Impacts of the 2014–2017 global bleaching event on a protected remote atoll in the Western Indian Ocean


The third global bleaching event caused prolonged elevated sea surface temperatures from 2014 to 2017 that heavily impacted coral reefs worldwide. This study determines changes in benthic community following this bleaching event at a remote UNESCO World Heritage Site in the Western Indian Ocean. Aldabra Atoll offers a rare opportunity to study global impacts in the absence of local anthropogenic stressors. Analysis of satellite-derived temperature data indicated that Aldabra was exposed to the highest bleaching-risk intensity of the past 20 years during this bleaching event. Bleaching-risk conditions lasted from December 2015 to June 2016 close to the 4 °C-week threshold, when bleaching is expected. Benthic cover was established pre- and post-bleaching from 21 transects across two reef locations (lagoonal reef, 2 m depth; seaward reef, 5 and 15 m depth). From a pre-bleaching benthic community in which living corals and epilithic algal matrix (EAM) predominated, Aldabra’s reefs switched to an EAM-dominated community 8 months after bleaching. Soft corals declined by 93% of their overall pre-bleaching cover to < 1%. Although overall hard-coral cover was also reduced, the decline varied among depths and might indicate local adaptations of the lagoonal reef, due to greater variability in sea surface temperature compared to the seaward reef. With the exception of Isopora palifera, all taxomorphic coral groups experienced a decline following bleaching. Overall, Rhytisma experienced a near-complete extirpation, Acroporids (excluding I. palifera) and branching Poritids declined by more than 80%, Merulinidae lost ca. 60% of their pre-bleaching cover, while massive Poritids cover slightly decreased. Aldabra’s benthic community therefore underwent substantial changes following the 2014–2017 bleaching event and showed that live coral cover declines significantly even in protected areas isolated from local anthropogenic pressures.

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A special thanks goes to the Seychelles Islands Foundation (SIF) and staff for their invaluable support and help, in particular: Rowana Walton and Anna Koester for the identification of benthic data, as well as the SIF staff who helped with data collection (Daig Romain, Rebecca Filippin, Sheril De Commarmond, Jude Brice, Samuel Basset and Terence Mahoune for pre-bleaching data; Terence Mahoune, Ella Nancy, Joel Bonne, Marvin Roseline for post-bleaching data). Anna Koester is further thanked for cleaning the temperature dataset. We thank the Global Environment Facility for funding for the diving and research equipment, and the development of the ARM programme. We further thank Anthony Bernard and Nick Riddin from the South African Institute for Aquatic Biodiversity and SIF staff who assisted in the establishment of permanent benthic transects; and Rebecca Klaus for advice on benthic transect set-up and photoquadrat methodology. Prof. Gabriela Schaepman-Strub was supported by the University of Zürich Research Priority Programme on Global Change and Biodiversity. Prof. Bernhard Schmid and Prof. Pascal A. Niklaus are further thanked for their advice on the statistical analyses. Finally, we would like to thank the editor and reviewers for their constructive feedback.

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Cerutti, J.M.B., Burt, A.J., Haupt, P. et al. Impacts of the 2014–2017 global bleaching event on a protected remote atoll in the Western Indian Ocean. Coral Reefs 39, 15–26 (2020) doi:10.1007/s00338-019-01853-1

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  • Coral bleaching
  • El Niño
  • Marine protected area
  • Aldabra Atoll
  • Seychelles
  • UNESCO World Heritage Site