Coral bleaching happens when corals lose their vibrant colors and turn white. Corals are bright and colorful because of microscopic algae called zooxanthellae. The zooxanthellae live within the coral in a mutually beneficial relationship, each helping the other survive. But when the ocean environment changes—if it gets too hot, for instance—the coral stresses out and expels the algae.
As the algae leaves, the coral fades until it looks like it’s been bleached. If the temperature stays high, the coral won’t let the algae back, and the coral will die.
The leading causes of coral bleaching are:
- A warming planet means a warming ocean, and a change in water temperature—as little as 2 degrees Fahrenheit—can cause coral to drive out algae.
- Corals exposed during daylight hours are subjected to the most ultraviolet radiation, which can overheat and dry out the coral’s tissues.
- Corals may become so physiologically stressed that they begin to expel their symbiotic zooxanthellae, which leads to bleaching, and in many cases, death.
- Reefs also are threatened by tidal emersions. Long periods of exceptionally low tides leave shallowwater coral heads exposed, damaging reefs. The amount of damage depends on the time of day and the weather conditions.
- Increased sea surface temperatures, decreased sea level and increased salinity from altered rainfall can all result from weather patterns such as El Niño. Together these conditions can have devastating effects on a coral’s physiology
- Improperly treated sewage, fertilizers and top soil are elevating nitrogen levels, which are causing phosphorus starvation in the corals, reducing their temperature threshold for “bleaching.” These coral reefs were dying off long before they were impacted by rising water temperatures.
- In addition to weather, corals are vulnerable to predation. Fish, marine worms, barnacles, crabs, snails and sea stars all prey on the soft inner tissues of coral polyps. In extreme cases, entire reefs can be devastated by this kind of predation.