The Imperiled Future of the Great Barrier Reef

Back to Map Page March, 2002: Northeastern Coast of Australia The Great Barrier Reef is an expanse of coral in the Coral Sea, off the northeast coast of Australia. The reef is over 1,200 miles long, making it the largest deposit of coral in the world. The reef provides a home for countless organisms and also protects coastal regions from the harsh winds and waves characteristic of the Coral Sea. However, the future of this coral reef is in jeopardy because of land clearing, coastal development, and overfishing. In addition, the coral is often coveted for jewelry and other ornamentation, since it exists in a range of colors, shapes, and sizes. Further compounding the degradation of this natural wonder is global warming, which causes an increase in sea surface temperatures and subsequent bleaching of the reef. There are three main types of hermatypic coral, the only type of coral that builds reefs: Fringing reefs extend from an island or mainland; atolls are islands made entirely of coral; and barrier reefs occur offshore, generally with a channel between the reef and the shoreline. Coral. Coral reefs are composed of animals and algae (specifically, dinoflagellates) living together in a symbiotic relationship deemed mutualistic, since both members benefit from the association. Polyps, the animal component, are members of the Coelenterata, a phylum of invertebrate animals that includes sea anemones and jellyfishes. The polyps secrete a calcareous exoskeleton to which they are basally anchored and into which they can withdraw for protection. At the anterior end of the polyp are both the mouth and the anus, surrounded by tentacles and cilia. Some polyps have eight tentacles, and others have six or multiples of six, depending on the subclass. The cilia and tentacles are used to seize zooplankton from the water column, offering the polyps much-needed nutrients, such as phosphorus. Dinoflagellates with characteristic cellulose plates include (a) Ceratium and (b) Gonyaulax. Within the polyps live endosymbiotic dinoflagellates called zooxanthellae, or more commonly, algae. While dinoflagellates characteristically have an armor of thick, stiff cellulose plates, dinoflagellates that are symbionts lack such plates. Zooxanthellae are unicellular, round, and taxonomically classified in the kingdom Protista, order Dinoflagellata. Most pigmented zooxanthellae contain chlorophylls a and c and carotenoid pigments, which enable them to photosynthesize and then to share those products with the polyps. In return, the polyp-produced exoskeleton provides protection for the zooxanthellae as well as access to light (coral containing zooxanthellae generally live in waters less than 100 meters deep). Nutrients, including nitrogenous waste from the polyps, cycle between these two organisms, thereby reducing nutrient loss to the surrounding water. Global warming. Over the past 20 years, mass coral mortality has been linked to global warming and to subsequent elevation of sea temperatures. Global warming is the modification of climates that results from increased retention of terrestrial radiation by certain atmospheric gases. These "greenhouse gases," principally water vapor, carbon dioxide, and methane, are mainly the by-products of anthropogenic (human-related) activity. For example, since wetlands and forests are huge carbon sinks, the destruction of these areas releases large amounts of carbon into the atmosphere. Increased carbon dioxide concentrations have been shown to change water chemistry, thereby reducing the ability of the exoskeletons to calcify. Moreover, mortality due to "coral bleaching," in which zooxanthellae are expelled from the polyps, is a response to high temperature and/or high-incident solar radiation. (The phenomenon is called bleaching because, once the polyp dies due to lack of nutrients, a white, calcareous skeleton is left behind.) While this trend has been noted over the past two decades, the highest rate of coral mortality occurred in 1998, the year of El Nino. As recorded by the U.S. National Oceanographic and Atmospheric Administration, temperatures that rarely exceed 29°C in the central area of the Great Barrier Reef were recorded at 31.5°C in 1998. It has been documented that an increase of only one degree can trigger zooxanthellae expulsion and subsequent polyp death. Some scientists feel that if sea temperatures continue to rise 1 to 2 degrees each century, most coral reefs will be a thing of the past by the year 2100. To test the effect of increased sea temperature, researchers from the University of Sydney placed six test