Coral Bleaching 101

Coral bleaching is the whitening of corals, due to the loss of zooxanthellae, or the microscopic algae that reside within coral tissues that give corals their color.  The zooxanthellae are expelled in response to stress, and in the case of bleaching the stress is most often heat. 

Corals turn pale before they bleach and different corals bleach differently though most turn white.  The massive starlet coral turns light blue when it bleaches and some corals fluoresce bright orange/red. 

Paling is the initial response when corals bleach and parts of the colony will be lighter in color than normal. Part bleached is when part of the coral colony becomes bright white and whole bleached is when 90% or more of the coral colony is bright white.

The first sign of bleaching is "paling" where the corals become lighter in color.

At the onset of bleaching, coral reefs also appear to be more "colorful" which is due to the fact that their normal drab olive green and brown tones lighten as a result of the loss of their zooxanthellae.

When corals bleach they first lighten in color, or pale, then begin to whiten.  hard and soft corals and even sponges can exhibit signs of coral bleaching.

Prolonged bleaching can lead to invasion by algae and coral diseases, and death.  One colony is not a problem, but this can happen over very large scale and lead to the collapse of the whole reef structure, i.e. loss of habitat for fishes and other reef organisms.  When this happens, it has a great impact on ecosystem services and the people that rely on these habitats for their livelihood, such as persons in the fishing and tourism industries.   By volunteering with the Coral Watch Program you can help conserve reefs for future generations by helping to identify resilient reefs. 

This is fully bleached finger coral growing among a healthy boulder star coral. Here the boulder star coral colony is more resilient to thermal stress than the finger coral.

If corals remain bleached and under stress for too long disease can invade and take over the coral colony. Here is black band disease.

​http://oceanservice.noaa.gov/facts/coral_bleach.html