q Deforestation is also a threat to coral reefs because without the tree roots to absorb water and hold the soil together, heavy rainfall washes excess soil into rivers and out into the sea. The silt and consequently reduction of light, chokes coral polyps. Algae feed on the silt and so reproduction of algae gets out of balance with the natural checks placed on it by the reef.
q Ocean pollution such as oil slicks, pesticides and other chemicals, heavy metals and rubbish can also cause great damage to the reefs. Traditionally, spilled oil is treated with detergent to cause it to break up, but often the detergents can do more harm than good.
q Coastal development and dredging can destroy reefs, i.e. Building hotels, houses and harbours.
q Fishing with dynamite, cyanide or bleach has killed refs in the Indo-Pacific region. Between 1986 and 1991, half the coral reefs in the Philippines were destroyed using these fishing methods.
q Fishermen over harvesting the shrimps and lobsters can damage the reef because of careless handling of nets, lines, and lobster traps and because they are disturbing the reefs ecosystem.
q International aquarium and seashell traders have put a strain on the coral reef and the reef inhabitants. Excessive farming on the reef decimates reef species and upsets the balance of the reef ecosystem. In 1990, the world consumption of corals for the souvenir trade was estimated to be around 2,200 tons a year. Coral skeletons are used to make jewellery and decorations in aquariums, and also living, natural biological filters in saltwater aquariums.
q The rise in popularity of diving and a larger number of tourists taking long-haul holidays means that some of the most well known reefs are being damaged. Too many divers, the anchors of dive and snorkel boats and the litter and sewage that inevitably accompany them, all contribute to the loss of diversity on the reefs.
q Changes in sea level are hazardous to the reef because a decrease in sea level exposes the corals, and a rise in the sea level decreases the amount of sunlight available for the photosynthetic algae. Rises in sea level can also release nutrients trapped in the soil.
q Disease can also wipe out huge amounts of coral. This could be connected to a rise in the sea level and/or nutrients.
q Coral bleaching (explained in more detail below.)
q Major tropical storms can strip corals from miles of reef habitat.
Coral bleaching is the whitening of coral colonies due to the loss of symbiotic algae from the tissues of polyps. This loss exposes the white calcium carbonate skeletons of the coral colony. The exact mechanism by which corals bleach is unknown though two hypotheses have been suggested to answer this. One mechanism is that the algae are released into the gut of the polyp and are then expelled from the polyp through the mouth. Another mechanism is that stressed corals give fewer nutrients to the algae and thus the algae leave the polyp. Algae may produce oxide toxicity under stress, and these toxins may affect the polyps. Coral bleaching is caused by many stresses or environmental changes, such as disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures. In recent years, large scale bleaching has been reported on the reefs around the Maldives and on a smaller scale in the Carribean and on the Great Barrier Reef.