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Coastal and Marine Geology

  U.S. Geological Survey Fact Sheet 025-02

**U.S.   Coral Reefs—Imperiled National Treasures**

 

| **Abstract**

Coral reefs         are home to 25% of all marine species. However, the tiny colonial animals         that build these intricate limestone masses are dying at alarming rates.         If this trend continues, in 20 years the living corals on many of the         world’s reefs will be dead and the ecosystems that depend on them         severely damaged. As part of the effort to protect our Nation’s extensive         reefs, U.S. Geological Survey (USGS) scientists are working to better         understand the processes that affect the health of these ecologically         and economically important ecosystems. |
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| The beauty       and abundance of life in coral reef habitats, like this one in Florida,       attract a variety of visitors, including sport divers and recreational fishermen.       Such activities in U.S. waters alone generate billions of dollars a year       for the tourist industry. (Copyrighted photograph courtesy of Sandra Edwards.) |

In recent decades, many   of the world’s coral reefs have been severely damaged by both natural processes   and human activities. The reasons for the catastrophic decline in the health   of these complex marine ecosystems are poorly understood. Coral reefs are home   to 25% of all marine species, and it is crucial that they be protected. Coral   reef habitats contribute hundreds of millions of dollars annually to the United   States fishing industry and help generate billions of dollars a year for the   Nation’s tourist industry.

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| A large school       of jacks swims in Poka‘i Bay, O‘ahu, Hawaii. Coral reefs       in Hawaii and elsewhere provide      shelter and nursery grounds for many commercially and culturally important       species of fish and invertebrates. Such reef habitats contribute hundreds       of millions of dollars a year to the United States fishing industry. (Photograph       courtesy of      National Oceanic and Atmospheric      Administration/James P. McVey.) |

To preserve and protect   the biodiversity,  health, and social and economic  value of coral reef ecosystems in the  United States and its territories, the U.S.  Coral Reef Task Force was established by  Executive Order in 1998. The task force  is a cooperative effort among Federal  agencies, state agencies, and academic  institutions and is co-chaired by the U.S.  Secretaries of Interior and Commerce.  

There are extensive coral   reefs in the waters of the United States and its territories, covering more   than 4 million acres of the sea fl oor in the Atlantic Ocean, Gulf of Mexico,   Caribbean Sea, and the Pacifi c Ocean. More than 60% of the Nation’s coral   reefs are found in the extended Hawaiian Island chain. Most of these are included   in the recently designated Northwestern Hawaiian Islands Coral Reef Reserve,   the largest U.S. nature preserve.

 

| U.S.       Coral Reefs |
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|                         | Extensive       coral reefs are found in the waters of the United States and its territories.       In the Atlantic Ocean, Gulf of Mexico, and the Caribbean Sea these include       reefs off Florida, Texas, Puerto Rico, and the U.S. Virgin Islands. In the       Pacific Ocean, they include those of the Hawaiian Islands, Wake Island,       Johnston Atoll, the Northern Marianas, Saipan, Guam, Kingman Reef and Palmyra       Atoll, Howland Island, Baker Island, Jarvis Island, and American Samoa.       More than 60% of the Nation’s coral reefs are found in the extended       Hawaiian Island chain. |
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**What is   a reef?**

The word “reef”   has different meanings to different people. To a mariner, the term “reef”   applies to shallowly submerged navigational hazards. To a surfer, a reef is   an undersea obstruction that can make waves (and surfboards) break. Geologists   and biologists generally restrict the defi nition of a reef to rigid biological   constructions that have formed in shallow water. The Earth’s fi rst reefs   were built by photosynthesizing cyanobacteria about 2.5 billion years ago. From   fossil remains, it is known that a variety of organisms have constructed reefs,   including bivalves (clams and oysters), bryozoans (coral-like animals), and   sponges. Corals have been found in fossil reefs as old as 500 million years,   but corals similar to modern colonial varieties have constructed reefs only   during the past 60 million years. 

