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Dr David Wachenfeld
Chief Scientist, Great Barrier Reef Marine Park Authority

Coral Bleaching

Climate change, caused by global emissions of greenhouse gases, is the greatest threat to coral reefs worldwide. The greenhouse effect is a natural process that warms the Earth’s atmosphere by trapping some of the sun’s energy that would otherwise be radiated back into space. Increased greenhouse gas emissions mean more heat is trapped in the Earth’s atmosphere and temperatures increase, causing the climate to change. Emissions from human activities are largely responsible for enhanced levels of greenhouse gases. Over the last five years, global carbon dioxide levels have increased at a rate similar to that of the last fifty years. The most common cause of coral bleaching is sustained heat stress, which is occurring more frequently as our climate changes and oceans become warmer. Coral is an incredibly small and simple animal. Imagine an animal, just like a jellyfish, only imagine that that animal is upside down, that soft, squishy animal is living now on a hard limestone skeleton. One of the things that makes coral such a special animal is that it’s not just an animal. Inside the living tissue of the coral are microscopic plants called zooxanthellae, and those plants generate energy sugars, just the way that any plant does. A lot of the sugars that they produce are used by the coral animal for its own energy supply. So the singlecelled plants are incredibly important to the corals because they provide their food, but the plants also provide most of the colour that you see in a coral, most of the time. Healthy corals, most of the time, are a brown-green colour.

Bleaching is a stress response. Under the wrong conditions when things get stressful for the corals, the relationship between the coral animal and the tiny single-celled plant starts to break down. Instead of the plant producing sugars and chemicals that are good for the coral, the plant starts to produce chemicals that are harmful to the coral. Consequently, the coral starts to expel the single-celled plants from its tissue. The more of those plants the coral spits out the paler, the coral becomes. The coral will then start to look, eventually, either white or it can look a brilliant shade of yellow or pink or blue or purple because now you’re seeing some of those pigments that are produced by the coral itself. They are showing through as the coral is expelling the single-celled plants because it is in stressful conditions. So coral bleaching is a stress response. Quite a few things can cause corals to bleach: if it gets too hot; if it gets too cold; if they’re exposed to fresh water in a flood plume; or exposed to excessive nutrients or other pollutants that might be in a flood plume. Although different things can cause individual corals to bleach, the only thing that we know that causes mass bleaching-very large numbers of coral colonies bleaching over very large areas of reef-is temperature stress. If the temperature is a long way above average, or if the above average temperature stays around for a long time, then a lot of corals can die, they’ll be very badly stressed, and then they die. But if the water cools down or the temperature stress doesn’t stay around for too long, they can recover. They can get their microscopic plant cells back again and get their colour back again and move forward.

There is another kind of recovery that happens on a coral reef. Imagine a reef with a very bad bleaching. That reef isn’t dead. The reef is still a reef ecosystem. The corals that are left reproduce. Corals on other reefs nearby will reproduce, and given enough time, a reef itself can recover. The critical point is that phrase, ‘given enough time’. With climate change the frequency and severity of severe events like bleaching events is increasing and reefs don’t get as long to recover, as they would have had twenty, thirty, or forty years ago. As severity and frequency of events is increasing, we increasingly see that reefs are struggling to recover the way they used to.

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