Lately many reports have been published on how badly the Great Barrier Reef in Australia has been hit by the latest El Niño. Also the Maledives have experienced another large bleaching event during the last months. So why do coral bleach?


What are coral?

Even looking like plants they are animals, more exactly marine invertebrates. Coral are colonies of many identical small polyps. The polyps are basically a sac with a mouth opening and tentacles around it.

Hard coral build up a skeleton by secreting calcium carbonate (limestone) from calcium and bicarbonate ions from the seawater. Generations of polyps build up on the previous limestone skeleton. This is how a reef is built.

Many coral live in a symbiosis with photosynthetic single-cell organisms (zooxanthellae) that produce most of the energy and nutrients for the coral. While the polyps are usually transparent and the skeleton is white the zooxanthellae are responsible for the brilliant colors.


coral reef


What are the main threats to coral?

  • Temperature: Coral are sensitive to temperature changes. Water only a few centigrades higher than normal can already threaten coral. Causes for such changes in temperature are the global warming and El Niño. 90% of the additional heat from global warming is absorbed by the oceans
  • Pollution: Besides the obvious pollution of the sea directly this also includes agricultural and urban runoff through increased coastal development
  • Algae: Certain fish species like parrotfish keep the reef clean from algae. Overfishing of these species leads to reefs covered in algae. The coral then won’t get enough light and die off
  • Ocean acidification: Through absorption of the carbon dioxide from the air the pH of the ocean decreases. Only a few types of coral can tolerate high acidity
  • Direct physical damage: By coral mining, blast fishing, digging canals, boats (dropping anchor) and swimmers/snorklers/divers


How can coral bleach?

When coral are distressed the symbiotic zooxanthellae are expelled from the polyps. Without the little organisms the coral turn white and might die off as they do not get enough nutrients. The coral bleaching can be caused by different types of threats like pollution, changes in water temperature, pH and changes in UV light.

If the conditions don’t improve and the zooxanthellae don’t move back to the coral it will die. The skeleton remains and will get covered by algae.

Why do Coral Bleach

Credit: XL Catlin Seaview Survey


Why do we need reefs?

We do not only need coral reefs because they look nice. These reefs are among the most biodiverse and productive ecosystems on earth. Many countries and millions of people depend on the food source ocean and on the income from tourism.

  • Food: An estimated number of 500 Million people live mainly from sea food caught at close-by reefs
  • Protection of the coast: Coral reefs buffer the coastline from waves and storms. Without the reefs many islands are under risk to disappear
  • Sandy beaches: A huge part of the sandy beaches is made of small coral grains
  • Tourism: The tourism industry of about 100 countries benefits from coral reefs
  • Nurseries of the ocean and food source for marine life: An estimated 25% of all marine species depend on coral reefs


What can we do?

Reefs At Risk Revisited issued the following list of how individuals can help to protect the reefs:

If you live near coral reefs:

  • Follow local laws and regulations designed to protect reefs and reef species.
  • If you fish, do it sustainably, avoiding rare species, juveniles, breeding animals, and spawning aggregations.
  • Avoid causing physical damage to reefs with boat anchors, or by trampling or touching reefs.
  • Minimize your indirect impacts on reefs by choosing sustainably caught seafood and reducing household waste and pollution that reaches the marine environment.
  • Help improve reef protection by working with others in your area to establish stronger conservation measures, participating in consultation processes for planned coastal or watershed development projects, and supporting local organizations that take care of reefs.
  • Tell your political representatives why protecting coral reefs is important.

If you visit coral reefs:

  • Choose sustainably managed, eco-conscious tourism providers.
  • Dive and snorkel carefully, to avoid physically damaging reefs.
  • Tell people if you see them doing something harmful to reefs.
  • Visit and make contributions to MPAs to support management efforts.
  • Avoid buying souvenirs made from corals and other marine species.

Wherever you are:

  • Choose sustainably caught seafood.
  • Avoid buying marine species that are threatened or may have been caught or farmed unsustainably.
  • Help to prioritize coral reefs, the environment, and climate change issues within your government
  • Support NGOs that conserve coral reefs and encourage sustainable development in reef regions.
  • Educate through example, showing your family, friends, and peers why reefs are important to you.
  • Reduce your carbon footprint.


coral timor leste



Title image credit XL Catlin Seaview Survey

THE 3RD GLOBAL CORAL BLEACHING EVENT – 2014/2016 on Global Coral Bleaching. Retrieved 20/09/2016 from

Reefs at Risk Revisited on World Resources Institute. Retrieved 20/09/2016 from