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Coral bleaching

This image shows the coral of Airport Reef off American Samoa taken by the XL Catlin Seaview Survey team in 2014. Healthy coral reefs provide a number of what are known as ecosystem goods and services. These are the benefits that humans derive from ecosystems, in terms of goods, such as fish stocks, and services, such as coastal protection.

Although covering less than 0.1% of the ocean, coral reefs support 25% of all marine life. Healthy coral reefs also play an important role in erosion prevention, absorbing the power of coastal storms; a ‘service’ that is valued at over $150,000 per hectare per year.

Airport Reef was revisited by the team and rephotographed at the height of the coral bleaching event in 2015.

Coral bleaching is caused by rising sea temperatures. Tropical coral reefs are created by the coral polyp. These tiny animals derive between 70% and 90% of their energy from photosynthetic algae that live inside their tissue. When the sea temperature rises, there is a breakdown in the symbiotic relationship between the algae and the coral polyp.

The algae leave the coral polyp and the coral starts to starve. As this algae gives the coral its coral, when it leaves, the coral turns white. This is why the process is known as bleaching.

Coral bleaching does not mean that the coral dies. If sea temperatures return to normal within a few weeks, the algae will return and the coral will recover. Airport Reef has bounced back from previous bleaching events.

Six months after the bleaching event in 2015, the XL Catlin Seaview Survey team returned to American Samoa. They were shocked by what they saw.

When coral dies, it soon becomes covered with algae and start to crumble. The reef structure, and the habitat for fish and marine life, can change state from a healthy coral-dominated environment to an algae-dominated ‘dead reef’.

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