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Coral Reefs Thomas J. Goreau Coral reefs are often called "the rain forests of the' oceans" because they are among the richest marine ecosystems in species, productivity, biomass, structural complexity, and beauty. Like rain forests, reefs provide evolutionary lessons from intricate interactions between organisms. Both rely on structural frameworks built by a single group of living organisms: trees in forests and corals in reefs. Like rain forests, coral reefs thrive in nutrient-poor habitats by containing many species whose complex food chains recycle essential nutrients with great efficiency, making reefs especially sensitive to any process that disrupts recycling. Almost every group of marine organisms reaches its greatest species diversity in coral reefs. Over a quarter of all marine fish are found in reefs, and estimates offish productivity suggest that around 10% to 15% of the total worldwide catch comes from there. Reefs occupy only around 600,000 square kilometers, less than 0.2% of the ocean surface, making their biodiversity and productivity many times greater other marine ecosystems. Most reefs form long, narrow strips along the edge between shallow and deepwater, rather than occupying large areas, like rainforests; oceans cover more than two thirds of the Earth yet tropical rain forests cover over ten times the area of reefs. Because biodiversity and productivity of marine ecosystems are so much more concentrated, comparing areas of damaged reefs to deforested areas on land seriously understates their great vulnerability. Corals: Architects of the Reef Corals are simple, bottom-dwelling animals whose fundamental unit is the polyp, which has a common opening to take up food and excrete wastes, surrounding a ring of tentacles. Each polyp sits in its own cup in a limestone skeleton, which the coral constantly builds as it grows upwards. Polyps use weak stinging cells in their tentacles to capture small animal plankton from the water. Reef-building corals live in large colonies made by repeated divisions of genetically identical polyps. These colonies come in an astonishing variety of branching, leafy, or massive forms, which may grow continuously for thousands of years. They are related to jellyfish, sea anemones, and a variety of soft corals that lack massive limestone skeletons. The cells of reef-building corals contain symbiotic algae that release most of the organic matter they make from photosynthesis to their coral hosts. Corals consequently look much like plants and grow over each other in competition for light. They are also able to take up dissolved and particulate organic matter from seawater. This wide range of potential food sources places corals at many levels of the food chain simultaneously acting like a producer, herbivore, carnivore, and decomposer. The algae remove carbon dioxide and excreted nutrients, while supplying food and oxygen, and greatly enhancing the rate at which the corals deposit their skeleton. Virtually all coral skeletons are white, but they are hidden beneath tissues with a wide range of colors, derived from pigments in the symbiotic algae. Reef Structure and Natural Stresses Coral reefs—dynamic wave-resistant structures built by the skeletons of living organisms—absorb the energy of breaking waves that transport food and nutrients to corals and clean their surfaces of sediment and waste matter. Because the most active growth is in the wave-breaking zone, reefs form linear structures facing waves, often parallel to shorelines, protecting them from erosion. Only corals are able to build these structures. Other associated species live in nooks and crannies in the reef, in surrounding beds of sand made up of the remains of skeletons of corals and other reef organisms, and in associated protected ecosystems as sea grass beds and mangroves. Fish and turtles from adjacent ocean waters use reefs as feeding or breeding sites. Coral reefs are periodically devastated by storms or earthquakes, and take decades to recover. They are also subject to a variety of natural stresses, such as population explosions of the coral-eating crown-of-thorns starfish in the Indian and Pacific Oceans. Corals and other reef organisms are also subject to mortality from a variety of diseases whose impacts are spreading worldwide It is uncertain whether these diseases are natural events or caused by pollution. As a result of their dependence on symbiotic algae, coral reefs can grow only in conditions suitable for the algae. Coral symbiosis requires warm, bright, clean marine waters, confining reefs to shallow well-lit tropical waters, free from excessive turbidity (muddiness) and pollution. Extremes of temperature, salinity, or light can cause corals to expel their algae, losing most of their food supply and capacity for rapid growth. This phenomenon is called bleaching because the corals lose color. Thus corals and coral reefs are extremely sensitive to environmental change and habitat degradation. Corals and Geological History Reefs have been built over the last500 million years by a variety of marine organisms, including corals, sponges, algae, clams, and worms. Modern corals probably build the largest and fastest-growing reef structures in history because of the remarkable enhancement of skeleton growth caused by symbiotic algae. Many ancient reef builders are thought not to have had this advantage, but 65 million years ago, reefs were constructed by massive clams that did. After a period without reefs, modern corals and reefs spread. Over the last several million years oscillating ice age and non-ice-age conditions have caused ocean levels to swing up and down by 100 to 150 meters, forcing corals to migrate up and down in response Around 130,000 years ago global climate was one to two degrees Celsius warmer than today, conditions similar to those projected in coming