What is Bleaching?
Bleaching refers to the appearance of “whitened” coral where there was once-colorful coral, a symptom of stress in corals and other reef animals with symbiotic algae.
These algae are known as zooxanthellae and are present in most healthy reef-building corals.
Zooxanthellae provide nutrients and oxygen to the coral through photosynthetic activities, allowing their host to direct more energy toward growth and constructing its calcium carbonate skeleton.
The host coral polyp in return provides zooxanthellae with a protected environment and a constant supply of carbon dioxide needed for photosynthesis.
Bleaching causes corals to lose their zooxanthellae and turn white. In extreme cases of bleaching, corals die.
What Causes Bleaching?
The primary cause of mass bleaching is high water temperature. Other stressors can have a cumulative impact that weakens corals, making isolated bleaching and eventual death possible. Sources of stress on coral communities include:
- Elevated water temperatures and bright sunlight
- Urban and agricultural run-off pollution
- Salinity shock from heavy rains or floods
- Sedimentation from activities such as dredging
The photos at right illustrate dramatically how bleaching can be caused by a host of human-induced and natural factors.
What is Coral Bleaching (Great Barrier Reef Marine Park Authority)