Mass Coral Bleaching


    In News

    • Recently, the management authority of Australia’s Great Barrier Reef confirmed that the reef is experiencing a mass coral bleaching event.

    Image Courtesy: NOAA 

    Key Findings

    • Repeating Events: 
      • 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. 
    • Past Occurrences:
      • 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. 
      • Global temperature in 2017, was the third-highest to ever be recorded. 
      • In the 2014-17 event, more than three times as many reefs were exposed to bleaching-level heat stress as compared to 1998.
    • Threat to Corals:
      • 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.
    • In Sync with La Nina:
      • It’s 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. ?

    Coral Reefs

    • About:
      • 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.
    • Symbiotic Relationship:
      • Corals share a symbiotic relationship with single-celled algae called zooxanthellae
        • The zooxanthellae also 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. 
    • Types: 
      • Hard coral:
        • They are 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:
        • Soft coral polyps, however, borrow their appearance from plants, attach themselves to such skeletons and older skeletons built by their ancestors. 
        • They 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.
    • Great Barrier Reef: 
      • Australia’s Great Barrier Reef is the world’s largest reef system stretching across 2,300 km. 
      • It hosts 400 different types of coral, gives shelter to 1,500 species of fish and 4,000 types of mollusc.

    Significance of Corals

    • Supports Marine Biodiversity: 
      • Coral reefs support over 25% of marine biodiversity, including fish, turtles and lobsters; even as they only take up 1% of the seafloor. 
    • Fishing Industry:
      • The marine life supported by reefs further fuels global fishing industries. 
      • Even giant clams and whales depend on the reefs to live. 
    • Tourism & Employment:
      • 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. 
    • Protection from Storm: 
      • Coral reefs also provide protection from storm waves.

    Coral Bleaching 

    • Process: 
      • 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. 
      • This also ends the symbiotic relationship that helps the corals to survive and grow.
    • Causes:
      • Increased ocean temperature caused by climate change is the leading cause of coral bleaching. 
      • Storm generated precipitation can rapidly dilute ocean water and runoff can carry pollutants — these can bleach near-shore corals. 
      • When temperatures are high, high solar irradiance contributes to bleaching in shallow-water corals. 
      • Exposure to the air during extreme low tides can cause bleaching in shallow corals.
    • Mass Bleaching events:
      • 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. ?
    • Declining Corals: 
      • 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.

    Way Ahead

    • 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. 
    • Bleached corals can survive depending on the levels of bleaching and the recovery of sea temperatures to normal levels. 
    • If heat-pollution subsides in time, over a few weeks, the zooxanthellae can come back to the corals and restart the partnership.

    Source: TH