The process by which corals expel their zooxanthellae.  The corals lose their colour under stressful environmental conditions. The major causes are unusually high water temperature and light intensity.
Coral Bleaching is the loss of colour from corals under stressful environmental conditions. While any stress can cause corals to bleach, high water temperature has been the major cause of coral bleaching events worldwide in recent decades. In 1998 and again in 2002, a major bleaching event in reef waters of the Great Barrier Reef raised concerns about the long-term health of the reef. With predictions of continued rising temperatures as a result of global warming, the future of tropical corals reefs is causing concern worldwide.
- Karleskint G. 1998. Introduction to marine biology. Harcourt Brace College Publishers. p.378