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Coral Reefs

Types of Reefs

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There are three basic types of coral reefs; fringing, barrier and atoll. Fringing reefs are located very close to shore, and because of water run off they are typically high in nutrients and the water has a high turbidity. Barrier reefs are further from shore, with a lagoon between the reef and the shore. And finally atolls are a circular reef with a central lagoon and possibly small islands formed on the reef.


Barrier Reefs

Fringing reefs

Types of Corals


Atolls form when a volcanic island subsides below sea level leaving the coral reef as a ring around a central lagoon. Coral growth maintains the circular reef but reduced turbulence in the lagoon leads to sedimentation and reduced coral growth in that area. Hundreds of atolls are attached to seamounts in the south Pacific Ocean.


The windward side of the coral reef is on the eastern side due to the direction the Trade Winds blow. The Trade Winds, dominating on Earth around the equator from about 30 degrees north to 30 degrees south, blow pretty consistently from east to west. Thus the eastern sides of tropical islands have the biggest waves and receive the brunt of most of the storms moved from east to west by the Trades.

The leeward side of the reef is on the western side and is protected by the algal ridge. In fringing and barrier reef stages it is also protected by the crustal oceanic island. This is the side of the island where the buttress and groove formations are most prevalent. As one descends the side of a typical reef four different areas are usually encountered - the buttress zone, transition zone, the living base, and the dead base.

The buttress zone is found from low tide to 20 meters deep. It may extend upwards from the outer rim of the lagoon but not to as shallow a depth as the algal ridge. This is an area of some surf, high oxygen, and lots of light. Often some of the hardier coral live here and there may be many buttresses and grooves. Tunnels may be present leading from the lagoon through the buttress zone either from the coral reef growing together across a groove or from the erosive forces of sand from the lagoon. Some of these tunnels provide divers with amazing short journeys from the lagoon, through the inside of the reef, to the open ocean.

The transition zone is found from 20 to 50 meters deep and has a variety of species. Light is decreasing in this area and zooxanthellae do not do as well as farther up in shallower water. The number of filter feeders (like sponges) is increasing in this area and the number of reef building corals is decreasing. There is a good deal of organic material, from the upper reef, that falls here, providing food for filter feeders (like sponges).

The living base is found from 50 to 150 meters deep and is dominated by sponges. Living species are common here but the light is so reduced that it is hard for the reef building corals to survive. Some corals do live here especially those that grow very flat and plate-like with their polyps on the top facing the surface (to collect the most light for their zooxanthellae).

The dead base is found below 150 meters and has few living species as compared to the reef areas closer to the surface. This base is almost solid calcium carbonate from the corallites of the reef building corals - as well as ahermatypic corals, snails, sea urchin spines, and coralline algae. This is the area where the original fringing reef started, when the oceanic island first formed.

The algal ridge is made almost entirely of pink encrusting coralline algae (left). Encrusting coralline algae

An algal ridge is common on the windward reef. It extends up to the surface of the ocean and forms a protective cap on the coral reef. The algal ridge is composed mostly of coralline algae. Coralline algae secrete large amounts of calcium carbonate in their tissues. Encrusting corallines ( common on algal ridges) adhere this calcium to the reef and cover the coral like a layer of pink cement. The coralline algae can withstand extensive wave pounding and needs a high oxygen content in the water. So, the areas of breaking waves are the best for it. As it grows up to the surface, this windward algal ridge actually serves as a protection for the lagoon behind it. At low tide the top of the ridge may be exposed but only a couple of times a month at extreme low tides. The pink (mauve) areas shown in these pictures show an extensive algal ridge exposed at an extreme low tide.

Typical coral reef zonation (buttress - to 20 meters, transition - between 20 and 50 meters, living base - from 50 to 150 meters, and dead base - below 150 meters) showing the algal ridge on the windward side (east) of a tropical island.

Barrier Reefs, if the land mass starts to subside, then continued coral growth will produce a barrier reef separated from the land by a lagoon. The Great Barrier Reef is the best known.

Fringing Reefs produced along the submerged edges of islands or continents where larval corals settle out and develop. Common in the Caribbean Sea.



Hermatypic coral

Types of Corals

Corals can be either ahermatypic of hermatiypic

Ahermatypic corals do not form reefs and can grow in the absence of light. They live can live in deep water and are distributed world wide. They can do this because they do not need zooxanthallae to live.

Hermatypic coral forms reefs in tropical regions and possess zooxanthallae, therefore they need to be shallow enough where the zooxanthale can have access to light, since they are photosynthetic. The picture shown to the right iis a hermatypic coral. The circular star shaped depressions are called polyps. The zooxanthale live with in these polyps. Some ahermatypic corals have polyps but they are very small.

hermatypic coral
Questions? Contact Ms. Kman at Page last updated Saturday, October 30, 2004