CORAL-REEF BLEACHING - FACTS, HYPOTHESES AND IMPLICATIONS

Coral reef bleaching, the temporary or permanent loss of photosyntheti c microalgae (zooxanthellae) and/or their pigments by a variety of ree f taxa, is a stress response usually associated with anthropogenic and natural disturbances. Degrees of bleaching, within and among coral co lonies and across reef communities, are highly variable and difficult to quantify, thus complicating comparisons of different bleaching even ts. Small-scale bleaching events can often be correlated with specific disturbances (e.g. extreme low/high temperatures, low/high solar irra diance, subaerial exposure, sedimentation, freshwater dilution, contam inants, and diseases), whereas large scale (mass) bleaching occurs ove r 100s to 1000s of km(2), which is more difficult to explain. Debilita ting effects of bleaching include reduced/no skeletal growth and repro ductive activity, and a lowered capacity to shed sediments, resist inv asion of competing species and diseases. Severe and prolonged bleachin g can cause partial to total colony death, resulting in diminished ree f growth, the transformation of reef-building communities to alternate , non-reef building community types, bioerosion and ultimately the dis appearance of reef structures. Present evidence suggests that the lead ing factors responsible for large-scale coral reef bleaching are eleva ted sea temperatures and high solar irradiance (especially ultraviolet wavelenths), which may frequently act jointly.