Although it is called the "Great Barrier Reef" and is often referred to simply as "the reef," both titles are misleading. The names suggest the existence of a single barrier reef, but the ecosystem really is made up of a number of different reefs -- only some of which are true barrier reefs. The rest are a combination of barrier, fringing, platform and patch reefs. Not sure which is which? We can tell you.
- Barrier reefs form along the outermost edges of the continental shelf (the underwater border of a continent) and are separated from land by a wide, deep lagoon. They echo the coastline and sometimes break the water's surface at their shallowest areas.
- Fringing reefs resemble barrier reefs in that they also parallel coastlines, but they hug the shore more. They border islands or extend outward from the mainland and are separated from land only by a narrow, shallow lagoon.
- Platform and patch reefs are both small, isolated reefs that grow on top of the continental shelf, often on underwater hills that provide a good surface for coral growth. Platform reefs tend to be oval in shape and anywhere from 1.9 miles to 6.2 miles (3 km to10 km) long [source: CRC]. Patch reefs are smaller and shallower than platform reefs.
- Ribbon reefs are long, narrow reefs (hence the ribbon name) that develop along the edge of the continental shelf. These reefs lack a lagoon. They may be up to 16 miles (25 kilometers) long but only 1,640 feet (500 meters) wide [source: Microsoft Encarta].
If you were to fly over the Great Barrier Reef, you would notice that its topography changes as you travel south. The reef is sometimes characterized as having three major sections: Far Northern, Cairns/Central, and Mackay/Capricorn (farthest south).
The northernmost section is known for having the most diversity due to its remote location and nearness to the equator. Here, the ribbon reefs dominate. Their windward sides deflect strong waves and currents, providing a calm inner section wtih scattered fringing reefs and patch reefs.
As the plane headed south, you would approach the central section. You would see the continental shelf widen and find that the reefs are farther from the mainland. The broad, shallow area created by the wider shelf hosts patch reefs and small coral islands. This section is the most accessible and gets the most tourist traffic.
Moving on to the southern section, the shelf continues to widen before narrowing and bringing the reefs back closer to land. Many submerged platform and patch reefs dot the underwater landscape. If you kept heading south, you'd see the reefs gradually disappear as the shelf narrows and temperatures dip to levels unsuitable for coral growth.
Scattered among the 3,000-plus reefs are low-lying coral or sand islands called cays. These cays support many different types of plant communities including mangrove, rainforest and grassy, depending on the rainfall they receive. The islands form from debris such as coral sand, shells and hard algae piling up against the tops and edges of the reefs.
Both the reefs and the cays benefit from this arrangement. Barrier reefs enable inner sea grass beds and mangroves to thrive by buffering them from rough seas. In turn, these formations prevent contaminants from entering the fragile ecosystem and smothering the coral by capturing nutrients and sediments in runoff. They also serve as a nursery for many of the reef's residents. You'll meet some of those interesting residents next.