Coral bleaching occurs when the photosynthetic pigment of the coral, the zooxanthellae, is expelled. This is a result of the coral experiencing high stress levels. Zooxanthellae are algae, which have a mutualistic relationship with the coral polyp. The zooxanthellae photosynthesise and provide energy to the polyp and the polyp provides shelter, nutrients and carbon dioxide. Most corals have 1,000,000 to 5,000,000 zooxanthellae per centimetre-squared of the surface tissues, 60-90% of which is lost during bleaching (Glynn, 1996). Bleached corals appear pale (Figure 5) because the translucent calcareous coral skeleton is visible without all the coloured zooxanthellae (Glynn, 1996).
The high stress levels which trigger bleaching can be caused by changes in temperature, solar radiation and salinity. Temperature increases are most commonly attributed to bleaching (Brown, 1997). Only a 1°C rise above the seasonal maximum sea surface temperature is needed to cause bleaching (Goreau and Hayes, 1994; Brown, 1997).
Since the 1980s there has been a steep increase in the rate of coral bleaching (Baker et al., 2008), The majority of coral reefs have witnessed bleaching (Glynn, 1993). The most severe bleaching event was the 1997/1998 El Nino-Southern Oscillation, where bleaching was documented in 32 countries, including locations where bleaching had not previously recorded (ISRS, 1998).
If climate change predictions are correct and sea surface temperatures rise, the frequency of coral bleaching will increase (Hoegh-Guldberg and Salvat, 1995). However corals may possess the ability to acclimatise to higher temperatures, meaning they are less likely to bleach (Hoegh-Guldberg, 1999). Bleaching causes dramatic effects to the coral reef ecosystem and with weakened resilience from other factors the corals are more likely to bleach (Hughes et al., 2003).