Coral Reefs 101
Importance of Coral Reefs
Coral reefs are some of the most diverse and valuable ecosystems on Earth. Coral reefs support more species per unit area than any other marine environment, including about 4,000 species of fish, 800 species of hard corals and hundreds of other species. Scientists estimate that there may be another 1 to 8 million undiscovered species of organisms living in and around reefs (Reaka-Kudla, 1997). This biodiversity is considered key to finding new medicines for the 21st century. Many drugs are now being developed from coral reef animals and plants as possible cures for cancer, arthritis, human bacterial infections, viruses, and other diseases.
Storehouses of immense biological wealth, reefs also provide economic and environmental services to millions of people. Coral reefs may provide goods and services worth $375 billion each year. This is an amazing figure for an environment that covers less than 1 percent of the Earth’s surface (Costanza et al., 1997).
In the 1890s, harvesting sponges was second only to cigar-making in economic importance in the Florida Keys. Nets of recently harvested marine sponges are drying on the top of the boat's wheelhouse. Click the image for a larger vew. (photo: Scott Larosa)
Healthy reefs contribute to local economies through tourism. Diving tours, fishing trips, hotels, restaurants, and other businesses based near reef systems provide millions of jobs and contribute billions of dollars all over the world. Recent studies show that millions of people visit coral reefs in the Florida Keys every year. These reefs alone are estimated to have an asset value of $7.6 billion (Johns et al., 2001).
The commercial value of U.S. fisheries from coral reefs is over $100 million (NMFS/NOAA, 2001). In addition, the annual value of reef-dependent recreational fisheries probably exceeds $100 million per year. In developing countries, coral reefs contribute about one-quarter of the total fish catch, providing critical food resources for tens of millions of people (Jameson et al., 1995).
Coral reefs buffer adjacent shorelines from wave action and prevent erosion, property damage and loss of life. Reefs also protect the highly productive wetlands along the coast, as well as ports and harbors and the economies they support. Globally, half a billion people are estimated to live within 100 kilometers of a coral reef and benefit from its production and protection.
Natural Threats to Coral Reefs
Coral reefs face numerous threats. Weather-related damage to reefs occurs frequently. Large and powerful waves from hurricanes and cyclones can break apart or flatten large coral heads, scattering their fragments (Barnes & Hughes, 1999; Jones & Endean, 1976). A single storm seldom kills off an entire colony, but slow-growing corals may be overgrown by algae before they can recover (UVI, 2001).
Reefs also are threatened by tidal emersions. Long periods of exceptionally low tides leave shallow water coral heads exposed, damaging reefs. The amount of damage depends on the time of day and the weather conditions. Corals exposed during daylight hours are subjected to the most ultraviolet radiation, which can overheat and dry out the coral's tissues. Corals may become so physiologically stressed that they begin to expel their symbiotic zooxanthellae, which leads to bleaching, and in many cases, death (Barnes & Huges, 1999).
In addition to severe weather, corals are vulnerable to attacks by predators. Large sea stars like this crown-of-thorns (Acanthaster planci) slowly crawl over coral reefs consuming all of the living coral tissue they come into contact with. Click the image for a larger view.
Increased sea surface temperatures, decreased sea level and increased salinity from altered rainfall can all result from weather patterns such as El Niño. Together these conditions can have devastating effects on a coral’s physiology (Forrester, 1997.) During the 1997-1998 El Niño season, extensive and severe coral reef bleaching occurred in the Indo-Pacific and Caribbean. Approximately 70 to 80 percent of all shallow-water corals on many Indo-Pacific reefs were killed. (NMFS Office of Protected Resources, 2001).
In addition to weather, corals are vulnerable to predation. Fish, marine worms, barnacles, crabs, snails and sea stars all prey on the soft inner tissues of coral polyps (Jones & Endean, 1976). In extreme cases, entire reefs can be devastated by this kind of predation. In 1978 and 1979, a massive outbreak of crown-of-thorns starfish (Acanthaster planci) attacked the reef at the Fagatele Bay National Marine Sanctuary in American Samoa. Approximately 90 percent of the corals were destroyed.
Coral reefs may recover from periodic traumas caused by weather or other natural occurrences. If, however, corals are subjected to numerous and sustained stresses including those imposed by people, the strain may be too much for them to endure, and they will perish.
