Advertisement

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
Note

Abstract

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.

Keywords

Coral reef Caribbean Coral bleaching Coral mortality Coral disease Diversity indices 

Introduction

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.

Methods

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

Location

Depth (m)

Temperature (°C)

# Transects (n)

# Coral species

Margalef (d)

Pielou (J′)

Shannon (H′)

Simpson (1 − λ)

Tunnel Rock

8

25

7

20

2.7 ± 0.23

0.8 ± 0.03

1.9 ± 0.12

0.8 ± 0.04

Miller’s Hole

8

25

6

25

2.8 ± 0.29

0.9 ± 0.02

2.2 ± 0.11

0.9 ± 0.02

Schooner Cays

5

27

6

20

2.4 ± 0.22

0.9 ± 0.02

1.9 ± 0.07

0.8 ± 0.01

Bamboo Point

6

27

6

25

2.8 ± 0.23

0.8 ± 0.03

2.0 ± 0.12

0.8 ± 0.03

Cotton Bay

3

26

8

21

2.2 ± 0.17

0.9 ± 0.03

1.7 ± 0.13

0.8 ± 0.04

Eleuthera

6

26

33

37

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

Location

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

0.0

8.7 ± 3.8

4.8 ± 2.1

52.5 ± 10.7

Eleuthera

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).

Notes

Acknowledgements

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.

References

  1. Ainsworth TD, Heron SF, Ortiz JC, Mumby PJ, Grech A et al (2016) Climate change disables coral bleaching protection on the Great Barrier Reef. Science 352:338–342CrossRefPubMedGoogle Scholar
  2. Baker AC, Glynn PW, Riegl B (2008) Climate change and coral reef bleaching: An ecological assessment of long-term impacts, recovery trends and future outlook. Estuar Coast Shelf Sci 80:435–471CrossRefGoogle Scholar
  3. Bourne DG, Garren M, Work TM, Rosenberg E, Smith GW, Harvell CD (2009) Microbial disease and the coral holobiont. Trends Microbiol 17:554–562CrossRefPubMedGoogle Scholar
  4. Brainard RE, Oliver T, McPhaden MJ, Cohen A, Venegas R et al (2018) Ecological impacts of the 2015/16 El Niño in the Central Equatorial Pacific. BAMS 99:21–26CrossRefGoogle Scholar
  5. Bruno JF, Valdivia A (2016) Coral reef degradation is not correlated with local human population density. Sci Rep 6:1–8CrossRefGoogle Scholar
  6. Clarke KR, Gorley RN (2006) PRIMER v6: User manual/tutorial. PRIMER-E, Plymouth, p 192Google Scholar
  7. Cróquer A, Weil E (2009) Spatial variability in distribution and prevalence of Caribbean scleractinian coral and octocoral diseases. II. Genera-level analysis. Dis Aquat Organ 83:209–222CrossRefPubMedGoogle Scholar
  8. Dahlgren C, Sherman K, Lang J, Kramer PR, and Marks K (2016) Bahamas coral reef report card volume 1: 2011-2013Google Scholar
  9. Eakin CM, Liu G, Gomez AM, De La Cour JL, Heron SF et al (2017) Ding, dong, the witch is dead (?) – Three years of global coral bleaching 2014-2017. Reef Encounter 45:33–38Google Scholar
  10. Edward JKP, Mathews G, Diraviya Raj K, Laju RL, Selva Bharath M et al (2018) Coral mortality in the Gulf of Mannar, southeastern India, due to bleaching caused by elevated sea temperature in 2016. Curr Sci 114:1967–1972CrossRefGoogle Scholar
  11. Food and Agriculture Organization, FAO (2016) Fisheries and aquaculture in The Bahamas: A review. FAO of the United Nations/Department of Marine Resources Nassau, The BahamasGoogle Scholar
  12. Gardner TA, Côté IM, Gill JA, Grant A, Watkinson AR (2003) Long-term region-wide declines in Caribbean corals. Science 301:958–960CrossRefPubMedGoogle Scholar
  13. Hughes TP, Anderson KD, Connolly SR, Heron SF, Kerry JT et al (2018) Spatial and temporal patterns of mass bleaching of corals in the Anthropocene. Science 359:80–83CrossRefPubMedGoogle Scholar
  14. Hughes TP, Kerry JT, Álvarez-Noriega M, Álvarez-Romero JG, Anderson KD et al (2017) Global warming and recurrent mass bleaching of corals. Nature 543:373–377CrossRefPubMedGoogle Scholar
  15. Jackson JBC, Donovan MK, Cramer KL, Lam VV (eds) (2014) Status and Trends of Caribbean Coral Reefs:1970-2012. Global Coral Reef Monitoring Network, IUCN, Gland, SwitzerlandGoogle Scholar
  16. Kennedy EV, Ordoñez A, Diaz-Pulido G (2018) Coral bleaching in the southern inshore Great Barrier Reef: A case study from the Keppel Islands. Mar Freshw Res 69:191–197CrossRefGoogle Scholar
  17. Komyakova V, Jones GP, Munday PL (2018) Strong effects of coral species on the diversity and structure of reef fish communities: A multi-scale analysis. PLoS ONE 13:e0202206CrossRefPubMedPubMedCentralGoogle Scholar
  18. Lang JC, Marks KW, Kramer PA, Kramer PR, and Ginsburg RN (2010) AGRRA protocols version 5.4. Atlantic and Gulf Rapid Reef Assessment Program, Florida, USAGoogle Scholar
  19. Lesser MP (2006) Oxidative stress in marine environments: Biochemistry and physiological ecology. Annu Rev Physiol 68:253–278CrossRefPubMedPubMedCentralGoogle Scholar
  20. Muller EM, Bartels E, and Baums IB (2018) Bleaching causes loss of disease resistance with threatened coral species Acropora cervicornis. eLife.7: e35066Google Scholar
  21. Muller EM, van Woesik R (2011) Black-band disease dynamics: Prevalence, incidence, and acclimatization to light. J Exp Mar Bio Ecol 397:52–57CrossRefGoogle Scholar
  22. Nyström M, Folke C, Moberg F (2000) Coral reef disturbance and resilience in a human-dominated environment. Trends Ecol Evol 15:413–417CrossRefPubMedGoogle Scholar
  23. Pandolfi JM, Connolly SR, Marshall DJ, Cohen AL (2011) Projecting coral reef futures under global warming and ocean acidification. Science 333:418–422CrossRefPubMedGoogle Scholar
  24. R Core Team (2016) R: A language and environment for statistical computing. https://www.r-project.org
  25. Rodgers KS, Bahr KD, Jokiel PL, Richards Donà A (2017) Patterns of bleaching and mortality following widespread warming events in 2014 and 2015 at the Hanauma Bay Nature Preserve, Hawai‘i. PeerJ 5:e3355CrossRefPubMedPubMedCentralGoogle Scholar
  26. Shaver EC, Burkepile DE, Silliman BR (2018) Local management actions can increase coral resilience to thermally-induced bleaching. Nat Ecol Evol 2:1075–1079CrossRefPubMedGoogle Scholar

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

Personalised recommendations