Coral and Coral Reef

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 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.

Even tourism can be extremely damaging to coral reefs. Physical damage may be caused by boat anchors, or by rarer events like ship groundings. Snorkelers swimming over or walking on shallow reefs may cause significant damage, because the less robust species of corals are easily broken. Also, some visitors like to collect pieces of coral as souvenirs, a practice that can contribute to the degradation of frequently visited reefs. Local residents may also harvest corals for sale to tourists or in the aquarium trade. Many coral reefs that support heavy recreational usage by tourists are in a relatively degraded condition, compared with more remote reefs that are less intensively visited.

The U.S. State Department estimated in 1999 that 58% 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.