June 7th, 2016

Third mass coral bleaching in 18 years threatens Great Barrier Reef


by Amanda Wilcox

One of the seven wonders of the natural world is currently in peril: Australia’s Great Barrier Reef, which spans 2,300 miles of the Queensland coast, is experiencing its most extensive bleaching event in recorded history. It is the third mass coral bleaching in 18 years. Formerly, mass coral bleachings occurred approximately once every thousand years.

The damage that this “rainforest of the sea” has sustained could have a devastating impact on its biodiversity and ability to provide ecosystem services. Here’s a brief summary of how coral bleaching occurs and its implications for the future of the Great Barrier Reef.

Coral reefs are composed of billions of tiny invertebrate animals called coral polyps, which secrete hard calcium carbonate exoskeletons.

coral reef

Coral reefs near Jarvis Island. The reefs are part of the Pacific Remote Islands Marine National Monument, which President Obama pledged to expand and protect from fishing and drilling. Courtesy of Jim Maragos via the U.S. Fish and Wildlife Service.

These exoskeletons allow the polyps to conduct a symbiotic relationship with zooxanthellae algae: the corals provide protection to the algae while the algae supply the reef with nutrients and give corals their vibrant hues.


A tourist swims on the Great Barrier Reef in this undated file picture. Amid pressure of losing its World Heritage status, the government has decided to expand areas subject to curbs on shipping. Photo by HO/Great Barrier Reef National Park Authority/Reuters.

As important primary producers, algae form the foundation of the ocean food web.

However, the symbiotic relationship between coral polyps and zooxanthellae algae is only functional within a narrow range of water temperatures and pH. When ocean conditions become excessively warm or acidic, the zooxanthellae algae produce toxins that cause the coral polyps to expel them from their tissues.

As a result, the corals turn a ghostly white and lose their source of nutrients.


BRISBANE, AUSTRALIA: Dr Paul Marshall from the Great Barrier Reef marine park photographs bleached coral heads off the Keppel Islands in Queensland in the this file picture from 23 January 2006. An international network of over 100 scientists is alarmed at the level of bleaching occurring worldwide resulting in a global loss of the tiny plant-like organisms. OVE HOEGH-GUIDBERG/AFP/Getty Images

If conditions return to normal, the zooxanthellae algae will return and the reef can recover. If they do not, the corals starve and their ability to fight disease is diminished; they could even die.

Ocean temperatures off the coast of Australia have been elevated over the past several months, which can be partially explained by a particularly strong 2016 El Nino Southern Oscillation event. In addition, most scientists concur that global ocean temperatures are rising as a result of anthropogenic climate change. This graph created by the U.S. Environmental Protection Agency shows an increasing trend in global sea surface temperatures.

This combination has proved deadly to corals: Scientists have estimated through aerial surveys that approximately 93 percent of the Great Barrier Reef has suffered at least moderate bleaching. About a third of corals in the northern Great Barrier Reef have died of malnutrition. This map from the Australian National Coral Bleaching Taskforce shows the extent and severity of the bleaching.

The Washington Post quoted the following from the Australian National Coral Bleaching Taskforce’s news release: “The most pristine section of the Great Barrier Reef is currently experiencing the worst mass bleaching event in its history.”

The damage is not wholly irreversible–if ocean temperatures decrease sufficiently, zooxanthellae algae could return to rehabilitate bleached corals and new coral polyps could succeed those that have died. However, most corals in the Great Barrier Reef are 50-100 years old and will therefore recover more slowly; in addition, severely traumatized corals are less resilient. According to this Washington Post article, “coral that has died is gone for good.”

To make matters worse, spikes in ocean temperature have become more frequent in past decades, giving damaged corals less time to recover.

Consequences of the damage that the reef has sustained are expected to be widespread; coral is a keystone species in the oceanic ecosystem. Reefs provide habitat and shelter for up to 25% of marine species, from tropical fish and mollusks to sea turtles, sea sponges and sharks.


Tropical fish swimming around reef from science nation (NSF) video.

Some of these species are endemic to the Great Barrier Reef, so they could become endangered or even extinct as a result of habitat loss. A biodiversity decrease in the Great Barrier Reef ecosystem could have ripple effects throughout the entire ocean food web.

Furthermore, the Great Barrier Reef provides invaluable ecosystem services to humans. It buffers the shoreline from tropical storms, protecting coastal homes and businesses from wave damage. Numerous fisheries depend on the reef’s health and productivity, as it acts as a nursery for juvenile fish. Damage to the reef, therefore, could potentially affect the global food supply. Ecotourism generates about $5 billion in revenue for Australia’s economy each year, and that figure could suffer if the splendor of the reef is compromised.

Even though the Great Barrier Reef is so large that it can be seen from outer space, it is one of the world’s most delicate ecosystems. Without immediate action to mitigate the effects of warming ocean temperatures, the world stands to lose a spectacular treasure.

For further information, please consult the following:

Coral bleaching is not solely plaguing the Great Barrier Reef. Watch this PBS NewsHour video story about threatened corals in Florida.

Watch this PBS NewsHour video story about threats the Great Barrier Reef faces in addition to bleaching.

‘And then we wept:’ Scientists say 93 percent of Great Barrier Reef now bleached’ –Washington Post


coral reef: colony of coral polyps and calcium carbonate deposits found in warm, shallow seas

bleaching: process by which corals expel zooxanthellae algae and turn white

biodiversity: variety and variability of species found in a particular ecosystem

ecosystem services: benefits provided by an ecosystem

invertebrate: organism lacking a backbone

coral polyps:  tiny invertebrate organisms related to jellyfish

exoskeleton: external skeleton that protects an animal’s body

symbiotic relationship: interaction between two species characterized by mutual benefit

zooxanthellae algae: photosynthetic algae contained in the tissue of reef-building corals

El Nino Southern Oscillation: change in trade wind patterns causing a band of unusually warm surface water in the Pacific Ocean

anthropogenic: caused by human activity

keystone species: species on which other species in an ecosystem heavily depend

habitat: area where a particular organism lives

endemic: an organism native only to one specific region

endangered: at risk of becoming extinct

extinct: no living members of a particular species remain

ecotourism: tourism focusing on plants, animals, and natural environments

Amanda Wilcox is a senior at T.C. Williams High School in Alexandria, Virginia. She will be attending Wake Forest University in Winston-Salem, North Carolina next fall.

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