Coral Reef and Coral Bleaching


Coral is actually a living animal.

has a symbiotic relationship (each gives something to the other and gets something back in return) With ‘zooxanthellae’ microscopic algae which live on coral [i.e. instead of living on the sea  floor, the algae lives up on the coral which is closer to the ocean surface and so that the algae gets lots of light.

The tissues of corals themselves are actually not the beautiful colors of the coral reef, but are instead clear (white). The corals receive their  coloration from the zooxanthellae living within their tissues.

  • There are two types of corals: hard corals and soft corals, such as sea fans and gorgonians. Only hard corals build reefs.
  • The builders of coral reefs are tiny animals called polyps. As these polyps thrive, grew, then die, they leave their limestone (calcium carbonate) skeletons behind. The limestone is colonized by new polyps.
  • found in tropical and sub-tropical water, there are also deep water corals in colder regions
  • The United Nations Environment Programme reports that there are more cold water coral reefs worldwide than tropical reefs.
  • There are only about 6 different coral species associated in building with these reefs.
  • The largest cold-water coral reef is the Rost ‘Reef off Norway occur in shallow tropical areas where the sea water is clean, clear and warm.
  • one of the most productive and complex coastal ecosystems with high biological diversity classified depending on their locations into fringing, patch, barrier and atoll.
  • The fringing reefs are contiguous with the shore and they are the most common – by occurring reef form, found in Andamans.
  • Patch reefs are isolated and discontinuous patches, lying shoreward of offshore reef structures as seen in the Palk bay, Gulf of Mannar and Gulf of Katchchh.
  • Barrier reefs are linear offshore reef structures that run parallel to coastlines and arise from submerged shelf platforms. The water body between the reef and the shore is termed as lagoon. Barrier reefs are seen in Nicobar and Lakshadweep.
  • Atolls are circular or semi-circular reefs that arise from subsiding sea floor platforms as coral reef building keeps ahead of subsidence. The examples are the atolls of Lakshadweep and Nicobar.
  • Among the four major reef areas of India, Andaman and Nicobar Islands are found to be very rich in species diversity followed by the Lakshadweep Islands, the Gulf of Mannar and finally the Gulf of Kachchh.

Coral Bleaching

Bleaching, or the paling of coral colour occurs; when (i) the densities of zooxanthellae decline and (ii) the concentration of photosynthetic pigments within the zooxanthellae fall.

Ecological causes of coral bleaching

  • Temperature (Major Cause)
  • Sub aerial Exposure-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.
  • Sedimentation
  • Fresh Water Dilution
  • Inorganic Nutrients(e.g. ammonia and nitrate)
  • Xenobiotics -Zooxanthellae loss occurs during exposure of coral to elevated concentrations of various chemical contaminants, such as Cu, herbicides and oil.
  • Epizootics