the last three decades in response to climate-induced shifts in environmental parameters across ecosystems, with incidences of bleaching events (Baker et al. 2008; Hughes et al. 2017) and disease epizootics (Bourne et al. 2009) becoming more frequent (Nyström et al. 2000). Global climate change has compounded effects on oceans, with rising sea surface temperature (SST) identified as the primary cause of regional and global bleaching events (Baker et al. 2008; Ainsworth et al. 2016; Hughes et al. 2017). While regional and global bleaching events have been previously documented, the global extent and severity of the most recent event (2014–2017) reached unprecedented levels (Eakin et al. 2017). In bleached coral colonies, endosymbionts are expelled from the coral host due to temperature stress (Lesser 2006), leaving the animal without the ability to gain energy via algal photosynthesis and resulting in high coral mortality or suppressed growth (Pandolfi et al. 2011). Furthermore, coral disease has been observed to correlate positively with bleaching, as bleaching weakens the coral host, and increased SST also stimulates bacterial proliferation, including pathogens leading to further coral mortality (Cróquer and Weil 2009; Muller and van Woesik 2011; Muller et al. 2018). In data-deficient locales, these impacts go largely unnoticed and therefore trends in coral reef health are often not reflected in local management, exacerbating the potential for ecosystem collapse. With coral reef health declining globally and at unprecedented rates (Hughes et al. 2018), monitoring of coral reef health metrics is of high importance. One method used to describe coral reef health is alpha diversity, which here explores the combination of the number of coral species present in the community (richness), how evenly distributed the abundance of those coral species are (evenness), and their phylogenetic relationships. Coral reef ecosystems with high alpha diversity and live coral cover, combined with low presence of mortality, bleaching and disease, are indicative of healthy coral reef ecosystems. Because increased coral reef health correlates positively with the biodiversity of fishes and invertebrates (Komyakova et al. 2018), baseline monitoring, e.g., through diver surveys, is a necessity for providing insights into the current status of local reefs, which influences their fisheries.

Hard coral cover in the Caribbean has been reported to have declined 50–80% in three decades, with remaining reefs dominated by macroalgae and fast-growing coral morphotypes (Gardner et al. 2003; Jackson et al. 2014). While several regions are surveyed annually in The Bahamas through government and non-government programs (Dahlgren et al. 2016), Eleuthera remains a data-deficient region despite its importance in supporting valuable commercial fisheries (FAO 2016). Local monitoring studies are critical for predictive modeling and long-term management to help reefs recover after bleaching events. By coupling standardized coral survey methods with biodiversity metrics, the present study describes the current status of Eleuthera’s patch reefs after the recent global bleaching event.


Sampling location and data collection

Diver surveys were conducted between November 2016 and January 2017 on five patch reefs in Southern Eleuthera, The Bahamas. Five sites (< 10 m) were surveyed, including locations on the Bahama Banks, the Exuma Sound, and Atlantic Ocean sides of Southern Eleuthera (Fig. 1). Schooner Cays (SC) are located on the Bahama Bank. Tunnel Rock (TR), Bamboo Point (BP), and Miller’s Hole (MH) are located on the edge of the Exuma Sound. Cotton Bay (CB), part of a barrier reef, is the only Atlantic Ocean site surveyed in the study.
Fig. 1

Coral bleaching survey locations in Eleuthera, The Bahamas. Stars indicate sampling locations: Schooner Cays (24°53′0.4092″, 76°23′3.707″), Tunnel Rock (24°48′53.64″, 76°20′59.64″), Bamboo Point (24°48′16.92″, 76°20′24.719″), Miller’s Hole (24°40′35.3244″, 76°12′58.8587″), and Cotton Bay (24°45′29.05″, 76°11′17.01″)

All survey data were collected on SCUBA using modified Atlantic and Gulf Rapid Reef Assessment (AGRRA v5) coral survey protocols (Lang et al. 2010). Weighted 10-m transect belts were used at each location, and a total of 33 transects was conducted. All corals (> 4 cm) within 0.5 m of either side of the transect were recorded and measured. Physical characteristics measured included: size (length and width), mortality (‘new’ showing coral skeleton and no algal growth; ‘transitional’ showing newly developed algal growth; and ‘old’ as developing macroalgal colonies), percent bleaching (showing non-necrotic bleached polyps), disease type and prevalence. As per the AGRRA protocol for coral surveying, colonies < 4 cm in diameter were defined as juvenile/recruits and were not recorded.

Coral health measurements and alpha diversity

To assess the status of coral reef health and extent of bleaching in Southern Eleuthera, summary statistics and alpha diversity for each site were calculated. Live coral cover (%) was calculated by multiplying the maximum width and length of each coral colony (relative coral cover surface area), summing the total for each transect, and dividing it by the total surveyed surface area for each transect. Coral bleaching, disease, and mortality prevalence (%) were calculated by dividing the observed frequency of each metric by the total coral count for each transect. Black band disease, dark spot disease, and white plague disease were the only diseases observed and were combined to calculate disease prevalence. Alpha diversity indices: Pielou’s evenness (J′), Shannon’s diversity (H′), Simpson’s diversity (1 − λ) and Margalef’s richness (d) were calculated (Clarke and Gorley 2006) for each site and used as further indicators of coral reef health.

