Coral reefs and the phenomenon of bleaching

(Mains GS 3 : Conservation, Environmental Pollution and Degradation, Environmental Impact Assessment.)

Context:

• Recently, the management authority of the world’s largest coral reef system, Australia’s Great Barrier Reef, confirmed that the reef is experiencing a mass coral bleaching event.
• This is the sixth time that the coral reef system is being hit by a widespread and damaging bleaching event and the fourth time in six years that such an event has occurred.

About coral reefs:

• Coral reefs are large underwater structures composed of the skeletons of colonial marine invertebrates called coral.
• The coral species that build reefs are known as hermatypic  or ‘reef building’  or "hard," corals because they extract calcium carbonate from seawater to create a hard, durable exoskeleton that protects their soft, sac-like bodies.
• Other species of corals that are not involved in reef building are known as “soft” corals which are flexible organisms often resembling plants and trees and include species such as sea fans and sea whips.

World’s largest reef:

• 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.
• Corals share a symbiotic relationship with single-celled algae called zooxanthellae and this zooxanthellae give corals their bright colour.
• The algae provides the coral with food and nutrients, which they make through photosynthesis, using the sun’s light. In turn, the corals give the algae a home and key nutrients.
• Australia’s Great Barrier Reef is the world’s largest reef system stretching across 2,300 km and hosts 400 different types of coral, giving shelter to 1,500 species of fish and 4,000 types of mollusc.

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 or food-producing algae living inside coral polyps 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 and ending the symbiotic relationship that helps the corals to survive and grow.

Impact of global warming:

• Bleached corals can survive depending on the levels of bleaching and the recovery of sea temperatures to normal levels.
• If heat-pollutions subside in time, over a few weeks, the zooxanthellae can come back to the corals and restart the partnership but severe bleaching and prolonged stress in the external environment can lead to coral death.
• Over the last couple of decades, climate change and increased global warming owing to rising carbon emissions and other greenhouse gases have made seas warmer than usual.
• Under all positive outlooks and projections in terms of cutting greenhouse gases, sea temperatures are predicted to increase by 1.5°C to 2°C by the time the century nears its end.

Mass bleaching event:

• The first mass bleaching event 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. In the past decade, however, mass bleaching occurrences have become more closely spaced in time, with the longest and most damaging bleaching event taking place from 2014 to 2017.
• This started with reefs in Guam in the Western Pacific region getting affected, then affecting the North, South-Pacific, and the Indian Ocean.
• In the 2014-17 event, more than three times as many reefs were exposed to bleaching-level heat stress as compared to 1998.

• A 2021 study by the Global Coral Reef Monitoring Network (GCRMN), which is supported by the United Nations, showed that 14% of the world's coral on reefs had been lost between 2009 and 2018, with most of the loss attributed to coral bleaching.

Broad importance:

• Coral reefs support over 25% of marine biodiversity, including fish, turtles and lobsters; even as they only take up 1% of the seafloor.
• The marine life supported by reefs further fuels global fishing industries and even giant clams and whales depend on the reefs to live.
• Besides, coral reef systems generate $2.7 trillion in annual economic value through goods and service trade and tourism.
• In Australia, the Barrier Reef, in pre-COVID times, generated $4.6 billion annually through tourism and employed over 60,000 people including divers and guides.
• Aside from adding economic value and being a support system for aquatic life, coral reefs also provide protection from storm waves.
• Dead reefs can revive over time if there are enough fish species that can graze off the weeds that settle on dead corals, but it takes almost a decade for the reef to start setting up again.

The Great Barrier Reef: 

• The United Nations Intergovernmental Panel on Climate Change (IPCC) in its report warned that the life of the Great Barrier is in grave danger.
• The 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.
• Just a couple of weeks after this warning, the Barrier Reef Authority confirmed a mass bleaching phenomenon affecting all pockets of the reef system.

Unexpected event:

• The Authority said that its a first 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.
• The chief scientist of the reef managing authority said that the climate is changing and the planet and the reef is about 1.5 degrees centigrade warmer than it was 150 years ago and because of that unexpected events are now to be expected.
• The Authority further stated that bleached reefs are affected at different levels from mild to severe, and while stressed, they are alive and could recover if temperatures moderate.

Conclusion:

• The UNESCO mission is currently in Australia to assess how much the country’s administration is doing to pull the reef out of danger.
• Thus climate activists demanded to show the true picture of the reef’s condition, rather than just being shown the vividly coloured unaffected reef patches.