Reef Structure
 

Summary

• Wave action creates a series of different habitats on a reef.
• These habitats help to create diversity.
• Waves create 3 main zones on a coral reef: Fore reef; reef crest; and reef flat.

Coral reefs are one of the most diverse ecosystems in the world, and are home to about ¼ of all marine species. Wave action helps to create this diversity. The reef bordering a lagoon is a very different habitat from the reef directly exposed to the open ocean. Waves create a patchwork of different habitats for many different marine organisms.

Reef zones

The outer parts of a coral reef are exposed to the pounding waves of the open ocean. Other parts are very sheltered. A predictable pattern of habitats forms because of this differing exposure to wave action.

image originally from USGS (http://pubs.usgs.gov/fs/2002/fs025-02/)

Fore reef zone

This part of the reef is closest to the ocean and is divided into 2 subzones:

• Deep fore reef zone

This is where the edge of the reef drops off into the depths. The deepest edge is very sheltered and can host a diverse array of coral species. Reef-building corals are gradually replaced by organisms like sponges, sea fans and solitary corals as depth increases.

• Buttress Zone

The shallow part of the outer reef is exposed to a great deal of wave action from the open ocean. This zone is a rugged area composed of spurs (buttresses) interspersed with deep channels. Buttresses help stabilize the reef and the channels funnel sediment away from the reef into deeper water.

Reef crest

This zone forms a ridge between the fore reef and the lagoon. Few coral colonies can live on the reef crest because it experiences strong wave action and is often exposed to the air at low tide. This habitat, also called the algal crest, provides a protective home for crabs, shrimps and other animals.

Reef flat zone

The reef flat zone is a very sheltered area located on the landward side of the reef. This zone is subdivided into two sections.

• Back Reef

The landward side of the reef is protected from the open ocean and forms one wall of the lagoon (with the land forming the other side).

• Lagoon

Lagoons are shallow pools of seawater with a substrate of coral rock and sand which may be exposed at low tide.

Conclusion

Coral reefs around the world differ greatly in their composition of species. However, all are subject to similar forces from the open ocean which create a similar pattern of habitats on these reefs.

References

CoRIS. (2008, August 7).  What are corals and coral reefs? NOAA's Coral Reef Information System (CoRIS). Retrieved 5 September 2008 from http://www.coris.noaa.gov/about/what_are/

Friedlander, A. et al.  (2003, August 16). Effects of habitat, wave exposure, and marine protected area status on coral reef fish assemblages in the Hawaiian archipelago. Coral Reefs 22: 291–305.