Coral Reefs

Coral reefs are the ocean’s most diverse and complex ecosystems, supporting 25% of all marine life, including 800 species of reef-building corals and more than one million animal and plant species. They are close relatives of sea anemones and jellyfish, as each coral is a colony consisting of many individual sea anemone-like polyps that are all interconnected.

Tropical coral reefs found in warm, clear water at relatively shallow depths are intricately patterned carpets of life growing on foundations formed primarily by calcium carbonate exoskeletons and coralline algae. These structures fuse over time, enlarging the reef and creating countless nooks and crannies. As the reef grows, species from nearly every major taxonomic group cover every square inch of these tightly integrated systems, providing food and shelter to a spectacular variety of fish species, including many of commercial value.

‘Hard’ corals use calcium carbonate from seawater to synthesize a hard, mineral protective shell around each polyp. These exoskeletons, along with shells formed by coralline algae, mollusks and tubeworms, spicules made by sponges, and shells of other calcifying species form the structural foundation of coral reefs. Corals catch plankton with their tentacles, but most of their nutrition comes from photosynthetic algae that live in their tissues, using the coral’s waste products for their own nutrition and feeding the corals with sugars and other nutritious compounds that leak through their cell membranes.

Deep water reefs, formed by large, long-lived but fragile, soft corals are also architecturally and ecologically complex and teem with life, but lack a calcium carbonate foundation. Though beyond the reach of sunlight, underwater lights reveal them to be nearly as beautiful and colorful as their tropical counterparts.

The condition of coral reefs is important to the Ocean Health Index because healthy reefs provide many benefits to people, including food, natural products, coastal protection from storms, jobs and revenue, tourism and recreation, biodiversity and others.

60% of reefs are already seriously damaged by local sources such as overfishing, destructive fishing, anchor damage, coral bleaching, coral mining, sedimentation, pollution, and disease. When these types of human threats are combined with the influence of rising ocean temperatures, 75% of reefs are threatened (Burke et al, 2011).

Major Sources Of Damage To Coral Reefs
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Which Goals Does This Affect?

How Was It Measured?

The extent of coral reefs are derived from the 500m resolution dataset developed for Reefs at Risk Revisited (Burke et al. 2011), in conjunction with a re-sampled version of the Ocean Health Index EEZ regions. The condition of reefs was estimated using data for percentage cover by live coral determined from 12,634 surveys conducted from 1975-2006 and summarized by Bruno and Selig (2007) and Schutte et al. (2010).

What Are The Impacts?

Corals that are exposed to elevated sea surface temperatures expel the symbiotic photosynthetic algae responsible for their nutrition and coloration [zooxanthelle] in a process known as coral bleaching. Corals can recover from occasional bleaching, but not from repeated bleaching. Increases in sea-surface temperature of about 1-3 °C are projected to result in more frequent coral bleaching events and widespread mortality (IPCC 2007).

Elevated sea surface temperatures cause increased damage to reefs from breakage as storm frequency and intensity increase.

Increasing concentrations of carbon dioxide in the atmosphere cause increased concentrations in surface waters, leading to ocean acidification (lowered pH). Acidification decreases the availability of calcium carbonate, making it harder for corals and other calcifying organisms to form their shells; it also dissolves existing shells.

By the end of the century, it is predicted that ocean pH will drop from its current value of about 8.1 by as much as 0.4 units; by 2050, conditions will not be sufficient for the formation of calcium carbonate (Hoegh-Guldberg et al. 2007).

Coral reefs that have been overfished can cause ecosystem structure to shift as fish populations decline. This can result in increases in disease and take-over by coral predators and aggressive algal species.

Overfishing threatens more than 70% of coral reefs in the Caribbean (Burke et al. 2011).

Hard corals develop and grow at a very slow rate, which does not allow for easy recovery when they are physically damaged by hurricanes, shipwrecks or anchors. Branching hard corals can recover more easily, but they are more susceptible to breakage by storms or physical contact. 
Corals and affiliated sponges contain bioactive chemical compounds that can be useful as cancer and virus-fighting drugs. For example, AZT, a compound generated by a Caribbean reef sponge, is an antiretroviral drug that effectively slows the spread of the HIV virus.
Almost 500 million people depend on coral reefs for coastal protection, food, and tourism income (Wilkinson, 2008).

Coral reefs help protect shorelines from storm damage and can absorb 70-90% of wave energy.

The economic benefit of reef protection can be substantial. A proposed wastewater treatment facility in the Florida Keys that would cost US $60-70 million to build and US $4 million annually to operate and maintain would lead to an estimated US $700 million benefit in Net Present Value (NPV) from reef ecosystem services and other reef industries (Cesar et al. 2003).

Total net benefits from coral reef shoreline protection are estimated at US $9 billion globally (Cesar et al. 2003).

Reef fisheries generate large potential net benefits and are relied upon by many for livelihoods and food; unsustainable fishing practices can have a severe impact upon reef-dependent economies.

Total net benefit of coral reefs to fisheries: US $5.7 billion (Cesar, Burke, and Pet-Soede, 2003).

Reef tourism provides long-term benefits although upfront costs to prevent reef degradation may initially be high.

Costs of coral bleaching to tourism in Net Present Value (NPV) is estimated at $10 - $40 billion (Cesar, Burke, and Pet-Soede, 2003)

What Has Been Done?

Get More Information

Reef Base
A global database for best practices in the management of coral reefs. 

Conservation International [CI]: Socioeconomic Conditions along the World’s Tropical Coasts: 2008
Synthesizes the latest knowledge regarding people and reefs based on socioeconomic monitoring around the world. 

Coral Reef Alliance [CORAL]
An international NGO founded to support local projects that benefit coral reefs and surrounding communities.

Global Coral Reef Monitoring Network [GCRMN]
 GCRMN works to improve coral reef conservation and management efforts and synthesizes the latest ecological and socioeconomic monitoring results into a biannual publication, Status of Coral Reefs of the World

International Coral Reef Initiative [ICRI]
A partnership between non-government organizations, governments and other international organizations, that works to implement international conventions and agreements.

International Coral Reef Action Network [ICRAN]
A network of science and conservation organizations that use scientific knowledge to conserve reefs on a global scale.

Reef Base: A Global Information System for Coral Reefs
A source for coral reef data, publications, maps, and other resources from around the world hosted by World Fish Center

Reef Environmental Education Foundation [REEF]
A foundation that preserves marines environments by educating and enabling divers and other people interested in ocean conservation to perform volunteer reef surveys.

Science to Action: Coral Health Index: Measuring Coral Community Health 
A guidebook to evaluating coral health and understanding impacts

Science to Action: Economic Values of Coral Reefs, Mangroves, and Seagrasses
A Global Compilation provides the latest statistics on the economic values of tropical marine resources

Science to Action: Living with the Sea
Local efforts buffer effects of global change: explains the benefits of protecting reefs at a local level.

World Resources Institute [WRI]: Reefs at Risk Revisited
A booklet detailing spatial and statistical data on current threats to coral reefs. 

World Atlas of Coral Reefs
A detailed assessment of the distribution and status of coral reefs

Spalding, Mark, Corinna Ravilious, and Edmund Green. 2001. World Atlas of Coral Reefs. Berkeley, CA: University of California Press and UNEP/WCMC.

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PHOTO(S): © Keith A. Ellenbogen
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