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Precious corals bleached to death

 Shreeshan Venkatesh |  2016-10-20 20:24:54.0  |  New Delhi

Precious corals bleached to death

The good news is that the current El Niño, a warm ocean current that drove temperatures up worldwide, is finally on its way out. But the bad news is that the unprecedented coral bleaching that El Niño triggered in 2014 shows little signs of slowing down. According to the October 3, 2016, update by the National Oceanic and Atmospheric Administration (NOAA), El Niño-Southern Oscillation (ENSO) is currently in a neutral phase and is likely to remain so during the winters. NOAA’s four-month coral bleaching outlook, however, says reefs in the eastern side of the Indian Ocean, the western side of the Pacific and the Caribbean are likely to sustain bleaching until January 2017. 

The current global coral bleaching event, which is the longest and most widespread event on record, began in mid-2014 from Hawaii and has so far left coral reefs in at least 38 countries and island groups ghost white. From Australia’s Great Barrier Reef—the largest in the world—to the Lakshadweep islands in India, the current bleaching event has spared none. Scientists fear the world stands to permanently lose over 15,000 sq km of coral reefs because of the current bleaching event. Aerial surveys in April this year reveal that about 93 percent of the Great Barrier Reef had suffered bleaching to some extent. Of the 911 surveyed reefs, around a third are severely bleached. With as much as 50 percent of the reef cover already estimated to have been lost, fears are that 50 percent of what is left might be lost during the current event. The northern section of the 2,300-km Great Barrier Reef has been the worst hit in the current bleaching event, with 81 percent of the 522 reefs surveyed indicating severe bleaching. “Though it has been predicted for many months, it’s not quite the same as seeing the devastation,” says Samantha Craven of UK-based Reef World Organisation.

Despite covering just 0.2-0.25 percent of the earth’s ocean area, coral reefs support up to 2 million species or a quarter of all marine life. For several island nations and tropical countries with long coastlines, corals provide protective services from severe tropical weather. Corals cushion the impact of tropical storms that would otherwise damage infrastructure close to the coast. It is estimated that for the Central American nation of Belize, the economic value of the infrastructure protected by coral reefs is equal to 10-15 percent of the nation’s Gross Domestic Product (GDP). Reef structures also act as indispensable genetic libraries and are often described as rainforests of the sea for their richness.

Decay over time
Experts have, for quite a while now, believed that corals are among the most susceptible organisms to climate change. All three of the mass bleaching events ever witnessed have happened in the past 20 years—the first in 1997-98, the second in 2010 and the third is ongoing. In fact, the large-scale devastation caused by the current event, particularly in the United Kingdom and the remote pacific atolls that had been relatively bleaching-free thus far, has triggered speculation of how far the world is from completely losing its coral cover. “In the first global bleaching in 1998, it was reported that the world lost 16 percent of its corals. The current bleaching event appears to be the most severe and therefore has the potential to cause greater coral mortality than that seen in 1998,” says Tyrone Ridgeway, professor at the University of Queensland. Considering that the world has already lost 30-40 percent of its total documented coral cover, the current bleaching event is expected to have considerable ramifications. 

The World Wide Fund for Nature (WWF) calculates the total global asset of reefs at US$800 billion, with around 850 million people dependent on reef-based ecosystems for food security and livelihood. Close to 100 countries benefit from coastline protection, tourism and fisheries, supported by the biodiversity hosted by coral reefs. And a quarter of the 100 countries benefitting from coral reefs depend on tourism for more than 15 percent of their GDP. According to a 2003 report made by WWF, the global annual benefit from coral reefs is close to $30 billion. It adds that fisheries and tourism sectors lose close to $5.7 billion and $9.6 billion a year respectively due to coral bleaching and death.

The greatest economic threat from global coral degradation is likely to be felt in Southeast Asia, Australia and the islands of Oceania. Reefs in the tropical Indo-Pacific stretch are estimated to account for more than 60 percent of the $30 billion contributed annually to the global economy by coral reefs. 

