Coral bleaching is defined as the loss of zooxanthellae by the coral host (van Oppen, 2008; Hughes et al., 2003). Photosynthetic pigment is lost with the symbiont, which leaves behind only translucent coral tissue and the more familiar white, or bleached, calcerous skeleton. This rejection of the symbiosis occurs from thermal stress. Specifically, this happens when sea surface temperatures exceed summer maxima by 1 to 2 C for 3 to 4 weeks (Hoegh-Guldberg et al. 2007).
The resiliency of a reef system is tested during a bleaching event, and symbiosis may be re-established given a less intense bleaching event or low levels of local stress.
However, researchers suggest that the system is weaker overall (Hoegh-Guldberg et al., 2007; Hughes et al. 2003):
Decreased growth rate
Increased susceptibility to other stressors