Threats to Coral Reefs
While coral reefs have survived and thrived for millennia, they are also very fragile ecosystems that can be devastated by natural events and human interaction. Tropical reefs grow in shallow waters near the coast. Their location exposes them to a number of threats including sedimentation, wave action, temperature changes, fishing, pollution and tourist activities.
Because corals live in shallow water they are susceptible to damage by wave action and changes in the water temperature. Storms produce strong waves that can break the corals, especially branching corals. Corals thrive in a very narrow temperature range. When the water temperature rises or falls outside of the corals preferred temperature, it may become susceptible to disease or it may expel the zoozanthelae (symbiotic algae) that live within the coral polyp. Water temperature changes may be brought about by rising or falling water levels, which either cause the coral to live in deeper water which is colder than it is used to, or exposes it to warmer water or the air. Water temperature may also be affected by wind action. El Nino and La Nina events in the Pacific Ocean regularly change the water temperature and threaten to harm the corals along the coast of Central and South America and Australia.
Corals evolved in water that has very few nutrients or sediment in the water column. The water is very clear, and the zoozanthelae that live in the corals use the sunlight that penetrates the clear water to produce nutrients for the coral. When the water column hold a higher than normal sediment load the sunlight cannot penetrate the water column for the algae and the corals may suffer from lack of nutrients. The sediment may also settle on the coral, rubbing against the delicate polyps and damaging them.
Sedimentation may also increase as a result of changes on land or at the interface of land and sea. When people cut down mangrove forests that live at the edge of the land, the mangroves are no longer able to trap sediment from the land. Ports and channels may need to be dredged to allow ships access to ports. Dredging disturbs the sediment which clouds the water. The sediment stays in the water and moves out to the reef where it blocks sunlight and settles on the corals.
In addition to trapping sediment, mangroves provide habitat for many juvenile reef species. When the mangroves disappear, the species that depend on the mangrove for habitat for some part of the life cycle are harmed. As a result, those species numbers change on the reef as well. Loss of species disturbs the balance in the ecosystem which can threaten the corals.
Development near the coast can also lead to pollution in the water. When the nutrient levels rise due to pollution algae blooms may occur. The algae may physically grow on the coral, blocking sunlight and trapping sediment on the coral. They may also cloud the water column, reducing the amount of light available to the zoozanthelae. The zoozanthelae may either die or be expelled by the coral, in either case the coral is damaged by lack of food.
People may physically harm the reef. Divers often touch corals, either with their bodies or pieces of equipment that trail in the water. Touching the coral damages the polyp and may kill the coral. Damage to the polys exposes the coral to disease. Dive boats may drag anchors that tear into the reef, physically tearing it apart. Suppliers in the aquarium trade may blast the reef with dynamite to stun the fish for easy collection. Not only does this stun the fish, but it damages the reef and raises sediment into the water. Collecting coral for tanks also damages the reef by removing specimens from the reef. When the collection is not done responsibly , either taking only parts of a specimen or growing specimens specifically for the aquarium trade, the reef looses the ability to grow.