Herald News

Rising sea temperatures are destroying swathes of coral reef

15 Apr 2017 12:16am IST

Report by
Freddy Dias

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15 Apr 2017 12:16am IST

Report by
Freddy Dias

Scientists estimate that by 2050, the oceans would be largely devoid of coral reefs as climate change and our relentless plundering of the sea set in motion geological changes not seen for millions of years. Coral reefs represent just one per cent of our oceans but support 25 per cent of the species that live in them. If millions of people, who depend on the reefs for their livelihood disappear, the attendant loss of food security would drive economic migration on a massive scale. 

Climate change and the consequent abnormal warming of the oceans, has been consistently killing marine life in the last few decades, according to a report of the World Wide Fund for Nature (WWF). Fish species essential to global food supply – especially in poorer nations where fish provides essential dietary protein – were among the hardest hit. The dwindling fish availability is mainly on account of destruction of large swathes of coral reef, mangroves and sea grasses as a result of rising sea temperatures. “Half of all coral has already disappeared, and the rest will vanish by 2050, if temperatures continue to rise at the current rate” the report warned.

For instance, studies have revealed that large sections of the Great Barrier Reef, the Earth’s largest living structure, are dying. Sustained high sea temperatures have stressed the corals to the point where they expel the brightly coloured algae that live within their tissues. This process is aptly named bleaching as it removes all pigment and exposes the shocking white calcium skeleton of the reef structures. The coral can survive in this state for up to a few weeks. Thereafter, if temperatures do not decrease than they die. 

Experts point out that the problem caused by mass bleaching on the Great Barrier Reef have continued to deepen, with the latest estimates based on results from recent surveys showing that 35 per cent of corals are now dead or dying along the northern and central sections of the reef. Biologists tracking this severe bleaching event have documented the damage as abnormally high water temperatures caused the coral to bleach – losing the algae that live within their tissues and supply most of the coral’s energy. From surveys, it has been estimated that this has happened to one-third of the coral reef’s section and half of the reef’s central section that was hit hardest by the bleaching event. 

The Great Barrier Reef has experienced bleaching events twice before. The first, in 1998, was bad enough, with 50 per cent of the reef affected. 2002 was even worse with 60 per cent of the reef bleached. Recently surveys of the reef evaluated 500 different sections. Given the record-shattering temperatures over the past year, coral researchers were preparing themselves for another mass bleaching. But the scale of this event has left many of the researchers stunned, particularly in view of the fact that there was not a single coral bleaching event on the Great Barrier Reef in the 400 years prior to 1998. It is now amply evident that the underlying driver in this latest episode of coral destruction is the increasing global temperatures as a consequence of human-made climate change, experts say. 

Corals are made up of tiny modules, called polyps, that are joined together to form colonies. Coral polyps and their colourful algae have a tightly coupled relationship, where each aids the other. The algae are autotrophic photosynthesizers, which means they make their own food by using sunlight to split carbon dioxide molecules and form sugars. Coral polyps are much more complex heterotrophs – they use fine-meshed sieves to sweep up suspended organic matter from sea water.

For the algae, a coral reef represents the perfect place to live. There, they are safe and secure within a strong structure, near the surface and so able to receive large amounts of energy from the sun along with coral polyp waste, which helps promote photosynthesis. The photosynthesis is so productive that the algae produce more food than they can consume. This surplus is gobbled up by the coral polyps. It’s a win-win situation – what biologists call a mutualistic symbiosis. However, high temperatures along with very bright sunlight can impact both coral polyps and their algae, as their relationship breaks down leading to the expulsion of the algae and bleached coral.

Other disturbances, like cyclones, can also cause destruction of algae and consequent bleaching of corals, but their effects are usually more localised, meaning that recovery is easier. Corals that are bleached totally white – having lost nearly all their symbiotic algae, have an extremely low chance of recovering because it takes several months for the algae to come back.

Biologists say that bleaching can also be driven by pollution, particularly agricultural run-off and sedimentation from activities such as dredging. These two factors threaten the more southern portion of the Great Barrier Reef which at least so far has dodged some of the very worst bleaching as sea temperatures have been lower. The northern stretch of the reef is more remote and better protected from human impacts. 

Experts are of the opinion that mass coral bleaching events happen mainly when ocean temperatures are abnormally warm. Consequently, reducing greenhouse gas emissions is the main way we can help to prevent more global coral bleaching events from striking in the future. Improving water quality and controlling outbreaks of crown-of-thorns starfish are also very important, because reducing coral loss due to these other factors can increase the resilience of a reef to bleaching. Technological approaches, like shading reefs or artificially restocking reefs, might be considered for specific small reef sites, but they are not feasible for the conservation of the 2300km Great Barrier Reef, or for coral reefs on a global scale. 

(The writer is a freelance journalist)
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