decades from global warming. Fossil reefs around the world show that sea levels were then around five to eight meters higher than today's levels. On a finer scale, annual growth bands in many corals contain information about tropical climate change over the past centuries that are not obtainable by any other means. Reefs also record changes in geological processes. Areas of rising crust are marked by reef skilled by being uplifted above the water. If the crust is subsiding or sea level rising slowly enough, reefs are able to grow upwards. Corals typically grow around a centimeter a year, but much of this material is eroded by waves or by boring organisms such as sponges, clams, and worms, so the reef framework itself accumulates at a few millimeters per year. Volcanoes forming near mid ocean spreading centers gradually subside as the crust on which they stand cools and moves away. Reefs around their edges grow upward as volcanic rock sinks, leaving behind circular reef structures, or atolls. Charles Darwin’s speculation that these were remains of drowned volcanoes was confirmed nearly a century later by drilling through atoll limestone. Atolls are the majority of reefs in the central Pacific and Indian Oceans. Human Stresses to Reefs Easy access by the rapidly expanding human populations of adjacent tropical coastal lowlands is causing mounting stress to reefs. Corals can be directly damaged by boats, anchors, handling by divers, dredging, reef mining for limestone, oil spills, and leakage of toxic chemicals from land sources or passing ships. Spear guns, trawls, explosives, and poisons are often used to collect fish or reef organisms for food, aquarium specimens, jewelry, or curios. In most regions favored reef fish are severely depleted, and fish are smaller and less diverse. Changes in fish populations may remove the species that control the abundance of other reef organisms such as seaweeds) thus allowing the spread of "weedy" species. Deforestation of watersheds and coastal habitats greatly accelerates erosion, allowing soil washed into the sea to smother reefs. The reef structure ceases growth and is gradually eroded by organisms boring into and weakening it. Drainage of canals in coastal wetlands damages reefs by freshwater and sediment discharges. If sewage generated by coastal populations does not undergo tertiary treatment to remove excess nitrogen and phosphorus, these nutrients stimulate prolific growth of seaweed, which overgrows corals. Expanding areas of dead or dying reef are seen around many coastal towns, resort areas, and populated areas in the tropics. Over 100 countries have reefs, and they are the most valued marine natural resource in most. Almost all have documented serious deterioration of reefs close to population centers in the past decade. Coral reef countries obtain the major part of their fish from reefs. Their tourism is largely based on diving, fishing, and boating in the reef waters, or on the sand produced by the death of reef organisms. If reefs die, sand replenishment is cut off and beaches are destroyed by erosion. Deterioration of shore protection, best provided by healthy reefs, can cause tremendous losses along coasts during storms. The economic value of reefs is often recognized only after it is impaired Many reefs are being damaged by poorly planned coastal development that was intended to capitalize on the beauty of reef-related habitats. Starting in the 1980s, massive bleaching of coral reefs was reported in the Pacific, Caribbean, and Indian Oceans. In extreme events almost all corals in a reef may bleach white, and many die. The survivors slowly recover, taking up to 10 months to regain normal color and growth. During this period they are starving, unable to grow or reproduce. These events appear to be due to unusually high water temperatures. They follow periods when water temperatures were one degree Celsius above Long-term averages in the warmest seasons of the year. It is still controversial whether these are linked to the record global temperatures of that period or to local weather extremes. Nevertheless there are clear signs that many reefs around the world have been under thermal stress. This suggests that coral reefs may be among the most climate-sensitive ecosystems. Before the mass bleaching of the 1980s, coral reefs were under increasing stress worldwide due to local causes. If bleaching continues, it may prove impossible for reefs to adapt to rising temperatures and sea levels from future global climate change. Hurricane strength could also increase with global warming, placing further stress on many reefs. Reef recovery requires abatement of external stresses, limiting change to rates within the capacity of the reef to adapt. This includes treating sewage, reforesting watersheds, controlling over fishing, stopping destructive utilization of reef organisms and materials, and preventing global warming. Unless such actions are taken together, many reefs may succumb in coming decades. Alterative technologies will have to be developed to restore reefs and provide shore protection Serious ecological damage to reefs and economic damage to many tropical countries could result from climate change and environment mismanagement, and the lowest-lying island nations on mid-ocean atolls could vanish entirely if sea level, temperature, and storm strength rise.
This article was originally published in The Encyclopedia of the Environment, RA Eblen& WR Eblen (editors), 1994, Houghton Mifflin Co., Boston. Further Reading. T. F Goreau. N I Goreau, and T. Goreau, "Corals and Coral Reefs," Scientific American 1979; B. Salvat, "Coral Reefs, a Challenging Ecosystem for Human Societies,'' Global Environmental Change 1992; United Nations Environment Program and International Union for the Conservation of Nature, Coral Reefs of the World 1988. |
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