Coral Bleaching

A coral is a marine organism characterized by a stone-like or leathery skeleton. The term “coral” includes any of the about 2300 species of cnidarians in the class Anthozao. Most corals grow in warm shallow waters. These waters are low in nutrients. Corals obtain nutrients in two ways. Firstly, they can capture small fish and planktons with their tentacles. Secondly, a coral has a mutually beneficial relationship with unicellular algae called zooxanthellae that live within its tissues. Zooxanthellae assist the coral in nutrient production through its photosynthetic activities. Coral tissues themselves are not colorful but are rather transparent. These tissues receive their color from the single-celled algae.

In the past few decades coral reef eco-systems have been subject to immense stress and degradation due to unchecked human activities. The corals may expel their Zooxanthellae under stress which exposes the white skeleton behind their transparent tissues giving them a bleached appearance. This phenomenon is known as coral reef bleaching. The corals are still alive and can recover fully if the stressful conditions are not very severe or prolonged.

The worst coral bleaching incident occurred in 1998 when nearly 16% of the world’s coral reefs were lost. It was sparked by an El Niño weather pattern. In the western Indian Ocean the effect was further compounded by regional currents. The result was the bleaching of 90 percent of the coral reefs there.

Coral bleaching occurs under two conditions. First, is the decline in the densities of zooxanthellae and secondly, when the concentration of photosynthetic pigments within the zooxanthellae fall.

Causes of coral bleaching:
It can be caused by various antropogenic and natural variations in the reef environment. The most common cause of bleaching is an increase or decrease in water temparatures. Next there can also be sub-aerial exposure. When reef flat corals are exposed to the atmosphere during events such as extreme low tides or tectonic lifts, they can come under stress inducing bleaching. Increase in photo synthetically active radiation and ultraviolet radiation, and increase in sedimentation can also force the zooxanthellate species to bleach. Coral bleaching can also result from the dilution of reef waters from storm-generated precipitation and runoff. Increase in acidification of water and infectious pathogens can also result in coral bleaching. Zooxanthellae loss can occur when corals are exposed to elevated concentrations of various chemical contaminants, such as copper, herbicides and oil. A Decline in zooplankton can lead to coral starvation. Coral reefs have also started showing early signs of stress due to global warming caused by green house gas emissions.

Global warming and coral bleaching:In the past few decades global climate has changed due to a variety of reasons. One of the primary reasons of climate change has been the emission of green house gases like carbon-dioxide and methane. About 30% of increased carbon-dioxide in the atmosphere has been absorbed by the oceans. This has had a two-fold impact in accelerating coral bleaching. Firstly, the increased carbon-dioxide has altered ocean chemistry and made oceans more acidic. Secondly, increased emissions of green house gases have warmed water temperatures. Global warming is only expected to worsen in the future posing a huge threat to the coral reefs of the world.

Linus Orakles
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