Coral Reef Bleaching
When a coral colony's living components die, all that remains is the exoskeleton. Since it is almost pure calcium carbonate it is white, hence the expression "coral bleaching" to describe the coral end-of-life processes. (Living coral derives its color from the zooxanthellae resident in the polyps. The polyps, themselves, are transparent.) In simplest terms, "Coral bleaching is the whitening of coral colonies due to the loss of symbiotic zooxanthellae from the tissues of polyps. This loss exposes the white calcium carbonate skeletons of the coral colony. Corals naturally lose less than 0.1% of their zooxanthellae during processes of regulation and replacement. However, adverse changes in a coral's environment can cause an increase in the number of zooxanthellae lost. There are a number of stresses or environmental changes that may cause bleaching including disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures."
It should be understood that coral bleaching is part of the natural life process of a coral community. However, during the past few decades, there appears to have been a marked increase in the rate of coral bleaching. This has been attributed to increased incidences of disease, increased UV radiation and rising ocean temperatures. It is possible, perhaps probable, that these various factors interact with one another.
The coral bleaching phenomenon has been long known and records of its occurrence extend back over a century. According to a Professor , "published records of coral reef bleaching events from 1870 to the present suggest that the frequency (60 major events from 1979 to 1990), scale (co-occurrence in many coral reef regions and often over the bathymetric depth range of corals) and severity (greater than 95% mortality in some areas) of recent bleaching disturbances are unprecedented in the scientific literature. A professor , however, argues that it would be premature to conclude that the specific cause has been determined. While small scale bleaching events can be reasonably attributed to specific problem areas, "Attempts to relate the severity and extent of large scale coral reef bleaching events to particular causes have been hampered by a lack of (a) standardized methods to assess bleaching and (b) continuous, long-term data bases of environmental conditions over the periods of interest. An effort must be made to understand the impact of bleaching on the remainder of the reef community and the long-term effects on competition, predation, symbioses, bio-erosion and substrate condition, all factors that can influence coral recruitment and reef recovery."