Coral and Coral Reef

Major controls: crustal subsidence and sea level change

Coral reefs are highly diverse ecosystems, supporting greater numbers of fish species and other organisms than any other marine ecosystem. Coral reefs are located in warm, shallow, tropical marine waters with enough light to stimulate the growth of the reef organisms. The primary reef-building organisms are invertebrate animals known as corals; corals secrete the bulk of the calcium carbonate (limestone) that makes up the inorganic reef structure, along with material deposited by coralline algae, mollusks, and sponges.

Corals are small (0.06–0.5 in; 1.5–12 mm), colonial, marine invertebrates. They belong to the class Anthozoa, phylum Cnidaria (or Coelenterata). Corals are subdivided into stony corals, which have six tentacles; and soft corals, sea fans, and sea whips, which have eight tentacles.

The limestone substrate of stony coral colonies develops because each individual animal, or polyp, secretes a hard, cuplike skeleton of calcium carbonate (limestone) around itself as a protection against predators and storm waves. These limestone skeletons, or corallites, make up the majority of the reef framework. Certain coral species produce distinctively shaped colonies, while others exhibit various shapes. Some species, such as staghorn coral, are intricately branched, and are sometimes called coral stands. Brain corals are almost spherical in outline and are often called coral heads; they often display surface convolutions reminiscent of those on a human brain.

Calcareous red, or coralline, algae also contribute to the framework of reefs by secreting their own outer skeleton that acts as cement, stabilizing loose sediment on the reef. Coralline algae often produce as much of a reef’s limestone as do the stony corals. Other calcareous organisms that contribute reef sediments include sponges, bryozoans (another colonial animal), tube worms, clams, and snails.

Adult corals are bottom-dwelling, attached animals usually found in single-species colonies. These colonies may house hundreds or thousands of polyps. The polyps are joined to one another by a thin tissue layer called the coenosarc (pronounced SEE-na-sark). The coenosarc connects the entire coral colony and covers the underlying coral skeleton. Reproduction through an asexual budding process results in development of duplicate daughter polyps and allows for growth of the colony. A single polyp can develop into a massive coral head through multiple budding episodes. Corals also reproduce sexually, producing multitudes of planktonic larvae that ocean currents disperse widely. This allows colonization of suitable habitats, resulting in development of new colonies and new reefs.

Single-celled dinoflagellate algae known as zooxanthellae live symbiotically within coral polyps. Chemical exchanges occur between the coral polyps and zooxanthellae, and both thrive in a mutually beneficial relationship (mutualism). The zooxanthellae, which are essentially tiny green plants that can produce food from sunlight, water, and dissolved minerals, supply some coral species with more than 90% of their nutrition on sunny days. In exchange for nutrients, the coral polyps supply a habitat and essential minerals to the algae. Another result of this relationship is more rapid development of coral reefs. During photosynthesis, the zooxanthellae remove carbon dioxide from the water, which promotes calcium carbonate production, in turn allowing the coral to more easily secrete its home.

In addition to the food provided by their zooxanthellae, corals prey on tiny planktonic organisms. Some corals paralyze their prey using stinging cells, or nematocysts, located on their tentacles. Other corals feed by creating weak water currents with cilia to draw food into their mouth, or by producing sticky mucus with which to trap tiny planktonic animals. Most species feed at night; during the day, they retract into their corallites for protection. The members of the colony share nutrients by passing them to their neighbors through the coenosarc.

While estimates of the total ocean floor area covered by coral reefs vary considerably because of difficulties in estimation due to their submarine location, a conservative estimate would be 235,000 sq mi (597,000 sq km)—only 0.1% of Earth’s surface—for reef areas at depths less than 100 ft (30 m). Coral reefs occur in shallow, warm-water locations, primarily below 30° latitude in the western Atlantic and Indo-Pacific regions. Their distribution is strongly influenced by the environmental preferences of the coral animals. Corals, or rather their symbiotic zooxanthellae, depend on light for growth. The algae need access to light to accomplish their photosynthesis. Too much sediment in the water also causes problems, by limiting light penetration or suffocating organisms, and thereby slowing reef growth. Consequently, the amount of light and the clarity and depth of the water are important influences on the development of coral reefs.

