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What is Coral Bleaching?

With the water temperatures at The Agincourt Ribbon Reefs, GBR, rising to 31 degrees on the surface this week, I decided it might be time to explain what coral bleaching is and how it effects the reef.



What is coral?


If I asked you whether coral was a rock, a plant, or an animal, which one would you choose? Well it doesn’t actually matter which one you choose because coral is a combination of all three.


Coral is in fact millions of tiny creatures called polyps. These are like upside down jelly fish and have tentacles which reach out and catch plankton floating passed in the water system.


Each polyp lives inside a hard outer skeleton which is made from limestone. This skeleton attaches to rocks or dead coral polyps to hold the polyp in place.


The polyps are also a host to a form of microscopic algae called zooxanthellae, which photosynthesises and provides the coral with oxygen and food. This is the corals main source of food.

Why does coral bleach?


When corals are stressed the zooxanthellae within them are expelled and the coral loses its colour and the limestone skeleton is exposed. This is the bright white colour you see, and what has given it the name bleaching. In some coral species the coral polyps may change colour before they turn transparent. These are often bright, florescent colours.


Without the zooxanthellae the coral polyps are likely to not get enough food and will eventually starve.


There are three main reasons for a coral polyp to become stressed:

  • If they are submitted to temperatures above 30 degrees or temperatures below 20 degrees.

  • If there is a change in the consistency of the water, for example chemicals are added from nearby farms or factories.

  • If humans are touching or standing on the coral.


Coral can survive in this bleached state for up to 6 weeks before they die. This therefore means that just because an expanse of reef has become bleached, it is not necessarily dead. Many of those bleached corals will survive, especially if the stressful conditions are removed within six weeks.



Luckily for The Great Barrier Reef, the temperatures are only just rising to above 30 degrees. This is also currently limited to the top couple of meters of water, which means that the number of corals effected is currently very low. It is also unlikely that these water temperatures will remain for another 6 weeks, so any corals currently bleaching are likely to recover.


Here are some photos taken this week at a few different dive sites across The Agincourt Ribbon Reefs, of corals where bleaching was starting to appear. From these pictures you can see that only a small area of the coral colony has been bleached, the majority of the polyps are alive and healthy.




I had to look quite hard to find these bleached corals, so I also took some general pictures of the reef just to prove that a tiny bit of bleaching has not affected the whole reef.


Please see the rest of my blogs in the series Is The Great Barrier Reef Dead?

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