• A recent drone analysis has revealed that the worst mass coral bleaching event in the Great Barrier Reef has destroyed corals at Australia’s Lizard Island in just three months.

Key Finding

• About 97 percent of the coral reefs around Lizard Island have suffered from bleaching and subsequent death.
• The intense bleaching highlights the rapid and devastating impact of prolonged heat stress on marine biodiversity.

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Corals

• About: Coral consists of genetically identical organisms called polyps, which contain microscopic algae known as zooxanthellae within their tissues. 
• Mutualistic Relationship:The corals and algae share a mutualistic relationship.
  • The coral supplies the zooxanthellae with the compounds needed for photosynthesis. 
  • In return, the zooxanthellae provide the coral with organic products of photosynthesis, such as carbohydrates, which the coral polyps use to synthesise their calcium carbonate skeletons. 
    • Besides supplying essential nutrients, zooxanthellae also give corals their unique and beautiful colors.
• Types of corals:  There are two types of corals:
  • Hard corals, also known as stony corals, are reef-building organisms that extract calcium carbonate from seawater to construct their rigid, white exoskeletons. 
  • Soft coral polyps mimic plant-like structures and attach themselves to existing and ancestral skeletons.
    •  Over time, soft corals contribute to these structures by adding their own skeletons, gradually forming expansive coral reefs through growth and reproduction.
• Coral Reefs: Coral reefs consist of colonies comprising hundreds to thousands of tiny individual corals known as polyps.

Coral Bleaching

• About: A prolonged rise in Sea Surface Temperature (SST) prompts zooxanthellae, which contribute pigmentation and fulfil 90% of corals’ nutritional requirements, to depart from their hosts, leading to the whitening of corals known as ‘coral bleaching’.
• Survival: Bleached corals may survive depending on the severity of bleaching and if sea temperatures return to normal levels. 
  • If conditions improve promptly, within a few weeks, zooxanthellae can return to the corals and resume their symbiotic partnership.
  • However, severe bleaching and prolonged stress in the surrounding environment can ultimately result in coral mortality.
• Occurrence: Coral bleaching has been a recurrent occurrence in the Caribbean, Indian, and Pacific oceans.
• Global Mass Coral Bleaching Events: According to the United States National Oceanic and Atmospheric Administration (NOAA), global mass coral bleaching events have been documented in 1998, 2010, and 2014-2017. 
  • The current period from 2023 to 2024 is now recognized as the fourth global mass coral bleaching event.

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Causes of Coral Bleaching

• Increasing Sea Surface Temperatures: Greenhouse gases, accumulating in the atmosphere, trap heat radiated from the Earth. Excess heat is absorbed by the oceans, causing Sea Surface Temperatures (SSTs) in tropical regions to rise nearly 1°C over the last century, with current trends indicating an increase of 1-2°C per century.
• Marine heatwaves: It causes a rapid increase in Sea Surface Temperatures (SST), resulting in the expulsion of zooxanthellae from corals and subsequent coral bleaching.
  • A recent underwater survey revealed that 85% of corals in the Gulf of Mannar near the Tamil Nadu coast experienced bleaching following the marine heatwave in 2020.
• El Nino: El Niño events cause specific areas to heat above their average temperatures, stressing corals and resulting in bleaching.
• Sedimentation: Sedimentation occurs when bottom trawling and waste dumping along coastlines deposit sediments that hinder the photosynthesis of zooxanthellae, ultimately leading to coral bleaching.
• Biological invasion: It occurs when invasive species, such as seaweeds, colonise bleached corals before they can recover from bleaching events. 
  • Outbreaks of crown-of-thorns starfish have caused significant damage to reef-building corals in the Great Barrier Reef. 
  • The invasive alga Kappaphycus alvarezii poses a threat in the Gulf of Mannar region.
• Xenobiotics: Coral can lose zooxanthellae when exposed to high concentrations of chemical contaminants like copper, herbicides, and oil.
• Epizootics: Pathogen-induced bleaching differs from other types of bleaching. Many coral diseases lead to patchy or entire colony mortality and the shedding of soft tissues, leaving behind a white skeleton.

Strategies to Mitigate Coral Bleaching

• Multilateral Cooperation: The International Coral Reef Initiative (ICRI) is a global informal partnership among nations and organisations dedicated to conserving coral reefs and associated ecosystems worldwide. Established in 1994 during the First Conference of the Parties of the Convention on Biological Diversity, ICRI includes India as a member.
  • The World Coral Conservatory project involves establishing coral banks in aquariums across Europe. These banks could potentially restore natural coral reefs affected by climate change or pollution.
• Biorock Technology: Biorock technology involves an innovative method of mineral accretion used to create natural building materials underwater, facilitating coral restoration efforts. For example, it has been employed in restoring coral reefs in the Gulf of Kachchh.
• Super Corals: The concept of “super corals” involves breeding corals ex-situ that are resilient to high temperatures, using a process known as “human-assisted evolution.”
• Long-term Structural Initiatives: Implementing sustainability measures to reduce carbon footprint and marine pollution through improved waste management.
  • Empowering coastal communities with awareness and training to build resilience, enabling them to engage in sustainable fishing and conservation efforts.

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