What are the major threats to corals?

Coral scientists collecting data on ocean acidification. Learn more and view a larger image.

Climate change is a major threat to corals. As carbon dioxide — the primary cause of climate change — increases in the atmosphere, it is readily absorbed by the ocean. This causes ocean water to become more acidic. The process is called ocean acidification. Ocean warming occurs when heat is reflected back to the Earth’s surface by increased levels of greenhouse gasses in the atmosphere (carbon dioxide, methane, etc.). The ocean absorbs about 90% of this heat energy. Both of these impacts to the ocean are especially significant in shallow water where many corals are found.

Our ocean absorbs excess carbon dioxide (CO₂) when we burn fossil fuels. This is making the oceans more acidic and harming reef ecosystems. Learn more and view a larger image.

Acidification
Ocean acidification occurs when carbon dioxide (CO₂) from the air gets absorbed by the ocean. When CO₂ dissolves in water, it forms carbonic acid, which makes the water more acidic. This increased acidity is harmful to many marine organisms, such as corals, shellfish, and small marine creatures like plankton.

Marine life relies on a specific balance of acidity in the water. When the ocean becomes more acidic, it affects the ability of many marine organisms to build and maintain their shells and skeletons, including stony corals, which are made of calcium carbonate. The acidified water makes it harder for them to grow and stay healthy.

Increased ocean acidity also weakens coral structures, making them more susceptible to damage and slower to recover. This puts the entire reef ecosystem at risk, including fish and other creatures that rely on corals for food and shelter.

Bleaching
When corals are stressed by changes in temperature, light, or nutrients, they expel the symbiotic algae (zooxanthellae) living in their tissues, causing them to turn completely white. This process is called coral bleaching. The loss of a coral’s symbiotic algae is usually a gradual process, increasing as the level of stress increases. Because the algae are so small, by the time we can actually see a change in a coral’s color, the bleaching process has already been taking place for quite a while. Once we see the coral’s color change, we say it is paling. Once it is white, we say it is bleached. The individual polyps become mostly clear, showing their white skeleton beneath.

Warmer water temperatures can result in coral bleaching. Learn more and view a larger image.

When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are much more likely to die if the stress that caused their bleaching continues. Whether or not the coral recovers, there are some long-term effects. Corals that recover from bleaching may look the same, but their overall health has been compromised. This leaves them more susceptible to disease. In addition, recovered corals have less energy for skeletal growth and reproduction. The same thing happens when you get a cold or the flu. Even after you recover, you are still not completely healthy for some time afterward, and it takes a while for your energy levels to return to normal. Dead corals, on the other hand, change the composition of the reef. This can ultimately result in a loss of habitat for so much of the marine life that depends on them.

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. Learn more and download the infographic.

Episodes of coral bleaching are happening more frequently and with greater severity as the ocean absorbs more heat energy, and its waters become warmer due to global warming.

Coral reefs may recover from periodic traumas caused by weather or other natural occurrences. If, however, corals are subjected to numerous and sustained stresses - including those imposed by people, the strain may be too much for them to endure, and they will perish.