Coral bleaching is a disease that occurs during times of stress. The coral reacts by excreting the algae living within its soft tissues. The soft tissue of the coral are transparent and do not possess any color. The vibrant and beautiful colors displayed by coral reefs are a direct result of the algae living within the coral polyps
In order to survive coral colonies enter into a symbiotic relationship with a specific algae species called zooxanthellae. Zooxanthellae live within the polyp's tentacles and digestive tract. The symbiotic relationship is based each species' mutual need for support from the other species. Coral polyps provide the algae with a safe environment in which to live and photosynthesize. In return algae offer the coral the necessary nutrients required for growth and development.
Under normal circumstances the coral polyps will only expel or excrete excess algae from its tissue in order to order to reduce and manage the photosynthetic production of the algae. However, when a coral polyp experiences excessive stress they display the tendency to over expel the algae from the soft tissues resulting in the discoloration or whiting of the skeletal structure of the reef.
Without the symbiotic relationship with zooxanthellae algae the coral reef colony will eventually die. Death of the reef colony depends upon the severity and duration of the coral bleaching. It is possible for the affected colonies to recover and again maintain a successful and healthy symbiotic relationship with the zooxanthellae algae.
Corals are extremely sensitive to water temperature, salinity, UV radiation, transparency and nutritional components. Bleaching can occur as a result of numerous variations that take place within the reef environment including:
1. Excessive or prolonged variances in water temperature
2. Changes to the salinity content of the water
3. Build up of various gases in the water including carbon dioxide and methane
4. Excessive exposure to increased U.V. radiation and higher than normal light levels
5. Reduction in light levels resulting in a lack of sunlight required for survival
6. Water turbulence and sediment disturbance
7. Water pollution resulting from various chemicals including nitrite, nitrate, ammonia, phosphate and other waste products
Coral bleaching was first observed in the early 1990's and has been steadily increasing over the past 19 years. It is estimated that global climate changes are playing a role in the increased frequency and extensity of coral bleaching which could lead to the degradation of numerous major reef structures as well as many of the coral polyp species.