Coral bleaching

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What are Coral reefs?

Coral reefs are large underwater structures composed of the skeletons of colonial marine invertebrates called coral.
The coral species that build reefs are known as hermatypic or “hard” corals because they extract calcium carbonate from seawater to create a hard, durable exoskeleton that protects their soft, sac-like bodies.
Other species of corals that are not involved in reef building are known as “soft” corals.
These types of corals are flexible organisms often resembling plants and trees and include species such as sea fans and sea whips.
Each individual coral is referred to as a polyp.
Coral polyps live on the calcium carbonate exoskeletons of their ancestors, adding their own exoskeleton to the existing coral structure.
Corals are found all over the world’s oceans.
The best suited environment for coral reefs is clear, shallow waters of the tropics and subtropics.

Coral bleaching:

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 because the loss of algae reveals the white color of the calcium carbonate structure underlying the polyps. This is called coral bleaching.

The 2014-2017 global-scale coral bleaching event killed several reefs, including large tracts of Australia’s Great Barrier Reef.
Severe or prolonged bleaching can kill coral colonies or leave them more vulnerable to other threats such as infectious disease.
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.
Not all bleaching events are due to warm water. In January 2010, cold water temperatures in Florida, USA caused a coral bleaching event that resulted in some coral death.
Other climate impacts such as sea level rise, increased frequency and intensity of tropical storms and altered ocean circulation patterns can also affect coral reefs.