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Mains   > Environment & Ecology   >   Biodiversity   >   Coral bleaching


  • GS 3 > Environment & Ecology   >   Biodiversity   >  Coral reefs


  • During the recently concluded TN Climate Summit 2.0, a report by the Suganthi Devadason Marine Research Institute (SDMRI), which is a pioneering institute in coral reef conservation, showed that the live coral cover in the Gulf of Mannar had declined from 37% in 2005 to 27.3% in 2021.
  • The report mentioned the reduction in the reef area from 11,060 hectares to 6,628 hectares in the region over the same period, which illustrates the magnitude of degradation affecting these vital habitats.

The decline of coral reefs in the Gulf of Mannar is attributed to several key factors:

  • Climate Change: Increases in sea temperatures and ocean acidification from rising CO2 levels cause coral bleaching and reduce coral's calcium carbonate production, essential for their structure.
  • Invasive Species: Kappaphycus alvarezii, a seaweed (alga) species introduced for commercial cultivation in the Gulf of Mannar about two decades ago, has significantly harmed its coral reefs. Recognized by the International Union for Conservation of Nature as one of the world's 100 most invasive species, its introduction has disrupted local marine ecosystems.
  • Anthropogenic Stress: Local activities, including mechanised trawling, net fishing, diving, and the collection of seaweeds, have caused direct physical damage to the corals and disturbed the ecological balance of the Gulf of Mannar's reef ecosystems.
  • Global Bleaching Events: The coral ecosystems in the Gulf of Mannar were severely impacted by global coral bleaching events, particularly those in 2010 and 2016, leading to a significant reduction in coral cover in the area.
  • Genetic Diversity Loss: The on-going decline in the health and habitat of the coral reefs in the region has resulted in a loss of genetic diversity, which is crucial for the resilience and recovery of coral populations.


  • Coral reefs are underwater structures made of coral, which are marine invertebrates. Each coral, known as a polyp, builds upon the skeletons of previous generations, forming reefs with hard, calcium carbonate exoskeletons. 
  • These reefs host a symbiotic relationship with algae called zooxanthellae, which live inside the polyps, providing them with energy and colour. 
  • Renowned for their biodiversity, coral reefs support about 25% of marine species, earning them the nickname "the rainforests of the sea."




  • Extensive submarine platforms for the formation of colonies by the coral polyps (not more than 90m below sea level.)
  • High mean annual temperature ranging 20-21°C. Hence, they are mainly seen in tropical seas and oceans.
  • Sufficient amount of sunlight and oxygen. Thus, they do not live in deeper waters i.e. not more than 60-77m below sea level.
  • Clean sediment-free water because muddy water or turbid water clogs the mouths of coral polyps resulting into their death.
  • Oceanic salinity ranging between 27-30 ppt. High oceanic salinity or fresh water is injurious for the growth of corals. Hence, they avoid river mouths and waters containing little amount of calcium carbonates.
  • Ocean currents and waves, as they bring food supply for the polyps.


  • World:  Most reef-building corals are in tropical and subtropical waters, between 30°N and 30°S latitudes. Over half of the world's coral reefs are in six countries: Australia, Indonesia, Philippines, Papua New Guinea, Fiji, and the Maldives. Corals are also found farther from the equator in places where warm currents flow out of the tropics, such as in Florida and southern Japan.
  • India: Gulf of Kutch, Gulf of Mannar, Andaman and Nicobar, Lakshadweep Islands, Netrani Island in Karnataka, Angria Bank in Maharashtra                   


  • Ecological:
    • Biodiversity: Coral reefs, less than 0.1% of the ocean's area, are home to 25% of marine species.
    • Carbon Sequestration: They mitigate global warming by removing CO2 from the atmosphere.
    • Environmental Indicator: Indicator of water quality due to their specific living conditions.
    • Air Quality: Regulate ocean and air CO2 levels through limestone formation.
  • Economic:
    • Fishing: Essential for fisheries and associated habitats like mangroves.
    • Tourism: Generate jobs and revenue through diving, hotels, and restaurants.
    • Coral Resources: Support blue economy via mining, aquarium trade, and jewellery.
    • Medicines: Source of potential drugs, including for cancer and bone grafts.
  • Disaster Management:
    • Coastal Protection: Act as barriers against storms and tsunamis, reducing coastline destruction.
    • Erosion Control: Stabilize coastlines and contribute to natural beach sand replenishment.
    • Wave Energy Reduction: Can diminish up to 97% of wave energy, protecting land and aiding sand accumulation.