Corals thrive in oligotrophic water; that is, water with low concentrations of nutrients such as phosphate, ammonium, and nitrate. Currents and wave activity help supply the continuous but low concentrations of nutrients that corals and algae require for survival, while also removing waste materials.

Water temperature is also an important environmental influence on the growth of stony corals. Typically, a water temperature of 74–78°F (23–26°C) is ideal for coral growth, and temperatures must generally remain above 67°F (19°C) throughout the year.

Stony corals also prefer marine waters with stable salinity. The salt concentration of the water must range between 35 and 38 parts per thousand, and the concentration of oxygen must remain high. Another important factor is the need for continuous submersion under water, although some corals can survive temporary exposure during low tide.

Reefs tend to develop a definite depth profile and associated coral zonation under the influence of constant wave activity. This results from the decrease in wave energy with water depth. The reef crest is the shallowest reef area and subject to the highest wave energy; here coral and algae encrust the substrate to avoid being broken and swept away. The reef crest is located at the top of the seaward reef slope and may be exposed at low tide. Waves and tides cut channels across the reef crest. As tides rise and fall, water moves back and forth through these channels between the open sea and the lagoon.

Wave and storm energy are important controls on the character of the reef crest. Coralline algae tend to dominate reef crests if hurricanes are frequent and average daily wave conditions are rough. Grazing fish, which would normally consume the algae, are deterred by the consistent high wave energy. In areas with infrequent storms and calmer daily wave conditions, encrusting corals or robust branching corals tend to inhabit reef crests.

Moving down the seaward reef slope, the reef front lies just below the reef crest. Corals here are diverse and show the greatest range of forms. At the top of the slope, wave energy is high and coral forms are usually encrusting to massive, such as brain corals. Further down the slope, in deeper water, massive corals dominate, then give way to delicate branching corals as wave energy decreases with depth. Finally, at the base of the reef front, plate-like corals take advantage of the low wave energy. By orienting their flat, upper surface toward the sun, they attain maximum exposure to the diminished light of the deep reef. Further downslope, the fore reef consists of limestone boulders, coral branches and smaller sediments, all transported from above, as well as sponges, soft corals and algae thriving in place.

Shoreward of the reef crest lies the shallow reef flat. Reef rubble occurs here in high-energy reefs. In lower energy settings, carbonate sand may be present. These sediments are supplied by storm waves breaking on the reef crest. Even closer to shore is the back reef area, where fine-grained sediment inhibits reef growth; however, scattered stubby, branching or low, knobby corals usually develop in water depths of 3–4 ft (1–1.3 m).

Beyond the back reef, the water begins to deepen again—to as much as 100 ft (30 m) or more—within the lagoon (i.e., water between the reef and the shore, or completely surrounded by the reef). Here the sea floor is generally protected by the reef from significant wave agitation, so fine-grained sediments compose the lagoon floor. Hardy corals occur in scattered clusters, known as patch reefs.

There are three major kinds of coral reefs: fringing reefs, barrier reefs, and atolls. Fringing reefs are located in shallow water close to shore, either with or without a shallow lagoon. Barrier reefs are also located in shallow water, but with a deep lagoon separating the reef from the shoreline and a steep reef front. Both fringing and barrier reefs form on shallow continental shelves and along island shorelines. Atolls are ring-shaped coral reefs centered around a deep lagoon. They are typically found in the vicinity of volcanic seamounts and islands located in the deep ocean.

Major controls: crustal subsidence and sea level change

In addition to the environmental requirements for coral growth described above, other factors play a role in coral reef character over long time intervals, that is, during geologic time spans. The two most important controls are both related to water depth—sea level change and crustal movement.

