Why are coral reefs called rainforests of the sea? What are the factors responsible for coral bleaching?

Why are coral reefs called rainforests of the sea? What are the factors responsible for coral bleaching?


  • Why coral reefs are called rain forest
  • Factors for coral reef to thrive
  • Factors leading to coral bleaching
  • Conclude appropriately
Model Answer :

Coral reefs comprise less than 0.1 per cent of the ocean’s total area. Scientists estimate that more than one million species of plants and animals are associated with coral reef ecosystems, making reefs one of the most diverse and important ecosystems on the planet. That is the reason Coral reefs are also called as the underwater equivalent of rainforests.

However, Corals are very sensitive and need a very specific environment to thrive:

  • Corals grow in shallow clear water where sunlight can reach them.
  • Corals are in a symbiotic relationship with zooxanthellae (algae) that grow inside of them. The algae provides oxygen and nutrition, along with the vibrant colours, to the corals, and since these algae needs sunlight to survive, corals also need sunlight to survive.
  • Reef-building corals also require warm water and saltwater conditions to survive.

Coral Bleaching:

Coral bleaching occurs when corals are stressed by changes in conditions such as temperature, light, or nutrients. When this happens, the zooxanthellae leave the corals’ bodies. This changes their color to white (as color of coral are due to algea) and can also in effect starve them of nutrients. If bleaching continues for too long, or if too many bleaching events occur in a row, corals die.

Coral reef ecosystems world-wide have been subject to unprecedented degradation over the past few decades. Disturbances affecting coral reefs include anthropogenic and natural factors:

Natural factors:

  • Increase in sea temperature : A slight increase in ocean temperature can harm corals. The El Niño in 1998 caused increase the sea temperature that resulted in the worst coral bleaching in recorded history, destroying about 15% of the world’s corals.
  • Ocean Acidification : There rise in carbon dioxide levels is increasing acidity of ocean water. This inhibits the corals ability to create calcareous skeletons, which is essential for their survival.
  • Diseases : Bacterial infections (V. Shiloi) attack zooxanthellae and cause bleaching, like in Mediterranean sea in 2003.

Anthropogenic factors:

  • Over fishing, especially using damaging practices like cyanide fishing, pollution from agricultural and industrial runoff, coral mining, development of industrial areas near coral ecosystems damage the corals.
  • Human induced climate change causes increased sea carbon dioxide levels and sea temperature, thus causing bleaching.

Coral reefs are vital for thriving ocean ecosystems. They also directly or indirectly provide food and livelihood to 500 million people apart from important services coastal protection, medicines, recreation, and tourism. All efforts must be made to conserve them and restore the habitat.

Subjects : Geography