Published on: March 19, 2022




Australia’s Great Barrier Reef is suffering widespread and severe coral bleaching due to high ocean temperatures two years after a mass bleaching event


  • Calcareous rocks, formed from the skeletons of minute sea animals, called polyps.
  • The polyps extract calcium salts from seawater to form hard skeletons which protect their soft bodies. These skeletons give rise to corals.
  • The corals live in colonies fastened to the rocky seafloor. New generations develop on skeletons of dead polyps. The tubular skeletons grow upwards and outwards as a cemented calcareous rocky mass collectively called corals.
  • The shallow rock created by these depositions is called a reef. These reefs, later on, evolve into islands.
  • The corals Occur in different forms and colours, depending upon the nature of salts or constituents they are made of.
  • The progressive development of corals appears over the sea surface in different forms over a period of time. Small marine plants (algae) also deposit calcium carbonate, thus contributing to coral growth.


  • Great Barrier Reef is the world’s most extensive coral reef environment, with over 2,900 distinct reefs and 900 islands.
  • The reef is located off the coast of Queensland, Australia, in the Coral Sea (North-East Coast).
  • It is visible from space and is the world’s biggest structure built by living organisms.
  • It was designated a UNESCO World Heritage Site in 1981.


  • Coral reef ecosystems worldwide have been subject to unprecedented degradation over the past few decades. Disturbances affecting coral reefs include anthropogenic and natural events.
  • Recent accelerated coral reef decline seems to be related mostly to anthropogenic impacts (overexploitation, overfishing, increased sedimentation, and nutrient overloading.
  • Natural disturbances which cause damage to coral reefs include violent storms, flooding, high and low-temperature extremes, El Nino Southern Oscillation (ENSO) events, etc.
  • Coral bleaching occurs when the relationship between the coral host and marine algae, which give coral much of their colour, breaks down. Without the marine algae, the tissue of the coral animal appears transparent and the coral’s bright white skeleton is revealed. Coral reef bleaching is a common stress response of corals to many of the various disturbances mentioned above.
  • Corals begin to starve once they bleach. While some corals are able to feed themselves, most corals struggle to survive without their algae. If conditions return to normal, corals can regain their algae, return to their normal colour and survive. However, this stress is likely to cause decreased coral growth and reproduction, and increased susceptibility to disease.
  • Bleached corals often die if the stress persists. Coral reefs that have high rates of coral death following bleaching can take many years or decades to recover.


  • Coral species live within a relatively narrow temperature margin and therefore, low and high sea temperatures can induce coral bleaching. Bleaching events occur during sudden temperature drops accompanying intense upwelling episodes, seasonal cold-air outbreaks etc.
  • Bleaching during the summer months, during seasonal temperature and irradiance maxima often occurs disproportionately in shallow-living corals and on the exposed summits of colonies.
  • Sudden exposure of reef flat corals to the atmosphere during events such as extreme low tides, ENSO-related sea level drops or tectonic uplift can potentially induce bleaching.
  • Rapid dilution of reef waters from storm-generated precipitation and runoff has been demonstrated to cause coral reef bleaching.
  • Other causes includes the increase in the concentration of inorganic Nutrients, sedimentation, oxygen starvation caused by an increase in zooplankton levels as a result of overfishing, ocean acidification, changes in salinity, sea level change due to global warming, cyanide fishing etc