Coral Bleaching

Coral reefs are sensitive to environmental change. Recently, the frequency and scale of coral bleaching have increased, and in 2002 the Great Barrier Reef experienced its worst case of coral bleaching on record.
Coral bleaching is the loss of color from corals under stressful environmental conditions.

http://www.reef.crc.org.au/discover/coralreefs/bleaching/coralbleaching.html

http://www.reefed.edu.au/explorer/bleaching/index.html

http://www.coral.noaa.gov/cleo/coral_bleaching.shtml