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Natural Concerns
Coral reefs are fragile ecosystems that are easily damaged by typhoons, cyclones and hurricanes. The effects of storms, earthquakes and tsunamis can dislodge and destroy sections of the limestone foundations of coral reefs. Shallow-living corals can be killed or injured by sudden exposure to the atmosphere caused by tectonic uplifts and extreme low tides. These subaerial exposures can upset the reefs' delicate ecological balance. Other natural threats include increased ultraviolet solar radiation and invasions of coral-eating predators such as the crown-of-thorns starfish.
Coral Bleaching
One of the greatest threats to the world's coral reefs is extensive coral bleaching. Coral bleaching occurs when the living coral polyps expel the symbiotic single-celled algae called zooxantheallae. These photosynthetic organisms live within the coral and assist them in nutrient production. The phenomenon of bleaching includes the reduction of chlorophyll in the zooxantheallae. The coral reefs then turn white as they receive their coloration from the algae. Bleaching is the coral's response to stress caused by changes of temperature, light levels or salinity in the reef's environment.
Pollution
Pollution is a major concern facing coral reefs. Runoff from coastal watersheds damage the reefs with pathogens, toxins, agricultural pesticides and fertilizers. Too much nitrogen and other nutrients cause the reefs to be overgrown with seaweeds. Sediment plumes from coastal urban over-development, deforestation and mismanaged watersheds smother the reefs and kill the living corals. Sediment particles suspended in the water inhibit the coral's growth, feeding and reproduction patterns as well as limiting the photosynthetic activity of zooxantheallae.
Other Concerns
Destructive fishing practices pose a serious threat to coral reefs. Over-fishing disrupts the delicate balance of the reef's ecosystem by removing fish species essential to the reef's survival. Bottom-trawling, dynamite and cyanide fishing are harmful to the reef's ecology. Coral mining for bricks, road-fill or cement damage the reef's infrastructure. Tourism negatively affects the reefs from careless boaters, divers and snorkelers. Resorts and cruise ships often dump their garbage and sewage into the waters directly over the reefs.