Coral Bleaching – The reefs are disappearing

Have you ever heard about the “rainforests of the sea”? One of the most diverse and valuable ecosystems, important to the survival of thousands of animals and plants – and heavily endangered. Even though coral reefs are crucial to human life and the sustainability and maintenance of our environment, they are facing a crisis. A crisis that sadly too few people know about.

Coral reefs are underwater ecosystems that have been around for 485 million years. The reefs are one of the earth’s most diverse ecosystems. Called “rainforests of the sea” they provide a home for about 25% of all marine species, all while only occupying less than 0,1 % of the total ocean area. Having been around for a long time the distribution of corals and coral reefs varied from time to time. However, since the 1950s the total amount of reefs declined by 50%. Causes for that concerning loss are changing water conditions, overfishing, land use practices, and other mostly anthropological reasons.

Corals are animals, perfectly adapted to their habitat. They depend on their habitat and other organisms in their habitat to survive. Usually, corals live in symbiosis with algae, zooxanthellae, in their tissue. Said algae forms a mutualistic relationship with the coral, meaning that both organisms depend on each other for survival. In their symbiosis, the zooxanthellae perform photosynthesis from which they produce certain products, such as sugars, lipids (fats), and oxygen. The zooxanthellae provide the corals with those nutrients, which the coral polyp uses to carry out cellular respiration and grow. In exchange for the nutrients, the algae receive carbon dioxide and a safe environment to live in.

However, lately, a phenomenon called “Coral Bleaching” has been occurring more and more frequently. The very sensitive corals react heavily to any kind of environmental stress. When experiencing stress they will expel the zooxanthellae, that have been living in their tissue. Resulting from that, the corals lose their colour and will appear white. Furthermore, the corals lose their main provider of nutrients and are more prone to disease, becoming immune compromised. If left in the bleached state for too long, the corals will starve or become sick.

While environmental stress is the reason for coral bleaching, the causes of stress vary. There are many natural reasons that influence algae. Changes in solar radiation can cause more bleaching in the summer months, and freshwater dilution occurring in oceans after heavy rainfalls may cause stress. Nevertheless, it must be acknowledged that most causes of coral bleaching are anthropological.

Humans impact the oceans and marine wildlife in many ways, directly and indirectly.  A direct human-induced cause for bleaching is sedimentation, the removal of substances such as seabed due to land clearing or coastal construction Another cause is contaminated waters, caused by chemicals. Especially leaks from petroleum probes, machines we use to obtain oil, are becoming more common. Even indirectly humans endanger corals by contributing to climate change. Climate change impacts water temperature or sea levels negatively, which makes coral bleaching more likely. Even epizootics, events of heavy disease within an animal population, are facilitated by climate change.

A bleached coral is not lost immediately but is heavily endangered. If bleached corals manage to stay healthy, they can recover. Said recovery usually takes about a decade if the corals do not experience stress during that time. However, currently, we are experiencing mass bleaching events of historic extent. During a warm-water period in 2014-2017 more than 70% of reefs around the world were damaged. If the number of damaged reefs does not become less, reefs will not manage to recover before more corals are destroyed.

Humans depend on coral reefs for multiple reasons. Reefs present a habitat for many species, which contributes to the conservation of marine wildlife. Furthermore, they stabilize the seabed and protect coastlines from erosion and storms. Without reefs, some parts of Queensland’s coastline would be uninhabitable by now. Reefs protect the coast also by reducing the energy of waves and therefore serve as a natural flood defense.  Lastly, coral reefs impact local economies heavily. Not only do they support tourism, but they provide support for local businesses and therefore protect jobs.

Now after reading my article, you might feel rather hopeless regarding the fate of our beloved corals. There are, however, things you can do to help our reefs.
How can you help protect coral reefs:

1. Conserve water
By using only as much water as you really need, you minimize wastewater that can pollute your local watersheds.
2. Consume seafood consciously
Even if you don’t want to go vegetarian or vegan, you can still help maritime life by doing your research and consuming sustainable seafood. Buy from local fishers!
3. Wear reef-friendly sunscreen
Many sunscreens contain oxybenzone and octinoxate, which are toxic to corals and other maritime wildlife. Once in the sea or ocean, these substances can travel fast and trigger diseases.
4. Volunteer and raise awareness
Draw attention to the topic in your daily life. Educate your friends and family, especially on what to do better.  

And when visiting coastal areas:
5. Do not pollute areas
If you visit a coastal area, make sure that you leave no trash or other unnatural things near or in the water. This is especially important when you are fishing.
6. Be responsible when visiting reefs
If you ever have the chance to visit a coral reef, make sure to not touch the corals and be delicate around the reef. Avoid taking pieces of corals home as a souvenir!

Tamina Schulze

S. Sully; D. E. Burkepile; M. K. Donovan: A global analysis of coral bleaching over the past two decades,
Barbara E. Brown; John C. Ogden: Coral Bleaching,,

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