When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white. Warmer water temperatures can result in coral bleaching. When water is too warm, corals will expel the algae (zooxanthellae) living in their tissues, causing the coral to turn completely white. This is called coral bleaching.
When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality.
In 2005, the U.S. lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event. The warm waters centered around the northern Antilles near the Virgin Islands and Puerto Rico expanded southward. Comparison of satellite data from the previous 20 years confirmed that thermal stress from the 2005 event was greater than the previous 20 years combined. Not all bleaching events are due to warm water.
In January 2010, cold water temperatures in the Florida Keys caused a coral bleaching event that resulted in some coral death. Water temperatures dropped 12.06 degrees Fahrenheit lower than the typical temperatures observed at this time of year. Researchers will evaluate if this cold-stress event will make corals more susceptible to disease in the same way that warmer waters impact corals.
Coral bleaching occurs when the relationship between the coral host and zooxanthellae, which give coral much of their colour, breaks down. Without the zooxanthellae, the tissue of the coral animal appears transparent and the coral's bright white skeleton is revealed.
Corals begin to starve once they bleach. While some corals are able to feed themselves, most corals struggle to survive without their zooxanthellae.
If conditions return to normal, corals can regain their zooxanthellae, return to their normal colour and survive. However, this stress is likely to cause decreased coral growth and reproduction, and increased susceptibility to disease.
Bleached corals often die if the stress persists. Coral reefs that have high rates of coral death following bleaching can take many years or decades to recover.
What Causes Coral Bleaching
The main cause of coral bleaching is heat stress resulting from high sea temperatures. Temperature increases of only one degree Celsius for only four weeks can trigger bleaching events. If these temperatures persist for longer periods (eight weeks or more) corals begin to die. High water temperatures can affect reefs at regional and global scale. Other stresses can also cause bleaching, including freshwater inundation (low salinity) and poor water quality from sediment or pollutant run-off.
Coral bleaching is a generalized stress response of corals and can be caused by a number of biotic and Abiotic factors which are given below:
1. Increased (most commonly), or reduced water temperatures.
2. Starvation caused by a decline in zooplankton levels as a result of overfishing.
3. Increased solar irradiance (Photosynthetically active radiation and ultraviolet band light)
4. Changes in water chemistry (in particular acidification)
5. Increased sedimentation (due to silt runoff)
6. Bacterial infections
7. Changes in salinity
9. Low tide and exposure
10. Cyanide fishing
11. Elevated sea levels due to global warming (Watson)