Bleaching Basics
The half purple half white Pocillopora (top left)—two colonies of Pocillopora appear as one, with one half bleached (white) and the other half (purple) beginning to bleach. Corals often brighten in color (top right—purple Pocillopora) as they begin to bleach. A part bleached Pocillopora (bottom right) may live or may die like the last photo (bottom left) overgrown by algae and invertebrates. Top right photo © R. Salm; all others © D.Obura
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.
Bleaching can be caused by a host of human-induced and natural factors such as (top) intense sunlight causing an increase in water temperature; (middle) diseases caused by bacteria, fungi, and viruses; and (bottom) city effluent that is dumped on or near coral reef habitats. Photos top to bottom © Great Barrier Reef Marine Park Authority, D. Obura, M. Erdman
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
- Disease
- 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.
Resources
What is Coral Bleaching (Great Barrier Reef Marine Park Authority)