CORAL-REEF BLEACHING - ECOLOGICAL PERSPECTIVES

Coral reef bleaching, the whitening of diverse invertebrate taxa, resu lts from the loss of symbiotic zooxanthellae and/or a reduction in pho tosynthetic pigment concentrations in zooxanthellae residing within th e gastrodermal tissues of host animals. Of particular concern are the consequences of bleaching of large numbers of reef-building scleractin ian corals and hydrocorals. Published records of coral reef bleaching events from 1870 to the present suggest that the frequency (60 major e vents from 1979 to 1990), scale (co-occurrence in many coral reef regi ons and often over the bathymetric depth range of corals) and severity (> 95% mortality in some areas) of recent bleaching disturbances are unprecedented in the scientific literature. The causes of small scale, isolated bleaching events can often be explained by particular stress ors (e.g., temperature, salinity, light, sedimentation, aerial exposur e and pollutants), but attempts to explain large scale bleaching event s in terms of possible global change (e.g., greenhouse warming, increa sed UV radiation flux, deteriorating ecosystem health, or some combina tion of the above) have not been convincing. Attempts to relate the se verity and extent of large scale coral reef bleaching events to partic ular causes have been hampered by a lack of (a) standardized methods t o assess bleaching and (b) continuous, long-term data bases of environ mental conditions over the periods of interest. An effort must be made to understand the impact of bleaching on the remainder of the reef co mmunity and the long-term effects on competition, predation, symbioses , bioerosion and substrate condition, all factors that can influence c oral recruitment and reef recovery. If projected rates of sea warming are realized by mid to late AD 2000, i.e. a 2-degrees-C increase in hi gh latitude coral seas, the upper thermal tolerance limits of many ree f-building corals could be exceeded. Present evidence suggests that ma ny corals would be unable to adapt physiologically or genetically to s uch marked and rapid temperature in creases.