tanks on the Great Barrier Reef, artificially increased the temperature, and then drew conclusions based on the data collected. Results showed a 40% drop in reproductive rate at 32°C and a nearly 0.0% reproductive rate at 34°C. Another likely consequence of global warming that will further compound the problem of coral bleaching is a rise in sea level, which can potentially make it more difficult for the solar wavelengths necessary for photosynthesis to reach the zooxanthellae. On the other hand, even with the publication of on-site experiments and the tracking of El Nino, not all scientists are convinced that coral bleaching is detrimental to the survival of coral reefs. Dr. Andrew Baker of the Wildlife Conservation Society, Osborn Laboratories of Marine Science, believes coral bleaching offers the opportunity for the reef to be colonized by more beneficial zooxanthellae. Since at least 10 different zooxanthellae are associated with corals, each showing strong zonal patterns that correspond to light intensity, bleaching may be a way for coral to rid themselves of suboptimal zooxanthellae in exchange for better-suited symbionts. Dr. Baker tested his theory by reciprocally transplanting several different types of coral in Panama between shallow and deeper waters. Following the subjects for a year, he noted that those transplanted to shallow waters bleached, but that they recovered when they were recolonized by a different species of zooxanthellae. The coral transplanted to deeper waters did not bleach; instead, many specimens outright died-probably because the zooxanthellae associated with them were not well suited to the different temperature and light intensity at that depth. If that is the case, concern over rising water levels is justified. While this research may show reefs to be potentially more resilient that previously thought, zooxanthellae are apparently environment-specific, and another type of zooxanthellae may not be available in the surrounding waters to colonize recently bleached coral. Evidently much research is needed in this area. In the meantime, many environmental and scientific groups are continuing to appeal to the United Nations Educational, Scientific, and Cultural Organization (UNESCO) to place the Great Barrier Reef on UNESCO's endangered list. References, Websites, and Further Reading Aronson, R.B., et al. May 4, 2000. Coral bleach-out in Belize. Nature 405, p. 36. Baker, A.C. June 14, 2001. Reef corals bleach to survive change. Nature 411, pp. 765-66. Brown, B.E., et al. March 9, 2000. Bleaching patterns in reef corals. Nature 404, pp. 142-43. Hoegh-Guldberg, O. 1999. Climate change, coral bleaching and the future of the world's coral reefs. Marine and Freshwater Research 50, pp. 839-66. Pockley, P. December 17, 1998. Budget block on U.S. bid to rejoin UNESCO as Australian minister hits out at agency over heritage sites. Nature 396, p. 606. Pockley, P. July 8, 1999. Global warming could kill most coral reefs by 2100. Nature 400, p. 98. Wilkinson, C.R. 1999. Global and local threats to coral reef functioning and existence review and predictions. Marine Freshwater Research 50, pp. 867-78. http://encarta.msn.com "Coral Reef," Microsoft® Encarta® Online Encyclopedia 2001 © 1997-2001 Microsoft Corporation. http://www.geo.ucalgary.ca/~macrae/palynology/dinoflagellates/dinoflagellates.html Department of Geology and Geophysics at the University of Calgary http://www.sprl.umich.edu/GCL/paper_to_html/coral.html Global Change Project at the University of Michigan http://www.epa.gov/owow/oceans/coral/ United States Environmental Protection Agency, Oceans and Coastal Protection Division Related Reading in Stern, Kingsley R. 2000. Introductory Plant Biology, 8th ed. New York: McGraw-Hill Companies. Chapter 3: Cells The cell wall and cellulose, pp. 33-36 Chapter 10: Plant Metabolism Photosynthesis, chlorophylls a and c, and carotenoid pigments, pp. 166-70 Absorption vs. wavelength and absorption spectrums, pp. 171-72; figs. 10.6 and 10.7 Chapter 17: Kingdom Monera and Viruses Symbiotic relationships and mutualism, p. 280 Chapter 18: Kingdom Protista Plankton, p. 306 Algae, pp. 304-33 Division Dinophyta, the dinoflagellates, pp. 321-22 Chapter 25: Ecology Wetlands, p. 462 Carbon cycle, pp. 467-68 Greenhouse effect (CO2 and CH4), p. 475 Loss of biodiversity, p. 476 Back to Map Page Copyright ©2000 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy. McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies. If you have a question or a problem about a specific book or product, please fill out our Product Feedback Form. 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