Threats to coral reefs

• Land development and pollution
• Dynamite fishing
• Bleaching
• Ocean acidification
• Overfishing
• Pollution
• Storms

Land development and pollution

Extensive and poorly managed land development can threaten the survival of coral reefs. Within the last 20 years,once prolific mangrove forests,which absorb massive amounts of nutrients and sediment from runoff caused by farming and construction of roads, building, ports, channels, and harbors, are being destroyed. Nutrient-rich water causes fleshy algae and phytoplankton to thrive in coastal areas in suffocating amounts known as algal blooms. Both the loss of wetlands and mangrove habitats are considered to be significant factors affecting warer quality on inshore reefs.

Dynamite fishing

Many fishing practices harm the reef by physically damaging the reef or by killing non-targeted reef fish or other reef organisms. Blast fishing, a method of fishing in pars of the Caribbean, East Africa, and Southeast Asia, uses underwater explosions to damage the swim bladders of fish so that they float to the surface where they are easily captured. The blast of the explosions destroys coral and flattens the reef structure. In some places fishermen use cyanide to stun fish so that they can be captured alive. Small fish and coral polyps may be killed by the cyanide. A style of fishing called muro-ami involves scaring fish into a net by pounding on the reef with sticks or other heavy objects that severely damage the coral reef.

Bleaching

During the 1998 and 2004 El Nino weather phenomena, in which sea surface temperatures rose well above normal, many tropical coral reefs were bleached or killed. Some recovery has been noted in more remote locations, but global warming could negate some of this recovery in the future. High seas surface temperature (SSTs0) coupled with high irrandiance (light intensity), triggers the loss of zooxanthellae, a symbiotic algae, and its dinoflagellate pigmentation in corals causing coral bleaching.

Ocean acidification

The decreasing ocean surface pH is of increasing long-term concern for coral reefs. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans. Carbon dioxide gas dissolved in the ocean reacts with water to form carbonic acid, resulting in ocean acidification.

Overfishing
In 1950 no fisheries were reported as being overexploited whereas by 1996 35% of fisheries were reporting overexploitation and an additional 25% were reported as being near overexploited. Overfishing can have a variety of negative effects on the environment. Overfishing can reduce genetic variation in a population making it harder for species to adapt to environmental change and mate. In addition, overfishing can alter tropic interactions and cause unexpected indirect effects on the environment. For example, in areas where predators have been removed increases in population size of their prey have resulted in unintended effect on the environment.

Pollution
Coral reefs can be damaged by a variety of pollutants that are produced by a variety of sources. Agricultural runoff can contain herbicides, pesticides, and nutrient fertilizers. Nirtogen and phosphorus addition can fertilize algae and result in algal blooms. Because algae can potentially grow so much faster than coral, they can out-compete corals.
Human sewage, often untreated, can add nutrients, microorganisms, and other pollutants to coral reefs. Nutrients in sewage can cause eutrophication. Bacteria added by sewage pollution are suspected causes of increased incidences of coral diseases such as white band disease.
Chemical pollution can also harm coral reefs. For example, oil spills, the result of spills from drilling or discharge of oil from vessels can harm reefs. They can be especially harmful if they occur during coral spawning because the oil can kill eggs and sperm.
Solid pollution such as plastics and discarded fishing nets (ghost nets) can also damage reefs.

Storms
Coral reefs can be damaged by natural events such as hurricanes, cyclones, and tsunamis. Wave activity can break apart corals; branching corals are more susceptible to storm damage than mound-building corals. Storms rarely kill all corals, and because storms are a natural part of coral reef ecosystems, coral species should be adapted to this type of disturbance and be able to recover following storms. However, human alteration to the environment may make recover more difficult. For example, the addition of nutrients or removal of herbivores might favor the growth of algae that is able to overgrow coral before they are able to recover.