Statistical analyses

Data on live coral cover, bleaching, disease and mortality (new, transitional, old) were compared among survey sites using a one-way ANOVA. If a significant site effect was found, Tukey’s post hoc analysis was used to assess differences between pairs of sites. A Pearson’s correlation test was used to test for all possible relationships among coral cover, bleaching, disease and mortality (new, transitional, old). All statistical analyses were performed in R 3.2.2 (R Core Team 2016).

Results and discussion

Local reef alpha diversity metrics

A total of 1232 individual corals were surveyed in Southern Eleuthera from November 2016 to January 2017. Of those 1232 corals, 37 unique hard coral species were identified among the five study sites. Overall, observed species diversity (Shannon’s diversity and Simpson’s diversity indices) was (± SE) 1.9 ± 0.09 and 0.8 ± 0.01, respectively (Table 1). Additionally, overall species richness (± SE) was 2.5 ± 0.12 and species evenness (± SE) was 0.9 ± 0.01 (Table 1). Variation in alpha diversity metrics except for Pielou’s evenness and Simpson’s diversity indices were found among study locations (Table 1).
Table 1

Environmental data for five coral survey locations in Eleuthera, The Bahamas and alpha diversity measures for each location


Depth (m)

Temperature (°C)

# Transects (n)

# Coral species

Margalef (d)

Pielou (J′)

Shannon (H′)

Simpson (1 − λ)

Tunnel Rock





2.7 ± 0.23

0.8 ± 0.03

1.9 ± 0.12

0.8 ± 0.04

Miller’s Hole





2.8 ± 0.29

0.9 ± 0.02

2.2 ± 0.11

0.9 ± 0.02

Schooner Cays





2.4 ± 0.22

0.9 ± 0.02

1.9 ± 0.07

0.8 ± 0.01

Bamboo Point





2.8 ± 0.23

0.8 ± 0.03

2.0 ± 0.12

0.8 ± 0.03

Cotton Bay





2.2 ± 0.17

0.9 ± 0.03

1.7 ± 0.13

0.8 ± 0.04






2.5 ± 0.12

0.9 ± 0.01

1.9 ± 0.09

0.8 ± 0.01

All diversity values are averaged for each site and reported with standard error. The Eleuthera location is a summation of all sites and weighted averages for the alpha diversity metrics

Coral cover variation among sites

Coral cover among sites in Southern Eleuthera ranged from < 10 to > 60% (Table 2). On average, live coral cover in Southern Eleuthera was (± SE) 28.2 ± 11.0% (Fig. 2a; Table 2). Tunnel Rock and CB had some of the lowest coral cover (8.7 ± 1.9% and 9.1 ± 2.3%, respectively). The Bahama Banks and Exuma Sound sites, SC and MH, had the highest live coral cover of the sites surveyed (49.1 ± 25.7% and 62.6 ± 11.3%, respectively). There was a significant difference in coral cover among sites (ANOVA, F4, 28 = 4.36, p < 0.01; Fig. 2a) with post hoc analysis revealing that MH had significantly more coral cover than both TR (Tukey’s pairwise comparison, p = 0.02), and CB (Tukey’s pairwise comparison, p = 0.02). Because live coral cover and diversity are an indication of a healthy reef, these data are a necessity for the development of regional and local baselines to support future management.
Table 2

Averages of coral health parameters recorded for five survey locations in Eleuthera, The Bahamas


Coral cover (%)

Bleaching (%)

Disease (%)

New mortality (%)

Transitional mortality (%)

Old mortality (%)

Tunnel Rock

8.7 ± 1.9

7.9 ± 2.4

2.4 ± 1.0

6.6 ± 1.3

20.1 ± 4.4

31.4 ± 3.1

Miller’s Hole

62.6 ± 11.3

9.8 ± 2.6

3.2 ± 1.6

15.2 ± 4.2

28.0 ± 8.5

36.3 ± 6.7

Schooner Cays

49.1 ± 25.7

11.5 ± 3.0

3.1 ± 1.1

6.2 ± 3.2

7.2 ± 3.4

50.4 ± 13.5

Bamboo Point

21.2 ± 5.6

7.8 ± 2.1

2.8 ± 1.2

5.6 ± 1.2

8.7 ± 2.8

34.7 ± 14.7

Cotton Bay

9.1 ± 2.3

16.6 ± 4.3


8.7 ± 3.8

4.8 ± 2.1

52.5 ± 10.7


28.2 ± 11.0

11.0 ± 1.6

2.0 ± 0.6

8.4 ± 1.8

13.4 ± 4.4

41.5 ± 4.3

Mortality is separated by classes New (showing no algal growth), Transitional (small amounts of algal growth over skeleton), and Old (established macroalgae). All values are percentages and the Eleuthera location is a weighted average of all sites combined

