Types of Coral Reefs
Most reef scientists recognize three types of coral reefs: (1) fringing reefs, (2) barrier reefs, and (3) atolls.
The traditional and most widely recognized basis for differentiating these reef types is large-scle reef morphology; the size and shape of a reef, and its relation to nearby land (if any).
This is usually (but not always) sufficient to clearly distinguish one type from the others.
Nonetheless, there is often a great deal of overlap among the major reef types (within a given biogeographic region) in terms of the dominant groups of animals and plants, as well as their ecological interactions.
Fringing Reefs (Shore Reefs)
Fringing reefs are reefs that grow directly from a shore. While there may be areas of shallow intertidal or sub-tidal sand bottom lying between the beach and the inshore edge of coral growth, there is no lagoon between the reef and shore.
The fringing reef is by far the most common of the three major types of coral reefs, with numerous examples in all major regions of coral reef development.
Without an intervening lagoon to effectively buffer freswater runoff, pollution, and sedimentation, fringing reefs tend to particularly sensitive to these forms of human impact.
In is no surprise then that increasing human populations in coastal areas - and the accompanying increases in coastal development and intensive agriculture - have resulted in the decimation of fringing reefs throuout the world in recent years.
Barrier reefs are linear reefs that parallel a shore, but are well-separated from it by an expansive lagoon (back reef).
Barrier reefs are far less common than fringing reefs or atolls, although examples can be found in the tropical Atlantic as well as the Pacific.
The 1200-mile long Great Barrier Reef off the NE coast of Australia is the world's largest example of this reef type. The GBR is not actually a single reef as the name implies, but rather a very large complex consisting of many reefs.
The second largest Indo-Pacific barrier reef lies off New Caledonia's NE coast - it is some 400 miles long with a lagoon 1-8 miles wide. Another very large barrier reef extends for nearly 170 miles to the north of Fiji and Vanua Levu.
This reef type is even rarer in the Caribbean region, where only 2 true barrier reefs are found. The largest of these runs off the coast of Belize, and the other off the north coast of the island of Providencia (east of Nicaragua).
An atoll is a roughly circular oceanic reef surrounding a large (and often deep) central lagoon.
In the Pacific, most atolls occur in mid-ocean. Examples of this reef type are common in French Polynesia, the Caroline and Marshall Islands, Micronesia, and the Cook Islands.
The Indian Ocean also contains numerous atoll formations. Examples are found in the Maldive and Chagos island groups, the Seychelles, and in the Cocos Island group.
In contrast, atolls are relatively rare in the Caribbean. Published counts range from 10-27, depending upon who is doing the classification.
The far greater number of atolls in the Indo-Pacific region (as opposed to the Greater Caribbean region) can be mainly attributed to the far greater size of the former, along with its unique geomorphology which is far more conducive to volcanic island formation and subsequent subsidence.
A Caveat on Reef Typology
Readers should be aware that identifying major reef types may not always be quite as simple as the above discussion may suggest. Some reefs seem to be intermediate versions that defy ready classification into one of the three major reef types.
This is especially true in the Caribbean region, where some of the "fringing reefs" - like that off the eastern shore of Andros Island, Bahamas (left) - are separated from shore by stretches of water that may reach 8m in depth and extend a kilometer or more from shore.
Is this truly a fringing reef, a barrier reef, or something else? Well, that depends on who you ask. Most say "fringing", but others just punt and simply call the Andros Island example a "fringing-barrier" reef.
Don't let exceptions like this throw you. Such issues are common in science because our human classification schemes often fail to account for the seemingly infinite variability found in nature.
Development Of Major Reef Types
The basic coral reef classification scheme described above was first proposed by Charles Darwin, and is still widely used today.
Darwin spent most of his coral reef explorations in the Indo-Pacific region, and viewed the three types of coral reefs he described as simply different stages in the geological 'evolution" of Pacific oceanic islands.
Darwin theorized that fringing reefs began to grow near the shorelines of new islands as ecological conditions became ideal for hard coral growth.
Then, as the island began to gradually subside into the sea, the coral was able to keep pace in terms of growth and remained in place at the sea surface, but farther from shore; it was now a barrier reef.
Eventually, the island disappeared below the sea surface, leaving only the ring of coral encircling the central lagoon; an atoll had formed (see right).
Darwin's general "reef evolution" theory was finally verified for Indo-Pacific reefs in the early 1950s after analyses of the results of deep core drilling at Bikini and Eniwetok Atolls.
However, it has also now become apparent that each of the three major types of coral reefs (described above) is often also formed by quite different geomorphic processes as well. The atoll-like Bahama Banks are a prime example of such alternate forms of reef development.
A Note On Patch Reefs
The term "patch reef" is commonly used to refer to comparatively small, isolated outcrops of coral surrounded by sand and/or seagrass (see photo, below).
While patch reefs have sometimes been described as a fourth "coral reef type", such comparisons are clearly not appropriate.
Rather, patch reefs are more properly considered regular microscale reef features that occur on all three of the macroscale reef types first described by Darwin - fringing reefs, atolls and barrier reefs.
In the sense that Darwin described coral reefs, patch reefs are not remotely comparable to the major coral reef types and should not be confused with them.