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March 2013

Coral bleaching

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Posted by Rahul Demello |

Coral reefs are the richest ecosystems on the planet. Giant formations built by some of the world’s tiniest creatures, these hidden worlds form tropical marvels that one has to see to believe. The biodiversity supported by coral reefs is greater than anywhere on earth, including the tropical rainforests. They form the largest structures built by any living thing, including man and no other realm can boast of as many species found in such concentrations.

Corals, the animals that build these reefs, typically live in colonies with many individual “polyps” that secrete exoskeletons made of calcium carbonate, the foremost building material for coral reefs. Being exclusively tropical, corals naturally thrive in conditions of warm water and strong sunlight. Recently, however, the limits of their endurance are being pushed further and further. As a result, coral bleaching is occurring on local, regional and global scales with alarmingly increased frequency and severity on reefs across the globe.

Bleaching of a coral is actually the reaction of coral to unfavorable conditions. The coral begins to expel the symbiotic algae (living within the coral’s tissue) which are responsible for producing up to 90 percent of its food. These algae are also what give the corals their amazing varieties of colour. By their elimination, the corals are effectively going on hunger strike.

While coral bleaching can be caused by high levels of UV radiation, changes in salinity as well as bacterial and viral infection, the most universal cause of coral bleaching is increased Sea Surface Temperature (SST). Thermal stress is considered to be responsible for most large scale bleaching events around the world. Coral has a very narrow range of temperature in which it can survive, and changes of just a few degrees can cause bleaching.

Several large-scale bleaching events have been documented at reefs around the world over the last two decades. This is important because significant declines in coral populations will ultimately lead to reef decay. This affects not only the corals themselves, but all associated flora and fauna which depend on the reef for food and shelter.

Ritchie’s Archipelago in the Andaman Sea (a part of what we know of as the Andaman Islands) experienced a period of extreme thermal stress in 1998, 2004 and again in early 2010 with Sea Surface Temperatures exceeding 32oC, which is a coral’s upper temperature threshold. As a result, mass bleaching followed and vast areas of the reefs turned white, died and were subsequently overgrown by algae. Storms later destroyed dead reefs that were once resilient to the action of the waves – South Button was one of the most badly-affected reefs.

Response to bleaching varies, but considering that corals grow only an inch or two a year, complete recovery will take decades. Despite extensive research into coral bleaching, there are very few usable data with which to predict the capacity of reefs to respond to changing environmental conditions.

This is why a Reef Monitoring Program on the reefs in the Andamans is essential to the future of the region. Concern needs to be shown about the future of our coral reefs, in the face of predicted global warming and climate change. Current predictions suggest that bleaching events will become more frequent and severe with phenomena like El Nino exacerbating the effects of global climate change. Preservation of the remaining coral is critical to maintaining the species richness and diversity in the waters of the Andaman Sea, one of the few places in this country where we can experience firsthand what most people only see on television.

The ultimate fate of corals in the region will depend on the capacity of corals to acclimatise or adapt to elevated temperatures. Recovery has proved to have been successful in North-West Australia and the southern United States and Caribbean. There are some documented species of coral in the Western US that can live in waters that are as warm as 38oC.

No one has more exposure to tiny changes than people who dive the reefs every day. Divers see the change that most people don’t and for us, it’s especially personal, as these are reefs that we’ve discovered and which we consider “ours”.

We have initiated a pilot project on a few reefs around Havelock Island to gauge the current state, recovery potential and speed of regeneration of the reefs in the area. Our work is designed to detect change in species richness and condition of the coral reef system. Of particular interest are changes in percent cover of live coral on the reef and at what rate recovery is progressing. With the data we gather, it will be a step toward determining the fate of our reefs more accurately. This allows us to learn from and influence the course of events rather than just being a spectator to the decline. We hope that we can collect some interesting scientific information which will improve our understanding of the dynamics of the coral reef and give us cause to be optimistic.

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