World-wide fluctuations in sea level can be caused by volume changes of fresh water in global reservoirs (lakes, groundwater, or glaciers) and by changes in the volume of ocean basins. If sea level rises while environmental conditions remain favorable for reef growth, coral reefs may grow upward rapidly enough to keep pace with rising sea level. If conditions are unfavorable, upward growth will be slow and light levels on the reef will slowly decrease as water depth increases, causing the reef to “drown.” If sea level drops, the crest of the reef may be exposed and eroded, while deeper zones will “back-step” down the reef slope as the water depth decreases.

Coral reefs occur in two distinct settings: oceanic settings and continental shelves. Deep water surrounds oceanic coral reefs, which generally lie hundreds of miles from continental shelves. These may be fringing reefs, barrier reefs or atolls. Charles Darwin, who started his scientific career as a geologist, developed a theory in the mid-nineteenth century about the origins of and relationships between fringing reefs, barrier reefs, and atolls in oceanic settings. Darwin visited Pacific atolls and also observed barrier and fringing reefs in the south Atlantic. He hypothesized that the first stage of development of an atoll is the creation of a fringing reef around a volcanic island. The volcano subsides under its own weight over millions of years, but the reef’s upward growth keeps pace with subsidence and so it remains in shallow water, developing into a barrier reef. If a volcanic island becomes completely submerged, the coral reef itself may be the only thing at sea level, forming an atoll.

Darwin was essentially correct, since the primary factor determining what types of reefs are present in oceanic settings is usually an island’s rate of subsidence. However this model is less applicable to shelf reefs, which usually experience less significant rates of subsidence. Continental shelves are typically fairly stable, so sea level changes tend to exert more control than subsidence on reef morphology (form). Shelf reefs occur on the margins of continents. A carbonate shelf is a broad, flat, shallow margin where conditions favor reef development. Most shelf reefs develop either on benches or banks or at the continental shelf’s seaward margin.

Bench reefs form on the outer edge of a submarine erosional terrace, or bench, produced by near-shore erosion during times of lower sea level. This bench provides a substrate for reef development. Generally, bench reefs form at water depths of 35 ft (10 m) or less. Bank-barrier reefs form on a shallow (less than 60 ft, 18 m) area of the shelf where, in the past, scattered corals trapped various coarse-grained skeletal material, forming a pile, or bank. Later, when water depths are suitable, corals use the bank as a substrate for reef development. Shelf-margin reefs are located at the outer edge of the shelf, beyond which water depths increase very rapidly.

When sea level rises or falls along a shelf, vast areas of land are flooded or exposed, respectively. If sea level drops on a shelf, bench or bank reefs cannot necessarily back-step—there may be no where to go, if the entire shelf becomes exposed. Shelf-margin reefs may backstep, however, sediments from erosion of the newly exposed shelf often overcome the reefs. Likewise, during rising sea level, water quality may be unfavorable to reefs due to coastal erosion of newly flooded land areas.

Coral reefs around the world have similar plants and animals. This means that the same families and genera tend to be present, although the actual species may be different. They have the highest biodiversity and greatest ecological complexity of any marine ecosystem. Many coral-reef organisms have established balanced, mutualistic relationships that help sustain a great richness of species and a tremendous complexity of ecological interactions. Coral reefs develop under environmental conditions characterized by a restricted supply of nutrients, yet maintain a high rate of productivity because of their efficient use of nutrients.

Some ecologists believe that coral reefs maintain high biodiversity as a response to a stable but periodically disturbed environment that allows for the accumulation of species over time. The most competitive species are prevented from dominating the ecosystem, while small-scale disturbances maintain a shifting mosaic of relatively young habitats for colonization by less competitive species. Natural disturbances to which coral reefs must typically adapt include intense windstorms such as hurricanes, events of sediment deposition or volcanism, unusually low tides, shortterm temperature extremes, and the population dynamics of reef species.

