coral bleaching, an introduction
Dr. Kelli Anderson
Photosynthetic zooxanthellae (algae) live within the tissues of some reef building corals, and provide compounds such as glucose and glycerol for the coral to use. In fact, zooxanthellae can provide up to 90% of the energy used by coral. In return, the zooxanthellae receive protection and can utilise the coral's organic waste (such as compounds containing nitrogen and phosphorus).
Bleaching occurs when the coral encounters stressful conditions, and the density of zooxanthellae decreases. Without pigmentation from the zooxanthellae, the coral will appear white as its 'skeleton' is revealed. Bleached coral is not necessarily dead, however if conditions don't improve the coral will eventually die.
The most well known cause of bleaching is elevated temperatures. An increase of just one degree over a few weeks can be enough to trigger bleaching events. Other stressors that may cause bleaching include exposure to freshwater (from large rain events), run off from land and other water quality issues.