Coral is made up of an animal called Polyps. They’re transparent organisms with tentacles and cover the coral’s exoskeleton working together as a unit.
The vibrant coral you see is due to the organisms living within the coral which is usually Zooxanthellae ( a type of algae). Algae and coral work together as one and both depend on each other. The coral shelters the algae and gives access to sunlight whereas the algae provide the coral with nutrients (90% of what the algae produce goes to the coral).
So why is the coral white?
Carbon dioxide is an acidic gas and as levels of carbon dioxide continue to increase more will dissolve into the oceans affecting the pH. The oceans are naturally alkaline but with more and more carbon dioxide dissolving the pH is now closer to neutral. A higher pH can stop algae from photosynthesising efficiently which will affect the number of nutrients the coral will receive.
Global warming has raised the temperature of the oceans by 2 degrees Celcius. This may not seem like much but water has a very high heat capacity meaning it takes a lot of energy to raise the temperature by one degree (acts as a buffer). The temperature rise has caused stress in the relationship between the coral and algae resulting in the coral to expel the algae. Eventually, there will be no algae within the coral, you will only be able to see the calcium carbonate exoskeleton thus the white exterior. The coral which is bleached is either already dead or is starving and will die soon.
On a more joyful note, there still is some hope it has been discovered that some coral seems to be resistant to bleaching and scientists are crossbreeding breeds of corals to try to restore the reef.
How does this affect the fish?
Fish and invertibrates that live in the reef rely on the coral reef for food and shelter without this populations will and have delcined.
The graph shows the rate of sea turtles observed on Ohama beach in Japan. As you can see the population dramatically drops in the early 2000s and even more so in 2017.
Graph showing the overall decline in the number of sea turtle spottings in Japan.