|  | Corals are       marine animals related to jellyfish and anemones. Coral polyps, the individual       animals that make up both colonial and solitary corals, catch plankton (microscopic       plants and animals) and other suspended food particles with arm-like tentacles,       which feed a centrally located mouth. Coral reefs are formed by colonies       of “hard” corals that secrete calcareous exoskeletons, giving       them structural rigidity. (Photograph courtesy of Florida Keys National       Marine Sanctuary.)                                                                                                                                                                                                                    |
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|  | To       scientists the word “reef” refers to rigid biological constructions       that have formed in shallow water. The Earth’s first reef-building       organisms lived about 2.5 billion years ago and were photosynthesizing cyanobacteria       that built stromatolites like these modern ones in Shark Bay, Australia       (top, courtesy of the Australian Heritage Commission). Much of North America       was once covered by tropical seas that were home to many species of coral.       Fossil corals, such as this one from Tennessee (bottom), have been found       in ancient reefs as much as 500 million years old, but modern colonial varieties       of coral have constructed reefs only during the past 60 million years. |
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**What is   a coral reef?**

Corals are marine animals   related to jellyfish and anemones. Both colonial and solitary corals catch plankton   (microscopic plants and animals) and other suspended food particles with arm-like   tentacles, which feed a centrally located mouth. Most hard corals also host   symbiotic algae, a long-standing and successful partnership. These algae provide   them with an additional food source through photosynthesis. Coral reefs are   formed by corals that secrete hard calcareous (aragonite) exoskeletons, giving   them structural rigidity. These colonial “hard corals” form elaborate   fi nger-shaped, branching, or moundshaped structures and can create masses of   limestone that stretch for tens or even hundreds of miles. 

Although corals have a wide   distribution in the world’s oceans, the varieties that form reefs are typically   restricted to relatively shallow, warm tropical waters between latitudes 30°   north and 30° south. Clean, clear water is essential to their health. Once   coral larvae settle on a hard substrate and become established, colonies can   arise if conditions are suitable for growth. Given enough time, coral colonies   become thickets. As coral thickets build upward on the skeletal remains of older   colonies, a reef is established. Today, richly diverse coral reefs are found   in the tropics along coastlines, on the margins of volcanic islands, and as   isolated coral atolls.

**Why are   coral reefs in peril?**

Coral reefs can be damaged   by natural processes, such as storms, but they are increasingly at risk from   human activities. Coral reef health can be seriously affected and the growth   of new colonies hampered by fi ne-grained sediment transported from land into   coastal waters as runoff. This sediment runoff can be greatly increased by nearby   land developments for agriculture, industry, and housing. Light necessary for   photosynthesis by symbiotic algae living in corals is diminished in waters made   cloudy or turbid by suspended sediment, reducing coral productivity. 

| **CORAL         REEFS DEVELOP IN CLEAR WARM SEAS . . .**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
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| Colonial       “hard corals” form elaborate finger-shaped, branching, or mound-shaped       structures and can create masses of limestone that stretch for tens or even       hundreds of miles. Many coral reefs fringing coasts consist of nearshore       inner reef flats that slope to deeper water fore reefs farther offshore.       The reef crest, between the inner reef flat and outer fore reef, lies in       extremely shallow water and may be exposed during the lowest tides. Waves       commonly crash against or break on the reef crest. Fringing reefs help to       protect harbors, beaches, and shorelines from erosion and wave damage by       storms. | **Atoll** |
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| Coral       atolls develop from reefs fringing volcanic islands (diagrams at right).       As first hypothesized by Charles Darwin, and confi rmed by ocean drilling       done by British scientists a century ago, reefs fringing volcanic islands       build vertically to sea level, forming steepwalled barrier reefs. As a volcanic       island subsides, or sinks, with time, the growing reef keeps pace with the       rising water level. When the island eventually submerges, the ring-shaped       reef forms an atoll with a central lagoon. The NASA Landsat 7 satellite       image at above shows Pacific islands with fringing reefs and an atoll.                       |  |

 

Corals face serious risks   from various diseases, including black-band, white-band, and yellow-band diseases   that have been reported from many localities worldwide. Black-band disease is   primarily caused by cyanobacteria, but the causes of white-band disease and   yellow-band disease are unknown. When corals are stressed, they often expel   the algal symbionts that are critical to their health in a process commonly   known as coral bleaching. One known cause of coral bleaching is increases in   ocean temperatures. Regional increases in sea-surface temperatures occur during   El Niño events, and ocean temperatures worldwide may be changing as a   result of global warming. 