As an underwater environment, coral reefs offer a wide variety of habitats for plants and animals. Phytoplankton, benthic algae, and bacteria are at the base of the food web. They serve as food for the large variety of animals in the coral-reef ecosystem. For example, as many as 500 species of fish may inhabit a given coral reef. If you ever visit a coral reef, you may find that greenery seems relatively scarce; however, six inconspicuous types of plants make coral reefs the marine community with the highest primary productivity. These are the calcareous and noncalcareous green algae, red algae, and brown algae; the hidden (“cryptic”) zooxanthallae living in coral tissue and green filamentous algae living in the porous limestone substrate; and the microscopic phytoplankton.

Many of these fish species defend a territory, while others occur in large schools. Many benthic species occur in and on the reef as well. These include the corals themselves, barnacles, oysters, clams, lamp shells, and polychaete worms. All told, as many as 150,000 species may live in some coral reefs.

Coral reefs are sometimes disturbed by natural forces, such as extreme rain events that dilute seawater, waves associated with hurricane-force winds, volcanism, earthquakes, and thermal stress from unusually warm water (such as El Ninö events). These natural conditions rarely destroy entire reefs, and the ecosystem can recover over time.

The crown-of-thorns starfish (Acanthaster planci ) has caused severe damage to coral reefs in the Pacific Ocean and elsewhere. These large, bottom-dwelling invertebrates feed on the corals, destroying them in the process. This damage has been well documented on the Great Barrier Reef of Australia, almost one-quarter of which was destroyed by a crown-of-thorns infestation in the 1980s. When corals are destroyed by starfish, algae and bacteria grow over the surface and inhibit the establishment of new coral.

Another threat to coral reefs, observed in early 1980’s in the Caribbean and the Florida Keys, was a blight that decimated the population of spiny sea urchins. These invertebrates are important because they feed on benthic algae, preventing them from overgrowing the corals.

A phenomenon known as coral bleaching is caused when coral polyps expel their zooxanthellae so that the coral becomes pale or white. Without their algae corals become weak, and after several weeks may die. For some years marine scientists have been observing major bleaching events in many parts of the world; the bleaching event of 1998 was the worst ever observed, affecting most of the world’s coral reefs simultaneously. In some cases, severe damage has been caused to the coral-reef ecosystem (e.g., 80% coral die off). Scientists believe that unusually warm water temperatures are responsible for these catastrophic bleaching episodes. The cause (or causes) of these unusually warm temperatures is not certainly known, but a majority of scientists believe that global warming caused by human alteration of the atmosphere (especially increases of atmospheric carbon dioxide [CO₂ ]) is responsible. Atmospheric CO₂ — expected to increase to twice its natural level (i.e., its level just prior to the Industrial Revolution) by 2065— is also expected to harm coral reefs by changing the chemistry of seawater in such a way as to make calcium carbonate less available to reef-building organisms.

Marine biologists also suspect that other devastating infectious coral diseases are also becoming more common. These diseases have names such as “black band,” “white plague,” and “white pox.” They are capable of wiping out much of the coral in an afflicted reef.

Events such as crown-of-thorns population explosions, spiny sea urchin population collapses, and coral diseases can all be considered natural events. However, in many cases, marine scientists suspect that human influences, such as pollution, ozone depletion, or global warming may ultimately be to blame.

Corals reefs provide extremely valuable environmental services for people, including the protection of shorelines from the full onslaught of storm-driven waves. Some of these services are of direct economic benefit to people, and they could be used on a sustained-yield basis, for example, through ecotourism or a controlled fishery.

Regrettably, many human uses of coral reefs damage their physical and ecological structure. The most devastating direct damage is caused by mining of reefs to provide calcium-based material for the construction of buildings, including the manufacturing of cement.

