In the Zone

No two coral reefs are exactly alike. Each one is a dynamic and ever-changing structure. Despite their differences, most coral reefs display several distinct zones that are created by environmental conditions such as wave and current strength, suspended sediment content, temperature, and depth of the water. Zones vary somewhat, depending on ocean location and type of reef, but most reefs have four typical zones: the reef flat, reef crest, buttress, and seaward slope.

The part of the reef that is closest to the shore is called the reef flat, or back reef. In this area, living things are protected from the full force of the breaking waves; however, water on the reef flat is relatively shallow, ranging in depth from a few centimeters to a couple of meters. Shallow-water inhabitants are exposed to wide variations in temperature and salinity. They must also deal with changing water levels and occasional periods when the low tide leaves them stranded without water. These factors limit the types of organisms that survive in the reef flat.

Moving from the shore toward the ocean, the second zone is the reef crest, also called the algal ridge. This is the highest point of the reef, and it is exposed to the full impact of waves that rush from the ocean toward the shore. During times of low tide, this area is fully exposed to the penetrating rays of the Sun. As in the reef flat, only a limited number of organisms can survive in this zone.

The third zone, traveling seaward, is the buttress. This area begins at the point where low tide waters cover the reef and continues out to a depth of about 65.6 feet (20 m). Jagged extensions of the buttress zone jut from the reef out into the ocean. The undulating shape of the buttress zone diverts water striking the reef into many direction. By spreading the impact of the waves, the reef buttress helps the structure withstand their full power and impact. Channels in the buttress drain debris and sediment out to sea. With plenty of sunlight and oxygen present, huge reef-building coral and algal colonies develop in the upper part of the buttress zone. The corals that grow on top of the buttress tend to develop short, thick branches, while those further under the water look like small shelves or branched plants.

In the Zone
The final zone, the seaward slope, begins where the buttress zone ends, just below 65.6 feet in depth. The upper section of this area, which receives the most sunlight, holds many different species of coral. Below 131.2 feet (40 m) deep, fewer are found because sediment builds up in the water, blocking the light. This deepwater region supports a lot of sponges and non-reef-building corals.

All of the zones of coral reefs support complex groups of living things, including more than 3,000 different kinds of animals. Competition among these organisms for available food and space is intense. Some species may overgrow and squeeze out others in an effort to utilize nutrients and light. Many other animals share the space effectively by limiting the times they forage for food; for example, some only come out at night, and others are active in the daytime.