2010 saw massive coral bleaching in Andamans

By Richa Sharma, IANS,

New Delhi : Rising sea surface temperatures due to global warming caused extensive coral bleaching in some of the Andaman & Nicobar Islands in 2010, which, according to a scientist, is the severest in over a decade.

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Scientists from Marine Research Laboratory, Central Agriculture Research Institute in Port Blair and Regional Remote Sensing Centre in Nagpur, which is under the Indian Space Research Organisation (ISRO), conducted a study to assess the extent of bleaching during 2010 at selected reef sites in the islands.

The sites surveyed January-June 2010 included Red Skin Island within Mahatma Gandhi Marine National Park, North Bay, Chidiyatapu – popular tourist destinations – Havelock Aquarium and Havelock Wall, a popular diving site, South Button Island and Nicolson Island, which are protected islands.

“Reef surveys conducted there revealed that the corals have been extensively bleached during April-May 2010, ranging from 37 percent to 70 percent in various sites,” P. Krishnan from Marine Research laboratory told IANS.

According to Krishnan, similar bleachings were reported in 1998 and 2002 in this region. However, the extent of the current bleaching surpasses earlier observations.

The Andaman and Nicobar Islands are bestowed with the richest coral diversity among all Indian reefs. A total of 177 species of hard corals falling under 57 genera have been reported from these islands.

Bleaching is one of the major threats which has significantly affected the reefs across the globe during different time-periods. Coral reefs form some of the most diverse ecosystems on earth and are home to over 25 percent of all marine species in the sea.

Corals are animals having symbiotic association with zooxanthellae, the algae which are responsible for the colouration of the corals.

Expulsion of the algae leads to the whitening of reef-building corals, widely referred to as bleaching. It is caused by physiological, algal, host-related stresses and various ecological and anthropogenic factors.

Sea surface temperature (SST) is a critical factor for the well being of symbiotic association of host animals like corals, giant clam and sea anemones with the microalgae.

“Three 100-metre transects were laid for surveys on the bottom topography, live coral cover and extent of bleaching. The atmospheric temperature and sea surface tempearture were measured using a mercury bulb thermometer,” Krishnan said.

Daily sea surface temperature maps for the period April 20 to May 12, 2010 were analysed to gauge the change in sea surface temperature during the period when bleaching was observed.

“The maximum atmospheric and sea surface temperature recorded were between 31-34 degrees Celsius. During summer, sea surface temperature rise of 2 to 3 degrees above the normal maximum can kill the corals,” Krishnan said.

The ideal temperature for corals to survive is between 25-29 degrees Celsius. The average sea-water temperature during the first week of May in all the study sites was 33.8 degrees Celsius, which resulted in extensive bleaching in different parts of the islands.

The atmospheric temperature showed a sudden increase in air temperature in the first, third and fourth weeks of April, the scientist said.

The coral cover estimated at the different study sites was analysed separately to determine the extent of bleaching.

It was observed that the branching corals were the worst affected due to bleaching.

It was also found that the fully bleached corals were maximum at Havelock Island-Wall (69.49 percent), followed by South Button Island (67.28 percent), Nicolson Island (56.45 percent), Havelock Island-Aquarium (43.45 percent), Red Skin Island (43.39 percent), North Bay (41.65 percent) and Chidiyatapu (36.54 percent).

Indian coral reefs have experienced 29 widespread bleaching events since 1989, but intense bleaching occurred in 1998 and 2002. The corals which were affected to the extent of 60 to 70 percent during the earlier events of bleaching recovered, but those fully bleached died.