Coral reefs occur around the shores of tropical islands and continents where the waters are shallow and poor in the supply of nutrients. They are composed mainly of the calcium carbonate that results from both living and dead corals, which results in a very diverse ecosystem which only has a certain amount of tolerance to light, the amount of salt in the water, the UV radiation that penetrates the depths and the quantities of nutrients. It is little wonder that they are sometimes called the “Rainforests of the Tropical Oceans.”
During the past few decades they have come under various kinds of threats to their existence, one of which is coral bleaching. Natural events, such as extreme weather, diseases of the living vertebrates and invertebrates that inhabit the reefs, and differences in temperature (by as little as one or two degrees), threaten the reefs, but there are also threats that come from man, such as overfishing, the overloading of the waters with nutrients and increased sedimentation. Any or all of these occurrences can bring about coral bleaching, which has increased in recent years.
Coral bleaching refers to the corals losing their color and become pale or whitish. It is one of the most visible signs that the coral reef is under stress. Once the bleaching starts, even if the trigger for the occurrence is removed, the process continues and may require long periods of time for the population of the reef to return to a normal density level. Once the bleaching ceases, though, the same colonies of species that inhabited the reef may return or a new species may claim the space as their own.
Some coral reefs are unable to withstand a large amount of bleaching, whereas others can rebound very quickly. The larger corals are able to sustain a much larger amount of damage and still survive the bleaching that occurs. Some of the fragile coral reefs are unable to withstand even the smallest changes in the environment on which they depend and so deteriorate quite quickly when bleaching begins.
Along Australia’s Great Barrier Reef, three major bleaching events have occurred – in 1998, 2002 and 2006. An increase in the temperature of the sea water has been determined to be the cause of the loss of color in the corals. Most of the reefs suffered minor damage in the low levels of death of the corals, whereas on some of the reefs about 90% of the corals died. It is predicted that coral bleaching will continue and possible worsen over the next two or three decades as the carbon dioxide levels in the waters around the reefs of the world increase.
Coral reefs in the Mediterranean suffered damages due to coral bleaching in 1996. In this case the cause was found to be an infectious bacteria that attacked and killed the symbiotic algae, essential to the life of the coral. Since this bacteria is only infectious during the warmest times of the year, it is expected that as global warming increases, the parasite will cause much more coral bleaching of the reefs in this part of the world.