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Coral reefs are created by animals and plants/ In warm tropical waters, with just the right combination of bottom depth, wave actions and nutrients, specialized algae and corals build reefs from their own calcium carbonate skeletons. Coral reefs are most abundant in tropical waters of the Pacific and Indian Oceans, the Caribbean, and the Gulf of Mexico as far north as southern Florida, where the maximum water temperatures range between 22°C and 28°C.
Reef-building corals are involved in a mutualistic relationship with unicellular algae called dinoflagellates, which live embedded in the coral tissue. These corals grow best within the photic zone at depths of less than 40 meters, where light can penetrate and allow their algal partners to photosynthesize.The algae benefit from the high nitrogen, phosphorus, and carbon dioxide levels in the coral tissues. In return, algae provide food for the coral and help produce calcium carbonate, which forms the coral skeleton. Coral reefs provide an anchoring place for many other algae, a home for bottom-dwelling animals, and shelter and food or the most diverse collection of invertebrates and fish n the oceans. The Great Barrier Reef in Australia is home to more than 200 species of coral alone, and a single reef may harbor 3000 species of fish, invertebrates, and algae.
Coral reefs are extremely sensitive to certain types of disturbance, especially silt caused by soil ?roding from nearby land. As silt clouds the water, light is diminished and photosynthesis reduced, hampering growth of the corals. The reef may eventually become buried in mud, the corals smothered, and the entire marvelous community of diverse organisms destroyed. Erosion from construction, roadways, and poor land management has produced enough silt to ruin several reefs near Honolulu, Hawaii. The reef inhabitants have been replaced by large numbers of sediment-feeding invertebrates such as sea cucumbers. In the Philippines. logging has dramatically increased erosion, so coral reefs as well as rain forests) are being destroyed. Another hazard is sewage and runoff from agriculture, which fertilizes near-shore ocean water, causing eutrophication and a dense growth of algae. The algae block sunlight from the coral's dinoflagellates and thereby deprive the corals of nutrients. Decaying algal bodies also deplete the water of oxygen, killing the coral.
Still another threat to the reef communities is overfishing. In at least 80 countries, a variety of species, including mollusks, turtles, fish, crustaceans, and even corals, are being harvested from reefs faster than they in replace themselves. Many of these species are sold to shell collectors and aquarium owners in developed countries. In some tropical countries, dynamite is used to kill coral reef fish, destroying entire sections of the coral reef community in the process. Tropical fish collectors often use poison to stun the fish before collecting them, having most dead. The removal of predators from reefs may disrupt the ecological balance of the community, allowing an explosion in populations of coral-eating sea urchins. For example, such sea urchins are destroying reefs along the Kenyan coast in Africa.
As with rainforests, both protection and sustainable use are crucial to the survival of these fragile and diverse ecosystems. Carefully regulated harvesting and tourism produce far more economic benefits than do activities that destroy the reefs. Some countries have recognized this, and there are now many areas where reefs are protected.