Satellites & Bleaching

Coral Bleaching

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Intro to Bleaching   |   What Is a Coral Reef?   |   Coral Bleaching   |
   Why Should We Care About Our Coral Reefs?
coral polyps

Bleached coral reef in the U.S. Virgin Islands. Click the image for a larger view and more information.

Reef-building corals need warm, tropical water. Generally, most tropical corals can't grow in oceans where the water temperature dips below 18°C (64°F) for extended periods in the winter. But how warm is too warm? In the 1980s, scientists discovered that corals start getting stressed if the water gets only 1°C warmer than the highest temperature expected in the summertime (Glynn and D'Croz, 1990). We call this temperature the "bleaching threshold" because the stress caused by warmer-than-normal water can cause the corals to bleach.

But what happens to a coral when it bleaches? We said that Polly has tiny algae that grow in her body tissue. Normally, these algae absorb energy from the sun and use it for photosynthesis. When the water gets too warm, these plants can't use the sun's energy as efficiently. The algae turn this excess energy from sunlight into chemicals (toxins called "reactive oxygen species") that can cause damage to themselves and to Polly. So even though Polly normally needs the algae, she has to get rid of them to survive this temperature stress. As a result, she will expel most of the zooxanthellae from her body. Her own body tissue is transparent, and the rock underneath her is white-- so what you see is her white skeleton instead of the normal golden-brown of the algae. The entire coral soon looks pale or white, so we say that it looks "bleached."

Can Polly survive the bleaching? If the warm temperatures don't get too high or last too long, the bleaching won't be too severe and Polly can recover. Polly may gradually rebuild her necessary stock of zooxanthellae over several months and go on to live a healthy life. However, if the temperature gets significantly above the bleaching threshold or stays high for an extended period of time, severe bleaching will occur and some corals can eventually die.

Why do the corals die? Corals without symbiotic algae are left without their main energy source, and there may be cellular damage from the reactive oxygen species. Even corals that survive bleaching are often likely to become infected with coral diseases, just like a stressed person is more likely to get sick. These diseases can also kill corals, sometimes many months after the bleaching. Scientists are still learning about why corals bleach, and why they are so sensitive to warm temperature and sunlight. This research is just starting to teach us ways to help corals like Polly survive or recover from bleaching, so that we can keep coral reefs healthy. Scientific researchers and marine resource managers from NOAA, Australia's Great Barrier Reef Marine Park Authority, and other institutions around the world wrote a book that can help managers take action to help save their coral reefs from bleaching -- it's called A Reef Manager's Guide to Coral Bleaching. You can learn more about this book in the last section of this tutorial. (Managers also can learn more about the top 10 actions to take before, during and after a coral bleaching event here.)

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