Coral diseases generally occur in response to biological stresses, such as bacteria, fungi and viruses, and nonbiological stresses, such as increased sea surface temperatures, ultraviolet radiation and pollutants. One type of stress may exacerbate the other (NMFS, 2001).
The frequency of coral diseases has increased significantly over the last 10 years, causing widespread mortality among reef-building corals. Many scientists believe the increase is related to deteriorating water quality associated with human-made pollutants and increased sea surface temperatures. These factors may allow for the proliferation and colonization of microbes. However, exact causes for coral diseases remain elusive. The onset of most diseases likely is a response to multiple factors (NMFS, 2001).
Yellow-band disease can rapidly spread over a coral, destroying the delicate underlying tissues. On the left is a massive coral in the early stages of attack by yellow band disease. On the right is the same coral several weeks later. Note how rapidly the area of destroyed tissue has expanded. Click the image for a larger view. (Photo: Andy Bruckner, NOAA)
While the pathologies, or mechanisms by which many diseases act upon the coral polyp are not well known, the effects that these diseases have on corals has been well documented. Black-band disease, discolored spots, red-band disease, and yellow-blotch/band disease appear as discolored bands, spots or lesions on the surface of the coral. Over time, these progress across or expand over the coral’s surface consuming the living tissue and leaving the stark white coral skeleton in their wake. Other diseases, such as rapid wasting, white-band, white-plague and white-pox, often cause large patches of living coral tissue to slough off, exposing the skeleton beneath. Once exposed, the coral’s limestone skeleton can be a fertile breeding ground for algae and encrusting invertebrates. The colonization and overgrowth of the exposed coral skeleton by foreign organisms often results in the health of the entire colony taking a downward spiral from which it seldom recovers.
Using color enhanced images of sea surface temperature scientists can observe how environmental changes on a global scale can affect coral reefs in specific regions. Click the image for an animation of sea surface temperature change over time.
Protecting Coral Reefs
Coral reefs are some of the most biologically rich and economically valuable ecosystems on Earth. They provide food, jobs, income, and protection to billions of people worldwide. However, coral reefs and the magnificent creatures that call them home are in danger of disappearing if actions are not taken to protect them. They are threatened by an increasing range of impacts including pollution, invasive species, diseases, bleaching, and global climate change. The rapid decline and loss of these valuable, ancient, and complex ecosystems have significant social, economic, and environmental consequences in the United States and around the world.
In 1998, the President of the United States established the Coral Reef Task Force (CRTF) to protect and conserve coral reefs. The CRTF is responsible for mapping and monitoring U.S. coral reefs; researching the causes of coral reef degradation including pollution and over fishing and finding solutions to these problems; and promoting conservation and the sustainable use of coral reefs. As a principle member of the CRTF, and as directed by the Coral Reef Conservation Act of 2000, NOAA has the responsibility to conserve coral reef ecosystems.
NOAA’s coral reef conservation efforts are carried out primarily through its Coral Reef Conservation Program (CRCP). Under this program, NOAA works with scientific, private, government, and nongovernmental organizations to achieve the goals of the CRTF.
Using high-resolution satellite imagery and Global Positioning Satellite (GPS) technology, NOAA has made detailed digital maps of reefs in Puerto Rico, the U.S. Virgin Islands, the eight main Hawaiian Islands and the Northwestern Hawaiian Islands. Satellite technology is also used to detect harmful algal blooms that can smother reefs and to monitor elevated sea surface temperatures, which can cause coral bleaching.
NOAA also monitors reefs using the Coral Reef Early Warning System (CREWS). This system consists of buoys deployed at reef sites that measure air temperature, wind speed and direction, barometric pressure, sea temperature, salinity, and tide levels. Every hour, these data are transmitted to scientists to help them understand conditions that may cause bleaching of coral reefs. In addition to the monitoring work conducted by satellites and buoys, NOAA conducts research, assessment, and restoration projects of coral reefs in marine reserves and among deep-sea coral banks. NOAA is also working to remove tons of marine debris from the Northwestern Hawaiian Islands and restore damaged reefs.
Monitoring, research, and restoration all are essential to safeguard coral reefs. However, to ultimately protect coral reefs, legal mechanisms may be necessary. One legal mechanism involves the establishment of marine protected areas (MPAs). Because MPAs have the added force of law behind them, a protected marine enclosure—such as a coral reef system—may stand a better chance for survival.