Climate change, leading to a warmer sea temperature and ocean acidification, is turning the rich coral reefs off India’s Andaman & Nicobar Islands into a graveyard
During April-May 2010, between 65% and 81% of live coral cover was bleached off the Andaman and Nicobar (A&N) islands.
Brown white-tipped horns of the Staghorn coral, named so due to its resemblance to deer antlers, have always been the home of dainty little damsel fish off the Andaman and Nicobar (A&N) islands. But this has changed in less than five years. Now the colourful underwater coral reefs off the A&N islands have turned into a graveyard. The damsel fish have disappeared. Coral reefs, referred to as the rain forests of the sea, due to the rich diversity of fish life they host, are dying due to ocean waters that have turned too warm and too acidic for them – a result of global warming.
“The ocean bed now seems like battlefield remains,” laments Siddhartha Ramachandra, an ecologist who trained as a scuba diver to record changes in the richest coral reef systems in the A&N islands. He has observed the disappearing coral reefs since 2010.
The observations are worrisome because the islands are home to 89% of India’s coral diversity. According to Zoological Survey of India (ZSI), out of 478 coral species in India, as many as 424 species are found here alone.
El Niño spells doom for corals
Coral colonies are crucial for the survival of marine life as one-fourth of the ocean life is dependent on them. These fragile invertebrates are found only in shallow waters in the tropics where the water temperature stays between 20 and 29 degrees Celsius. However, in the summer of 2010, the sea surface temperature went up to a devastating 34 degrees Celsius and the corals lost their colour in a process called bleaching, leaving them a ghostly white.
According to J. Ravindran, scientist at the National Institute of Oceanography (NIO), coral is a mutualism of plant cell and animal cell, and the drastic increase in the sea surface temperature due to the El Niño event in 2010 stressed the animal partner. “Bleached coral is the naked calcium skeleton. It could have recovered had the temperature returned to normal within a few days, but most corals died as the temperature consistently stayed high,” he added.
Corals are made of polyps, a type of spineless animal, which share a symbiotic relationship with algae from the genus Zooxanthella. The algae nourish the coral through photosynthesis while polyps in turn protect the algae and provide them carbon dioxide necessary for photosynthesis.
When stressful conditions like warmer sea temperature arise, polyps eject the symbiotic algae from their systems as a temporary survival strategy. This mass ejection of algae is called bleaching. As the algae which impart coloration to corals leave, the corals turn whiter and look bleached. The process is reversible if the situation returns to normal. But if the stressful conditions continue, the corals eventually die.
A ZSI study found that the 2010 El Niño was prominently felt by corals across India, mostly in the A&N islands and Lakshadweep islands. “Live coral cover came down drastically in Indian islands during the first mass bleaching in 1998 and then in 2010 due to El Niño events,” said R. Rajkumar, marine biologist with the ZSI.
According to ZSI, the first incident of coral bleaching occurred in July 1998, when the sea temperature exceeded 31 degrees Celsius leading to bleaching of 90% of the massive corals and 75% of the branching corals across India. More recently, during April-May 2010, between 65% and 81% of live coral cover was bleached.
And there is more trouble ahead for the corals. Scientists expect another bout of El Niño in the coming months as recordings of the ocean surface temperature show that 2014 was the warmest year since records began.
Last year, the National Oceanic and Atmospheric Administration (NOAA) of the United States predicted an El Niño. Although, it has not begun, most forecasters still expect it to arrive in the next couple of months. In December, Japan’s weather bureau and the Australian Bureau of Meteorology forecast that El Niño weather conditions would occur over the next three months.
In an interview to British newspaper The Guardian on December 19 last year, NOAA Coral Reef Watch coordinator Mark Eakin said that warm water will soon begin hitting reefs in the southern Pacific and the Indian Oceans. “On a global scale it’s a major bleaching event. What it may be is the beginning of a historic event.” He added that coral watch modelling predicts bleaching on Australia’s Great Barrier Reef as early as January.
The future looks grim for Indian corals as well. “We cannot protect coral from El Niño. Whenever El Niño comes, we can say bleaching will happen. The extent will be known when it happens,” says Rajkumar.
Ocean acidification adding to woes
Apart from the warming waters, Indian corals are facing another threat. With greenhouse gas emissions rising due to human activities, huge amounts of carbon dioxide released into the atmosphere are being dissolved in sea water and acidifying all oceans. The acid reacts with the coral structure, weakens it and eventually leads to coral destruction.
Indian scientists currently do not know to what extent the acidity in the Indian Ocean is affecting the corals. “Ocean acidification is not studied by the government but it is known that ocean acidification minimises the skeletal formation of the corals,” says Ravindran.
Ocean acidification can reduce the ability of corals to absorb calcium carbonate that gives strength to their skeletons. It can ultimately split and dissolve coral reefs.
A NIO study in a small area in coastal Bay of Bengal shows that the north-western coastal region has 3-5 times higher carbon dioxide levels than the global average.
A combination of El Niño and ocean acidification has resulted in disappearance of the fast growing Staghorn specie of corals along with severe decline in bolder soft and fire corals in A&N islands. The populations of damsel fish, longnose filefish, cornet, pipe and razor fish – all of whom depend on these corals for habitat have also significantly diminished.
“A layer of food chain has shut down due to dying corals, with cascading and sometimes unknown effects,” said Ramachandra.
But for corals present in deeper waters of the Andamans, there is still some hope. As one goes deeper into the sea, the water gets colder, increasing the chances of survival of corals and fish life. While the coral reefs that exist between 10 and 20 metres from the beach have suffered grave damage and are more vulnerable to bleaching due to increasing water temperature, those present at a distance of 30 metres are less susceptible to bleaching events.