Fig. 2

Comparison between means (open circle) and standard error for  % coral cover (a), and  % colonies showing bleaching (b), disease (c), and mortality (d). Mortality bars represent new (showing no algal growth; white bar), transitional (small amounts of algal growth over skeleton; gray bar), and old (established macroalgae; black bar) mortality across all locations (Tunnel Rock (TR), n = 7; Miller’s Hole (MH), n = 6; Schooner Cays (SC), n = 6; Bamboo Point (BP), n = 6; Cotton Bay (CB) n = 8; and all sites averaged for southern Eleuthera, The Bahamas (E), n = 33). Letters indicate significant differences between sites

Coral bleaching and disease extent after the 2014–2017 global bleaching event

Overall (± SE), 11.0 ± 1.6% of coral colonies were bleached at the time of surveys (Fig. 2b; Table 2). Among the Southern Eleuthera sites, CB had the highest incidence of bleaching at 16.6 ± 4.3%, and both TR and BP showed low incidences of bleaching at 7.9 ± 2.4% and 7.8 ± 2.1%, respectively (Fig. 2b; Table 2). No significant differences in percent of bleached corals were found among sites (ANOVA, F4, 28 = 1.45, p = 0.24). Interestingly, observed values for bleaching, overall, were lower than reported for some regions during the 2014–2017 bleaching event, e.g., 47.0% in the Hanauma Bay Nature Reserve (Rodgers et al. 2017), 23.9% on the southern coast of India (Edward et al. 2018), 21.0% on the Southern Great Barrier Reef (Kennedy et al. 2018), and 5–25% on Kanton Island in the republic of Kiribati (Brainard et al. 2018).

Across sites (± SE), 2.0 ± 0.6% of coral colonies showed signs of disease (Fig. 2c; Table 2). However, there was no significant difference in disease prevalence among sites (ANOVA, F4, 28 = 1.74, p = 0.17; Fig. 2c; Table 2). The percentage of corals showing disease was consistent with previously reported values for other locations in The Bahamas during 2011–2013, including the patch reefs of Andros, Cay Sal Bank, Little Bahama Bank, and Southern Bahamas varying from 1.5 to 2.5% (Dahlgren et al. 2016).

Coral mortality in Southern Eleuthera

Subclasses of coral mortality (new, transitional, and old) provided a further understanding of potential changes in coral mortality across years. Average values for new, transitional, and old mortality across Southern Eleuthera were (± SE) 8.4 ± 1.8%, 13.4 ± 4.4%, and 41.5 ± 4.3%, respectively, and significantly differed among sites for transitional mortality (ANOVA, F4, 28 = 4.70, p = 0.005; Fig. 2d; Table 2) but not for new or old mortality. Total mortality on Southern Eleuthera patch reefs was similar to other locales in The Bahamas (~ 55%), such as the patch reefs of Andros, Cay Sal, and Southern Bahamas (Dahlgren et al. 2016). Miller’s Hole had significantly more transitional mortality than BP, CB, and SC (Tukey’s pairwise comparison, p = 0.048, p = 0.008 and p = 0.034, respectively).

We speculate that localized factors (e.g., nutrient run-off/sedimentation) may play a role in the varying amount of transitional mortality across sites. New and old mortality prevalence did not vary among sites, and we hypothesize that values for new and old mortality may be better indicators of regional stressors such as bleaching and disease prevalence. However, results of our Pearson’s correlation analyses on coral cover, bleaching, disease, and mortality revealed no significant correlations (p > 0.05).

As regional and global stressors continue to degrade coral reefs (Bruno and Valdivia 2016), studies such as this are becoming of greater necessity to inform global policy decisions to mitigate the decline of coral reefs (Shaver et al. 2018). Therefore, regular monitoring of coral reef health is imperative for coastal tropical communities that rely on their coral reefs as a means of food resources and employment (e.g., local/commercial fisheries and tourism).



The authors thank two anonymous reviewers and the guest editors for their valuable comments on early drafts of the manuscript. The authors also thank Drew Hitchner for his contribution to data collection, both Brittany Munson and Callie Stephenson for boat support, and Logan Zeinert for creating the map of Eleuthera study sites. Lastly, the authors thank the Cape Eleuthera Institute for logistical support and the hard-working boat house staff for the countless hours required to ensure boats remained operable.

Compliance with ethical standards

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of BiologyMemorial University of NewfoundlandSt. John’sCanada
  2. 2.Cape Eleuthera InstituteRock Sound, EleutheraBahamas
  3. 3.Field Lab ConsultingPalmettoUSA

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