Impact on India
While it initially looked that coral reefs in India would escape the bleaching event, the story changed by April this year. Expeditions by the Natural Conservation Fund (NCF) in Lakshadweep in April and May revealed a staggering scale of bleaching. Shreya Yadav of NCF, a research organisation promoting the use of science for wildlife conservation in India, says, “Around 80 percent of the reefs in the seven islands visited by us have experienced bleaching. Water temperatures remained very high, between 32 and 34 degree Celcius, which for me was unheard of in this region. It was quite devastating to see the extent of bleaching, but we don’t have the numbers yet since the reports are being analysed now.” The Gulf of Mannar too has exhibited 40-50 percent bleaching, says P Krishnan, a researcher at the Union environment ministry’s National Centre for Sustainable Coastal Management. According to J K Patterson Edward, director of Tamil Nadu-based Suganthi Devadason Marine Research Institute, the percentage of reefs that experienced bleaching in the gulf is close to 60 percent. “We have again had intense heating with sustained temperatures up to 33.6 oC so we expect 10-12 percent of coral cover will be lost again this year. The cyclonic activity in May in south India brought respite but heating started early this year,” says Edward. So far, there has been good news from Andaman and Nicobar as the majority of the reefs surveyed have been spared large-scale devastation. 

Unfavourable climate
The most cited reason for the stress on corals has been global warming and associated climate change. The world has already witnessed a 0.8 oC rise in average global temperature since the pre-industrial era and all forecasts predict further increase at the same or higher rate in the foreseeable future. More than 90 percent of the accumulated anthropogenic heat ends up in the oceans. To give an idea of how much energy this is, the heat stored by the oceans in the past 30 years is roughly equivalent to the energy released by one nuclear bomb dropped into the ocean every second of the 30 years. This uptake of energy has been increasing in the past 40 years and according to most reliable climate models yet, the upward incline is set to continue for a few more decades at least. According to NOAA, sea surface temperatures (SSTS) have been higher in the past 30 years than at any other time since records were established in 1880 and around 70 percent of the tropical and sub-tropical oceans, where most of the reefs are located, continue to show a strong decadal heating trend.

Coral systems generally live in a symbiotic relationship with dinoflagellate algae called zooxanthellae. The coral skeleton provides the algae with a home and some nutrition while the photosynthetic algae, in exchange, provides the host with nutrition that enables reef-building and other critical activities. About 90 percent of the coral’s energy requirement comes through this arrangement. Algae, living in the nooks and crannies of coral skeleton, are what give reefs the magnificent vibrant appearance. Corals are stressed easily by disturbances in temperature, light, radiation and water composition. Under thermal stress, even with an increase of just 1-2 oC, the symbiotic relationship, vital to the health of both the coral and the algae, breaks down and the algae are systematically expelled. The expulsion causes the coral reefs to lose their colours and attain a pale and deathly look.

Bleaching, by itself, does not imply mortality. Small scale, short duration bleaching events have been reported from several locations over the past century and often these events are followed by a period of recovery when temperatures recede to normal levels. “If conditions return to normal quickly after the stress, the corals have the ability to recover. In doing so, the algae remaining in the coral tissue asexually reproduce and repopulate the coral tissue,” says Ridgewell.

Of late though, bleaching events have increasingly become a global affair with thermal stress conditions persist for several months and this is dangerous. NOAA imagery of the SSTS reveals a nearly uniform band along the equator that has experienced above normal temperatures in April and May this year. Bleaching leaves corals weak and without energy to continue with vital processes essential to their survival. So sustained bleaching drastically reduces the chances of recovery. Another problem is that dead reefs are often taken over by non-symbiotic algae, which are detrimental to the marine-life supported by the reefs. Reef Check Foundation, an international non- profit dedicated to conservation of two tropical coral reefs and California rocky reefs, has reported that dead coral reefs off Hong Kong are currently under a cover of red algae. Coral colonies, once dead, can still recover but take a longer time—ranging from a few years to decades. This depends on the local environmental factors as well as the type of corals. Carrie Man-frino, president of the Central Caribbean Marine Institute, cites coral recovery at Cayman Islands as an example. “(After the 1998 bleaching,) We conducted annual surveys and discovered that though the corals did not die immediately due to the thermal stress, a white plague disease set in and over a period of four years, we lost 40 percent of the coral community. It was not until 2009, when all of the sudden, we began seeing a rapid regrowth across the entire reef system and among all of the different species of corals,” she says. Manfrino adds that while having an extensive marine protected area did not stop bleaching; it definitely contributed to the recovery.

Even when bleached corals manage to recover, long-term survival is not guaranteed. The stress of bleaching leaves corals in a compromised position. Apart from increased susceptibility to any impending bleaching event, recovering corals suffer a reduced reproductive potential which adversely affects reef-building ability. Moreover, bleached corals are also more vulnerable to diseases and coral-eating organisms. In fact, after the mass bleaching episode in the Caribbean in 2005, much of the death associated with the event was not a direct consequence of bleaching, but due to diseases. “Once the corals regain their colour and full complement of algae, it does not necessarily mean that they are fully healthy straight away,” says Ridgewell.

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