Prelims level : Environment Mains level : Paper III – Conservation, Environmental pollution and degradation, environmental impact assessment.
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Why in News:

  • The corals off Lord Howe island has been hit by coral bleaching.

What are Corals?

  • Coral are made up of genetically identical organisms called polyps. These polyps have microscopic algae called zooxanthellae living within their tissues.
  • The corals and algae have a mutualistic relationship.
  • The coral provides the zooxanthellae with the compounds necessary for photosynthesis.
  • In return, the zooxanthellae supply the coral with organic products of photosynthesis, like carbohydrates, which are utilized by the coral polyps for synthesis of their calcium carbonate skeletons. In addition to providing corals with essential nutrients, zooxanthellae are responsible for the unique and beautiful colors of corals.
  • They are also called the “rainforests of the seas”.

What is coral bleaching?

  • Coral bleaching is the loss of intracellular endosymbionts (Symbiodinium, also known as zooxanthellae) from coral either through expulsion or loss of algal pigmentation.
  • Some scientists consider bleaching a poorly-understood type of “stress” related to high irradiance, environmental factors like sediments, harmful chemicals and freshwater; and high or low water temperatures.
  • This “stress” causes corals to expel their zooxanthellae, which leads to a lighter or completely white appearance, hence the term “bleached”.
  • Corals can recover if the stress-caused bleaching is not severe.
  • Coral bleaching has occurred in the Caribbean, Indian, and Pacific oceans.

Great barrier reef:

  • It is the biggest coral reef system in the world composed of over 2,900 individual reefs. It was recorded as a World Heritage site in 1981. The reef is located in the Coral Sea, north east of Australia and covers an area of approximately 348,000 sq km. It is credited as the world’s biggest single structure made by living organisms and is visible from the outer space.

Causes of coral bleaching:

  • Elevated sea temperature: This is considered to be the most common factor responsible for extensive coral bleaching. Corals grow in warm shallow waters that receive plenty of light. A few species can tolerate cooler and deeper waters. Slight increases in ocean temperature can harm corals. El nino:El nino elevates the sea temperature and destroys coral reefs. It destroyed about 15% of the world’s corals.
  • Solar radiation and ultraviolet radiation: UV radiation induces coral bleaching. Global warming causes changes in tropical weather patterns which results in less cloud cover and thus more radiation. In fact, reduced cloud cover also affects solar radiation. Ocean Acidification: Due to rise in carbon dioxide levels, oceans absorb more carbon dioxide. This increases the acidity of ocean water and inhibits the corals ability to create calcareous skeletons, which is essential for their survival.
  • Chemical Pollution: Increased nutrient concentrations affect corals by promoting phytoplankton growth, which in turn supports increased numbers of organisms that compete with coral for space.
  • Infectious Diseases: Penetration of bacterium like vibrio shiloi inhibits photosynthesis of zooxanthellae. These bacteria become more potent with elevated sea temperatures.
  • Increased sedimentation due to poor land use, deforestation, dredging makes the algae more prone to bleaching.
  • Human induced threats like over fishing, especially using damaging practices like cyanide fishing, pollution from agricultural and industrial runoff, coral mining, development of industrial areas near coral ecosystems.


  • Decline in genetic and species diversity may occur when corals die as a result of bleaching. Coral bleaching can cause large shifts in fish communities. This can translate into reduced catches for fishers, which in turn impacts food supply and associated economic activities.
  • Changes in coral communities can affect the species that depend on them, such as the fish and invertebrates that rely on live coral for food, shelter. Loss of such marine animals can disturb the entire food chain.
  • Coral reefs protect coastlines by absorbing constant wave energy from the ocean, thereby protecting people living near the coast from increased storm damage, erosion and flooding.
  • Healthy coral reefs attract divers and other tourists. Bleached and degraded reefs can discourage tourism, which can affect the local economy.

Way forward:

  • Harmful industrial waste must be treated before being disposed of in bodies of water.
  • Water pollution should be avoided wherever possible by not dumping chemicals or oils in water bodies.
  • Promoting sustainable fishing and providing opportunities for ecotourism can help conserve corals.
  • There is a need to minimize the use of chemically enhanced fertilizers, insecticides, pesticides, and herbicides which are non-degradable and harm corals.
  • Taking all possible measures to prevent actions that worsen global warming since climate change is the greatest global threat to coral reef ecosystems.
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