World Geography / Oceanography
Coral reefs are nothing but massive features that are found in the ocean bottom floor at shallow depths. There are 3 types
of coral reefs in the world namely, Fringing Reefs, Barrier Reefs and Atolls. They are formed due to large scale accumulation of mortal
remains, shells and secretions of Calcium Carbonate bearing organisms called Corals or Polyps which live in the form of very big colonies.
They rise above the sea levels and are known as Coral islands, e.g. Lakshadweep.
Different Types of Reefs
Depending on their structure, there are 3 different types of coral reefs in the world namely, Fringing Reefs, Barrier Reefs and Atolls.
- Fringing Reef - Here the coral reef is almost connected (fringes) to the coast or land.
- Barrier Reef - Here the coral reef is separated by a narrow oceanic stretch (lagoon) from the coast. It is regarded as barrier as ships cannot reach the
The largest Coral Reef in the world, Great Barrier Reef (GBR) in North East coast of Australia and the second largest coral reef in the world,
Mesoamerican Barrier Reef System comes under this category.
- Atoll - It is a circular Coral Reef enclosing a Lagoon within it. Atolls are favourable sites of tourism. Lagoons have uniform depth and are shallow in
nature (3-4m depth).
Lagoon acts like a natural swimming pool inside the midst of the ocean. In Lakshadweep, we have large number of Atolls.
Favourable Conditions for Growth of Coral Reefs
- Minimum of 210C sea surface temperature is required for their growth. Beyond 300 Latitude (both South and North), the
temperature is not sufficient for the formation of Corals.
So, we cannot find coral reefs all around the world. They are found mainly in Tropical and
- They are found only in shallow depth regions and not in deep sea areas. That is they are formed at Continental Shelf. If the depth is more, sun rays
cannot reach the ocean bottom which is an important condition for their growth.
- The water must be transparent enough so that sunlight can penetrate inside. They are not formed at the mouth of the rivers as these areas will be muddy.
- Flat table like features are crucial for their growth in the sea floor. Submarine platforms or bands are suitable for their development.
Significance of Coral Reefs
- They are regarded as rain forests of the ocean. They are rich in biodiversity and have huge biomass productivity. Coral Reef ecosystem is the unique
marine ecosystem. It plays an important role in the nutrient transfer (cycling) in the marine ecosystem.
- They support fishing and have high economic value (as they are important fishing zones).
- They support tourism because of the nature of their colour (coral organisms are having different colours).
Threats to Coral Reefs
- The main threats to survival of the Coral Reefs of the world are Oil Drilling, Oil Spills, Marine Pollution and Coral Bleaching (decolorisation).
- Coral Bleaching happens due to loss of pigment bacteria called Zooxanthellae. If there is very high mortality of this Zooxanthellae
species, it will result in decolouration. Due to this, the Reef's structure may become weaker, leading to physical disintegration.
- Coral Bleaching is directly connected to Global Warming. Increase in the sea surface temperature leads to Coral Bleaching. During El Nino years,
there is massive bleaching.
One third of the Great Barrier Reef is facing destruction due to bleaching. GBR Marine National Park Authority is conducting
Bleach Watch on GBR.