The third global coral bleaching — after events in 1998 and 2010 — in less than two decades, and the longest and most severe so far, is laying waste thousands of square kilometres of unique oceanic ecosystems. The US National Oceanic and Atmospheric Administration (NOAA) rang the alarm in October 2015, and frightening images emerged last week of ghostly white corals in the Maldives. The bleaching, which began in 2014 and is likely to last through 2016, will impact over a third of the world’s corals, and kill 12,000 sq km of reefs.
In the northern section of the Great Barrier Reef, site of the largest bleaching, only 4 of 520 individual reefs are now healthy. Damaged or dying reefs have been found from Réunion, off Madagascar, to East Florés, Indonesia, and from Guam and Hawaii in the Pacific to the Florida Keys in the Atlantic. The longest and most severe El Niño ever has added to relentlessly warming seas, killing in weeks or months swathes of an oceanic biosphere that took up to millions of years to come into existence. It is an ecological catastrophe that can destroy the livelihoods of 500 million people, rob them of incomes worth over $ 30 billion, and change the planet forever.
What are corals?
Small (0.25-12 inches), soft-bodied marine organisms called polyps that live in spectacular colonies called reefs that they build using a limestone skeleton (calicle) lying at their base. A polyp — which may live for 2 to several hundreds of years — starts building a reef by fixing itself to a sea-floor rock, and then budding into innumerable clones that fuse into each other to create a colony that acts as one organism. The colonies grow over thousands of years, and fuse into other colonies to become reefs. Some of today’s coral reefs started growing over 50 million years ago. Corals themselves are translucent animals related to sea anemones and jellyfish, but the reefs host zooxanthellae algae, which give them a range of dazzling colours. The algae have a symbiotic relationship with the polyps, capturing sunlight and carbon dioxide to make sugars that feed the polyps. The corals also feed on zooplankton and small fish.
Why do corals matter?
Coral reef ecosystems are less than 0.1% of the ocean area but provide food and shelter to 25% of all marine species. They support fish stocks on which some 500 million people are dependent globally, including an estimated 30 million smallscale fishermen and women whose livelihoods depend directly on the reefs’ survival. Losing a coral reef can have catastrophic consequences for local food, fisheries and livelihoods.
WHAT CAN bleaching Do?
With rising ocean temperatures, some bleaching is now reported every summer from across the world. In mass or global bleaching events, entire reef systems, and not just a few individual corals, bleach. This was first recorded in 1979 — when scientists reckon that short-term temperatures increases that normally accompany El Niño events started to exceed the temperatures that corals could tolerate. The 2015-16 mass bleaching has come along with possibly the worst and longest El Niño event ever. Mass bleaching can turn a coral dominated reef to an algae dominated one in the space of a few months — a process that can take decades or longer to reverse. The reefs in the Galapagos Islands, where mass bleaching and mortality were first documented, lost over 95% of their coral during the 1982 event. It is estimated that global warming, pollution and sedimentation could kill off more than 30% of the world’s reefs well before we are in the second half of this century.
THE REASON Corals GO WHITE
Warmer water: If the temperature of the ocean is just 1 degree C higher than the average summer maximum for 4 weeks, bleaching can start.
Strong sunshine: Excessive sunlight adds to the impact of rising ocean temperatures. It is made worse by calm seas and low tides.
Healthy: The brilliant colours of a healthy coral colony comes from tiny plant-like cells that live inside the clear body tissue of the animal. These plant-like algae convert sunlight into food for the corals.
Bleached: Heat or pollution stresses the symbiotic relationship between the polyps and the algae. As the algae turn toxic, they are expelled by the coral, resulting in the coral’s white skeleton showing through.
Dead: Without enough plant cells to provide the coral with food, it soon starves or becomes diseased. With time, the tissue of the coral disappears altogether, and the exposed skeleton is covered with algae.
How common is coral bleaching?
It is becoming increasingly common as a direct result of warming oceans, which are now significantly warmer than they were 50 years ago. Some bleaching has been reported every summer in recent years — a very visible indicator of climate change.
In the Maldives, ‘skeletons glowing white’
In May, the XL Catlin Seaview Survey captured undersea images in the crystal clear waters of the Maldives, which, released on June 1, showed reefs that had remained healthy through an intense earlier bleaching event, bleached a dazzling white. “The flesh of the corals had turned clear and we were seeing the skeletons of the animals glowing white for as far as the eye could see – it was a beautiful, yet deeply disturbing sight,” Richard Vevers, executive director of the project, told The Guardian newspaper. The livelihoods of the people of the Maldives are dependent on the coral atolls, and the associated tourism and fisheries industries. The atolls also break waves and prevent lowlying islands from being flooded.
THE REEFS SURVEY
The XL Catlin Seaview Survey, a partnership between the Irish insurance giant XL Catlin, The Global Change Institute at the University of Queensland, Google, and the ocean conservation non-profit Ocean Agency (formerly Underwater Earth), is a pioneering scientific expedition revealing the impact of environmental changes on the world’s coral reefs. The Survey, currently focused on the Pacific and Indian Oceans, aims to create a high-res 360-degree panoramic vision of coral reefs that would provide a vital scientific baseline for the study of the reefs. Other partners include the International Union for Conservation of Nature, the world’s oldest and largest environmental organization, and NOAA, of the US Department of Commerce. The Survey has so far studied 32 representative reefs along the Great Barrier Reef, and collected more than 1,00,000 stitched images.
The undersea cameras: Seaview SVII and SVII-S
The Seaview SVII does for the underwater what Google’s Street View did for dry land. Rapid-fire 360-degree images are taken every 3 seconds while travelling at about 4km/h. Images can then be stitched together and published online for viewers to do a ‘virtual dive’. The geo-location and camera direction is also recorded, so that an identical picture can be taken at a later date from an identical camera position. The SVII-S is a trimmed down version of SVII, the difference being that the camera orb and underwater tablet are attached to a pole (right) rather than being mounted on an underwater scooter, making it lighter and easier to use.
Illustration: Subrata Dhar;
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