The susceptibility of corals   to disease may be on the rise as a result of human activities. Many human activities   are known to directly and indirectly harm coral reefs. Oil spills and pollutants   can threaten entire reefs. Excessive nutrients from land sources, such as sewage   outfall and agricultural fertilizers, promote the growth of algae that can smother   corals. Such algae also thrive when fish that graze on them are overharvested.   Other organisms harmful to corals, such as crown-of-thorns starfish, multiply   when the species that prey on them are removed. The collection of live corals   and other reef organisms can directly degrade large areas of reef.

| **U.S.         GEOLOGICAL SURVEY SEA-FLOOR MAPPING HELPS ASSESS REEF HEALTH**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
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| The       data used to create this 3-D perspective image of West Flower Garden Bank,       in the Gulf of Mexico, were collected and processed by U.S. Geological Survey       scientists using multibeam sea-floor mapping techniques. Such techniques       provide important information about a number of coral reef characteristics       that can be used to help evaluate and monitor changes in reef health. The       shallowest areas shown in the image (red) reach to within 60 feet of the       water’s surface and support a coral reef environment, home to a richly       diverse web of life (see photos, copyrighted and courtesy of David M. Read). |  |
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| **CORAL       REEFS FACE MANY SERIOUS THREATS** |
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|                                                  | Corals       face serious risks from various diseases, including black-band disease.       The susceptibility of corals to disease may be on the rise as a result of       human activities. Many human activities are also known to directly and indirectly       harm coral reefs. Excessive nutrients from land sources, such as sewage       outfall and agricultural fertilizers, promote the growth of algae that can       smother corals. Such algae also thrive when fish that graze on them are       overharvested. Other organisms harmful to corals, such as crown-of-thorns       starfish, multiply when the species that prey on them are removed. [Photographs       courtesy of Florida Keys National Marine Sanctuary/ Page Gill (black-band       disease); National Oceanic and Atmospheric Administration (soil runoff,       Hawaii); and Fagatele Bay National Marine Sanctuary, American Samoa/Kip       Evans (crown-of-thorns starfish).] |
| **Black-band       disease, Florida**           |
| **Soil runoff,         Maui, Hawaii**           |
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|                                             | **Crown-of-thorns       starfish, American Samoa** |

**What is   being done to protect coral reefs?**

Scientists worldwide are   working to understand the impacts of natural processes and human activities   on the health and sustainability of coral reefs. In the United States, this   effort is being coordinated by the U.S. Coral Reef Task Force. As part of the   task force, the U.S. Geological Survey (USGS) is contributing to the effort   to understand the biological and geological controls that affect our Nation’s   coral reefs. USGS coral reef research focuses on detailed mapping of reefs,   the development of monitoring techniques, and studying reefs’ geologic   growth and development and how they are affected by sedimentary and hydrologic   processes, water quality, and fi shing. These efforts will help provide information   that is essential if coral reefs are to be saved.

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*Michael   E. Field, Susan A. Cochran, and Kevin R. Evans*

*Edited   By James W. Hendley II and Peter Stauffer*

*Graphic Design By Susan Mayfield and Sara Boore; Web layout by Carolyn Donlin*

For more   information contact:  U.S. Geological Survey  Pacifi c Science Center  1156 High Street  Santa Cruz, CA 95064  (831) 427-4745

http://coralreefs.wr.usgs.gov

  See also New Mapping   Techniques Help Assess the Health of Hawaii’s Coral Reefs (USGS   Fact Sheet 084-01).

Download this 4-page   fact sheet as a PDF file (1.2 MB)

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U.S. Department of the Interior | U.S. Geological Survey	URL: https://pubs.usgs.gov/fs/2002/fs025-02	Page Contact Information: Michael Diggles	Page Created: April 5, 2002	Page Last Modified: October 30, 2008 (mfd)

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