Stressed Corals Need to Chill Out

2010 hasn’t been a good year for the coral reefs of Southeast Asia. For the first time since 1998/1999, unusually high or prolonged summer sea temperatures caused mass coral bleaching, with reefs in Indonesia, Thailand and Malaysia most badly affected. In Indonesia, the distribution of bleaching was patchy. Coral reef scientists, NGOs, fishers and dive operators reported varying degrees of bleaching, possibly reflecting differences in thermal stress across different areas.

In areas like Aceh in western Sumatra, 90 percent of corals bleached. In Bali, around 20 percent of corals were affected. Other areas, like Raja Ampat in West Papua, have recorded little to no bleaching because higher water temperatures stayed clear of these regions.

In April 2010, I visited Wakatobi National Park, home to some of the world’s most diverse coral reefs and 100,000 people who depend almost exclusively on the sea for food and income. I was there to participate in the annual coral reef monitoring done by the National Park Authority with help from the Conservancy-WWF joint program and other partners. When we got in the water I was shocked to see that coral bleaching had also reached Wakatobi. At every site we monitored, corals were affected by bleaching down to depths of 30 meters or more.

Our surveys showed that some sites and some types of coral were affected more than others. But, overall, 20 percent of corals were fully bleached (albeit still alive) with another 30 to 50 percent becoming pale due to the coral tissue’s loss of a crucial symbiotic algae called zooxanthellae that provides nourishment. Swimming across fields of white coral is a very eerie experience.

We were more than a little worried that Wakatobi corals might not survive such an extensive and severe bleaching event, especially since colleagues from the World Conservation Society (WCS) had reported that the majority of the bleached corals in Aceh did not survive. Over the following months, field teams from Wakatobi continued to report bleaching but it seemed to recede slowly over time as water temperatures cooled and corals recovered.

In September, I returned to repeat the bleaching surveys to see if Wakatobi corals had indeed experienced widespread mortality. To our great relief, the reefs looked colorful and healthy, although close examination showed that a few corals were still bleached and that some had even died recently.  Wakatobi’s recent recovery was fortunate, but how will it fare with the upcoming La Niña event that’s already causing warmer-than-normal temperatures in Indonesia?

The Conservancy is now working with WCS and ReefCheck Indonesia to examine the bleaching’s impact on reefs in different parts of Indonesia. One of the main goals of our work will be to test theories of coral reef resilience and see if there are environmental factors that contribute to the differences in bleaching between sites or among regions. We hope this will allow us to better protect and manage coral reefs in the face of climate change.

Joanne Wilson is the Deputy Director for Science in the Conservancy’s Indonesia Marine Program.

(Image 1: Monitoring staff conducting a reef health survey in Wakatobi National Park. Credit: Joanne Wilson/TNC. Image 2: Bleached coral in Wakatobi National Park. Credit: Joanne Wilson/TNC.)

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