The phenomenon of coral bleaching

News: The management authority of the world’s largest coral reef system, Australia’s Great Barrier Reef, confirmed on 25th Mar that the reef is experiencing a mass coral bleaching event.

This is the 6th time that the coral reef system is being hit by a widespread and damaging bleaching event, and the 4th time in six years that such an event has occurred.

The United Nations Intergovernmental Panel on Climate Change (IPCC) in a recent report said that if temperatures continue to rise, bleaching events may occur more often and a large proportion of the remaining reef cover in Australia could be lost.

What are coral reefs?

Corals are marine invertebrates or animals not possessing a spine. Each coral is called a polyp and thousands of such polyps live together to form a colony, which grows when polyps multiply to make copies of themselves.

Types of corals: Hard coral and soft coral.

– Hard corals, also called hermatypic or ‘reef building’ corals extract calcium carbonate (also found in limestone) from the seawater to build hard, white coral exoskeletons.

– Soft coral polyps, however, borrow their appearance from plants, attach themselves to such skeletons and older skeletons built by their ancestors. Soft corals also add their own skeletons to the hard structure over the years and these growing multiplying structures gradually form coral reefs. They are the largest living structures on the planet.

• Read more about corals here

What is coral bleaching?

Bleaching happens when corals experience stress in their environment due to changes in temperature, pollution or high levels of ocean acidity.

Under stressed conditions, the zooxanthellae start producing reactive oxygen species, which are not beneficial to the corals. So, the corals expel the colour-giving zooxanthellae from their polyps, which exposes their pale white exoskeleton, giving the corals a bleached appearance. This also ends the symbiotic relationship that helps the corals to survive and grow.

Bleached corals can survive, depending on the levels of bleaching and the recovery of sea temperatures to normal levels.

But, severe bleaching and prolonged stress in the external environment can lead to coral death.

What have been the global coral bleaching events?

The first mass bleaching event had occurred in 1998 when the El Niño weather pattern caused sea surfaces in the Pacific Ocean to heat up. This event caused 8% of the world’s coral to die.

The second event took place in 2002.

The longest and most damaging bleaching event, took place from 2014 to 2017. This started with reefs in Guam in the Western Pacific region getting affected, to then affecting the North, South-Pacific, and the Indian Ocean.

​Why are coral reefs significant?

Coral reefs support over 25% of marine biodiversity, including fish, turtles and lobsters, while taking up 1% of the seafloor.

The marine life supported by reefs further fuels global fishing industries.

Besides, coral reef systems generate $2.7 trillion in annual economic value through goods and service trade and tourism.

Aside from adding economic value and being a support system for aquatic life, coral reefs also provide protection from storm waves.

Why the current bleaching event at Great Barrier Reef is significant?

It is the first time that the current bleaching event has occurred during a La Niña weather pattern, when warm areas in the Pacific Ocean shift, giving more cloud cover, rain and creating cooler weather conditions over the reef.

Source: This post is based on the article “The phenomenon of coral bleaching” published in The Hindu on 29th Mar 22.