Mass Bleaching Events
Most evidence indicates that elevated temperature is the cause of mass bleaching events. Sixty major episodes of coral bleaching have occurred between 1979 and 1990, with the associated coral mortality affecting reefs in every part of the world. Correlative field studies have pointed to warmer-than normal conditions as being responsible for triggering mass bleaching events. These studies show a tight association between warmer-than-normal conditions (at least 1°C higher than the summer maximum) and the incidence of coral bleaching.
Factors that influence the outcome of a bleaching event include stress-resistance which reduces bleaching, tolerance to the absence of zooxanthellae, and how quickly new coral grows to replace the dead. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce bleaching incidence. Coral and zooxanthellae health and genetics also influence bleaching.
Large coral colonies such as Porites are able to withstand extreme temperature shocks, while fragile branching corals such as table coral are far more susceptible to stress following a temperature change. Corals consistently exposed to low stress levels may be more resistant to bleaching.
Read more about this topic: Coral Bleaching
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