  • Local threats:
    • Physical damage
      • It includes destruction from coastal development, dredging, quarrying, destructive fishing practices and gear, boat anchors and groundings, and recreational misuse (touching or removing corals).
    • Pollution that originates on land but finds its way into coastal waters. This includes:
      • Sedimentation from coastal development, urban stormwater runoff, forestry, and agriculture.
      • Excessive nutrients (nitrogen and phosphorous) from agricultural and residential fertilizer use, sewage discharges and animal waste, which can lead to the growth of algae that blocks sunlight and consumes oxygen corals need for respiration.
      • Toxic pollutants from industrial discharges, sunscreens, urban and agricultural runoff, mining activities, and runoff from landfills.
      • Trash and micro-plastics from improper disposal and stormwater runoff.
    • Overfishing
      • It can alter the food-web structure and cause cascading effects, such as reducing the numbers of grazing fish that keep corals clean of algal overgrowth.
      • Blast fishing (i.e., using explosives to kill fish) can cause physical damage to corals as well.
    • Coral harvesting
      • Coral harvesting for the aquarium trade, jewelry can lead to over-harvesting of specific species, destruction of reef habitat, and reduced biodiversity.
  • Global Threats:
    • Increased ocean temperatures due to global warming.
    • Change in ocean chemistry, caused by increasing levels of Carbon Dioxide in seawater leading to Ocean Acidification.
    • Increases in ocean acidity reduce the availability of dissolved salts and ions needed by corals to form the calcium carbonate structure, thereby slowing the growth of coral reefs. If acidification becomes severe, coral skeletons will dissolve.
    • Changes in climate, which affects the flow of ocean currents and thereby impacts the nutrient availability for corals.


  • When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white. This phenomenon is called Coral Bleaching. When a coral bleaches, it is not dead. Corals can survive a bleaching event, but as they come under more stress, they are subject to mortality.


  • Change in ocean temperature: Increased Ocean temperature caused by climate change is the leading cause of coral bleaching.
  • Runoff and pollution: Storm generated precipitation can rapidly dilute ocean water and runoff can carry pollutants these can bleach near-shore corals.
  • Overexposure to sunlight: When temperatures are high, high solar irradiance contributes to bleaching in shallow-water corals.
  • Extreme low tides: Exposure to the air during extreme low tides can cause bleaching in shallow corals.
  • Xenobiotics: A xenobiotic is a chemical substance found within an organism that is not naturally produced or expected to be present within the organism. Zooxanthellae loss occurs during exposure of coral to elevated concentrations of various chemical contaminants, such as Cu, herbicide and oil.




  • UN Sustainable Development Goals – Goal #14 (Life below water)
  • Global Coral Reef Partnership - Initiated by UNEP and Regional Seas Conventions and Action Plans to support countries deliver internationally agreed coral reef commitments through ecosystem-based management of coral reefs.
  • International Coral Reef Initiative (ICRI) - a partnership of Nations and organizations which strives to preserve coral reefs and related ecosystems.
  • Global Coral Reef Monitoring Network. 
  • Global Coral Reef Alliance.


The first report on the “Status of Coral Reefs of the World” was published in 1998 by the Global Coral Reef Monitoring Network, edited by Clive Wilkinson. Since then, the report was published regularly with updates on reef status in different regions and countries of the world.


  • Legislative:
    • Coral reef is included in Schedule I of the Wild Life Protection Act, 1972 which affords it the highest degree of protection.
    • Marine Protected Areas (MPAs): Designated areas to protect marine biodiversity and restrict harmful activities.
    • Environment Protection Act 1986.
    • Water (Prevention and Control of Pollution) Act 1974.
  • Regulatory:
    • Coastal Regulation Zone (CRZ) Notification: Regulates activities in coastal areas to protect the environment, including coral reefs.
    • Zoological Survey of India has been undertaking studies on the serious threat to coral colonies in India. The mangroves and coral reefs areas are categorized as ecological sensitive areas (CRZ-I) where no new constructions are permitted with a few exceptions
  • Research:
    • Zoological Survey of India has been undertaking studies on the serious threat to coral colonies in India.
    • National Coral Reef Research Institute (NCRI): Focuses on research and conservation across India's coral regions.

Way forward:

  • Check marine pollution: Strong and coordinated action to address land-based pollution as 80% marine pollution originates from land addressing three priority source categories, namely marine litter, nutrient management, and wastewater
  • Sustainable use of marine resources: Encouraging sustainable use of the coral ecosystem by controlling overfishing, reef mining etc. and banning harmful practices like bottom trawling, cyanide fishing, blast fishing etc.
  • Ecosystem approach: Ecosystem approach to conserving the coastal ecosystem involving coral reefs, mangroves and sea grass beds.
  • Recognizing and classifying protected areas: Promoting effective marine protected areas and regulating shipping, tourist activities, coastal construction etc.
  • Awareness generation: Creating awareness among local people about the importance of coral reefs and involving them in the conservation of coral ecosystems.
  • Use of technology: Promoting the growth of corals through artificial means like Biorock/ Mineral Accretion Technology etc.
  • Research and development:Investing in research and development and documentation of genetic characteristics of reef communities to enable the management of reef ecosystems through modern techniques and international collaboration.


Q. What are the ideal conditions for the formation of coral reefs? Discuss the significance of and threats faced by coral reefs. (15 marks, 250 words)

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