Although fishing is a potentially sustainable activity, it is not often practiced as one. The most destructive fishing technique used in coral reef ecosystems involves the use of dynamite to stun or kill fish, which then float to the surface and are gathered. Dynamiting is extremely wasteful both of fish, many of which are not collected after they are killed, and of the coral reef ecosystem, which suffers serious physical damage from the explosions. Net fishing also physically damages reefs and depletes non-food species. Sometimes, poisons are used by divers to intoxicate fish so they can be collected by hand. This method is used both to catch fish as food for local people and for the international aquarium trade.

Coral reefs are also highly vulnerable to pollution of various kinds. Pollution by nutrients, or eutrophication, is most commonly associated with the discharge of sewage into the marine ecosystem as runoff from cities or coastal villages. Nutrients such as nitrate and phosphate that runs off from coastal agricultural land can cause phytoplankton to become highly productive and abundant, and their biomass can prevent sunlight from reaching the corals in sufficient intensity to sustain their zooxanthellae. Nutrients can also cause free-living algae on the reef surface to become highly productive, and in some cases these algae smother the corals, causing further decline of the coral-reef ecosystem.

Many species of corals and their zooxanthellae are highly sensitive to toxic contaminants, such as pesticides, metals, and various petrochemicals. Coral reefs can easily be degraded by these chemical pollutants, for example, through agricultural runoff into the near-shore environment. Sometimes, coral reefs are severely damaged by oil spills from wrecked tankers, or from smaller but more frequent discharges from coastal refineries or urban runoff. Corals and their associated species can suffer damage from exposure to toxic hydrocarbons, and from the physical effects of smothering by dense, viscous residues of oil spills.

Sedimentation is a type of pollution that occurs when large quantities of fine soil particles erode from nearby land areas and settle out of water in near-shore marine waters, smothering coral reefs. Corals are tolerant of a certain amount of sedimentation. However, they may die if the rate of material deposition is greater than the corals can cope with through their natural cleansing mechanisms: ciliary action and outward growth of the colony. Sedimentation may also cause damage if its associated turbidity significantly reduces the amount of light available for the symbiotic zooxanthellae.

Estimates are that approximately 60% of the world’s reef area may be directly threatened by human activities such as coastal development and pollution. This figure does not include the possible effects of global warming and atmospheric CO₂ increases, which threaten all of the world’s reefs.

Although coral reefs may be suffering a variety of ills, there is still hope. In the 1980s and 1990s, many countries began to realize the importance of coral reefs and to act accordingly. In response to requests from marine scientists for increased monitoring of reef condition, along with calls from environmental activists for enhanced reef conservation, several countries developed management plans for their reef areas.

In some cases, governments applied lessons learned elsewhere. For example, a small island north of Venezuela—Bonaire, in the Netherlands Antilles— established a marine park 1979. The park boundaries completely surround the island and ordinances provide some level of protection for all marine resources from the high water mark down to 190 ft (60 m). Bonaire’s reefs are now some of the healthiest in the Caribbean, even though they too have been affected at times by reduced water quality, coral disease outbreaks, declining spiny sea urchin populations, and storm damage.

Recent establishment of similar management zones with enforced limitations and controls on marine resource exploitation has resulted in significant improvement in the health of some reef systems. However,

KEY TERMS

Benthic— Dwelling in or on the ocean bottom.

Calcareous— Composed of calcite (calcium carbonate) (CaCO₃ ), a common mineral.

Mutualism— A mutually beneficial interaction (symbiosis) between two different species.

Polyp— An individual animal in a coral colony.

Sessile— Unable to move about.

Zooxanthellae— The single-celled, dinoflagellate algae that live in a mutualistic symbiosis with corals.

it remains to be seen if more restrictive measures may be necessary. In particular, it may be imperative to improve effluent water quality in areas of high population density, such as the Florida Keys.

See also Greenhouse effect; Oceanography.