<P> The corals that form the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae - like single - celled flagellate protozoa called zooxanthellae that live within their tissues and give the coral its coloration . The zooxanthellae provide the coral with nutrients through photosynthesis, a crucial factor in the clear and nutrient - poor tropical waters . In exchange, the coral provide the zooxanthellae with the carbon dioxide and ammonium needed for photosynthesis . Negative environmental conditions thwart the coral's ability to provide for the zooxanthellae's needs . To ensure short - term survival, the coral - polyp then expels the zooxanthellae . This leads to a lighter or completely white appearance, hence the term "bleached". As the zooxanthellae provide up to 90% of the coral's energy needs through products of photosynthesis, after expelling, the coral may begin to starve . </P> <P> Coral can survive short - term disturbances, but if the conditions that lead to the expulsion of the zooxanthellae persist, the coral's chances of survival diminish . In order to recover from bleaching, the zooxanthellae have to re-enter the tissues of the coral polyps and restart photosynthesis to sustain the coral as a whole and the ecosystem that depends on it . If the coral polyps die of starvation after bleaching, they will decay . The hard coral species will then leave behind their calcium carbonate skeletons, which will be taken over by algae, effectively blocking coral re-growth . Eventually, the coral skeletons will erode, causing the reef structure to collapse . </P> <P> Coral bleaching may be caused by a number of factors . While localized triggers lead to localized bleaching, the large scale coral bleaching events of the recent years have been triggered by increased sea surface temperatures . Coral reefs located in warm, shallow water with low water flow have been more affected than reefs located in areas with higher water flow . </P> <Ul> <Li> increased water temperature (most commonly due to global warming), or reduced water temperatures </Li> <Li> oxygen starvation caused by an increase in zooplankton levels as a result of overfishing </Li> <Li> increased solar irradiance (photosynthetically active radiation and ultraviolet light) </Li> <Li> increased sedimentation (due to silt runoff) </Li> <Li> bacterial infections </Li> <Li> changes in salinity </Li> <Li> herbicides </Li> <Li> extreme low tide and exposure </Li> <Li> cyanide fishing </Li> <Li> elevated sea levels due to global warming (Watson) </Li> <Li> mineral dust from African dust storms caused by drought </Li> <Li> pollutants such as oxybenzone, butylparaben, octyl methoxycinnamate, or enzacamene: four common sunscreen ingredients that are nonbiodegradable and can wash off of skin </Li> <Li> ocean acidification due to elevated levels of CO caused by air pollution </Li> <Li> being exposed to Oil or other chemical spills </Li> </Ul>

Where is coral bleaching most likely to occur