Coral Reefs and Coral Bleaching

by

Janice Lough, Clive Wilkinson

• LAST REVIEWED: 26 July 2017
• LAST MODIFIED: 26 July 2017
• DOI: 10.1093/obo/9780199363445-0080

Introduction

Tropical coral reef ecosystems are hotspots of marine biodiversity. Globally, coral reefs occupy less than 0.5 percent of the ocean floor—an area half the size of France—yet they support, for example, about 25 percent of all marine fish species. Aside from their aesthetic and biodiversity value, healthy coral reefs provide goods and services (food, income from fishing and tourism, coastal protection) to over 500 million people worldwide, and about 30 million people are totally dependent on coral reefs as they live on coral islands and atolls. Coral reefs occur in the warmest parts of the tropical oceans in shallow, well-lit waters with low amounts of sediments and nutrients. Their current distribution is also constrained by (1) their geological history since present-day sea level was reached eight to ten thousand years ago after the height of the last ice age, (2) water depth, and (3) the aragonite saturation state of sea water—a measure of how easily aragonite, the main form of calcium carbonate created by reef-building corals, can form depending on the degree of alkalinity/acidity (pH) of seawater. Reef-building corals are marine invertebrates (animals without backbones, phylum Cnidaria) that form colonies of many individual polyps. These extract calcium and carbonate from seawater to form hard skeletons which support and protect the polyps. The resulting spectacular variety of skeletal growth structures forms the backbone of tropical coral reef ecosystems and provides habitat for hundreds of thousands of reef-associated organisms. Within the living tissue layer reside single-celled photosynthetic algae (zooxanthellae, genus Symbiodinium) in a mutually beneficial, symbiotic relationship with their coral host. The microscopic plants gain protection and essential nutrients from the coral, and the coral gains photosynthetic products from the algae: this source of cheap energy allows the coral to calcify faster than the natural forces of physical and biological erosion. Photosynthetic pigments of the algae provide corals with their colors. If, however, the coral animal is stressed by unfavourable environmental conditions, they lose some or all of the algae and photosynthetic pigments, and the white skeleton becomes visible through the translucent tissue layer—a phenomenon called “coral bleaching.” The occurrence of mass coral bleaching events, when whole tracts of reef bleach, has increased in frequency in the early 21st century. As of March 2017, the world’s coral reefs have been experiencing the most protracted event on record, which began in mid-2014. This annotated bibliography introduces the science behind coral bleaching, its causes, and implications for the future of tropical coral reef ecosystems.

General Overviews

Coral reefs are not just about coral, they also create the backbone of reefs and thus provide habitat for hundreds of thousands of reef-associated organisms to form an ecosystem. Tropical coral reefs are dynamic and complex ecosystems that have biological, geological, and social dimensions. They are also recognized as ecosystems that are now in trouble due to a combination of local stressors (e.g., a history of overexploitation including overfishing, land-based pollution) and global stressors (e.g., current ocean warming, ocean acidification). Dubinsky 1990 is a useful early textbook covering tropical coral reef ecosystems. Knowlton 2011, an annotated bibliography, provides an excellent introduction to the various sources of literature relating to tropical coral reef ecosystems and the challenges they currently face. Hopley 2011 is a classic reference volume for all aspects of modern coral reefs, although only one entry specifically addresses bleaching, there are several references to this phenomenon. Rosenberg and Loya 2004 provides a useful introduction to various aspects of both coral diseases and the coral bleaching phenomenon including chapters on bleaching as an adaptive response and, more controversially, bleaching as a bacterial disease of corals. Dubinsky and Stambler 2011 provides a comprehensive and detailed overview of the various processes occurring on tropical coral reefs in an era of rapid changes to their environment, including a chapter devoted to coral bleaching. The publisher Springer is also progressively publishing a series of books under the theme “Coral Reefs of the World” (series editors Bernhard Riegl and Richard Dodge) covering the present state of knowledge of the world’s coral reefs. The edited volume van Oppen and Lough 2009 provides the only book specifically covering all aspects of the bleaching phenomenon.

• Dubinsky, Zvy, ed. 1990. Coral Reefs. Ecosystems of the World 25. Amsterdam: Elsevier.

  An edited early volume with nineteen chapters covering various aspects of coral reef ecosystems including one on the role of symbiotic algae.

• Dubinsky, Zvy, and Noga Stambler, eds. 2011. Coral Reefs: An Ecosystem in Transition. Dordrecht, The Netherlands: Springer.

  Edited volume containing 29 chapters written by respective experts. It covers the history, biology, ecosystems, disturbances and conservation and management of coral reefs. Four chapters specifically relate to coral bleaching: symbiosis, climate change, coral bleaching and potential for acclimatization.

• Hopley, David, ed. 2011. Encyclopedia of modern coral reefs: Structure, form and process. Dordrecht, The Netherlands: Springer.

  Described as in “the realm of classic literature on reefs, past and present,” this reference work contains 260 fully referenced entries from Ancathaster planci through zooxanthellae. Entries cover the biology, geology, geomorphology, and oceanography of reefs, and the book makes for a useful reference for students and established researchers.

• Knowlton, Nancy. 2011. Coral Reefs. Oxford Bibliographies.

  A comprehensive annotated bibliography covering the biology of corals, their biodiversity, coral reef ecosystems, human impacts and their conservation and restoration, including a short section on coral bleaching.

• Riegl, Bernhard, and Richard E. Dodge, series eds. Coral reefs of the world. New York: Springer.

  Edited series of books relating to world’s coral reef ecosystems; eight edited volumes published to date, six covering regional coral reef ecosystems and two more generally covering coral reef science and current coral reef crisis. Additional volumes planned or in progress.

• Rosenberg, Eugene, and Yossi Loya, eds. 2004. Coral health and disease. Heidelberg, Germany: Springer-Verlag.

  Edited volume from international meeting containing twenty-six chapters, focused on coral diseases but several chapters covering regional bleaching, symbiont diversity, thermal stress, hypotheses of bleaching, and future of reefs with climate change.

• van Oppen, Madeleine J. H., and Janice M. Lough. 2009. Coral bleaching: Patterns, processes, causes and consequences. Ecological Studies, Vol. 205. Heidelberg, Germany: Springer-Verlag.

  Edited volume containing eleven chapters written by respective experts ranging from the evolution of the coral:algal symbiosis, bleaching in space and time, detecting events, the diversity of the symbionts, impacts on other organisms and modeling the reef futures. Good synthesis for students and coral researchers. An updated second edition will be published in 2018.