The back reef zone (also commonly called the lagoon) of coral reef ecosystems lies immediately shoreward of the innermost margin of the reef crest zone, and extends all the way to the shore.
Although often not even considered part of “the reef” by the uninitiated, this zone (like the others) is an integral part of coral reef ecosystems. It contains a variety of shallow water habitats that play vital roles in the lives of many reef associated species and the coral reef food web.
Portions of the back reef zone may be exposed at low tide, and (compared to other coral reef zones) experiences comparatively large temperature and salinity variations, reduced water circulation, and considerable sediment accumulation. The main habitat types found in the back reef zone include patch reefs, sand flats, seagrass meadows, and sometimes mangrove forests.
Patch reefs are a key element of back reef zones, providing refuge and food for many species of coral reef invertebrates and fishes. The term “patch reef” is usually taken to mean a comparatively small assemblage of coral colonies, while isolated single colonies are generally called “coral heads”. Patch reefs typically take on a round to oval shape. They may be composed of just a few or a wide variety of corals, and range from the size of a small car or larger than a football field.
Patch reefs are by far the highest and most complex structures found amidst the comparatively flat featureless sand plains flats and seagrass meadows that make up most of the back reef. Thus, small as they may be in comparison with coral formations of the reef crest and fore reef zones, patch reefs (and to a lesser extent, isolated coral heads) play a vital role in providing adequate shelter for many coral reef fishes and invertebrate animals. The other habitat types of the back reef zone offer little protection from the voracious predators (e.g., barracudas, sharks, etc.) that constantly prowl these areas during the daylight hours in search of an easy meal.
Patch reefs occur in all three of the main types of coral reefs; atolls, barrier reefs, and fringing reefs. They also sometimes occur on the fore reef, but there they are generally close to much larger coral formations and take on less significant ecological roles.
Sand flats (also called “sand plains”) are expanses of nearly featureless coralline sand. In many cases sand flats occupy most of the area of the back reef (lagoon), only occasionally interrupted by smaller patches of the other habitats types of the zone.
Sand flats may appear on casual inspection to be more or less barren marine “deserts”, but in reality are themselves home to a variety and abundance of marine animals. The vast majority of these remain are small invertebrates that remain well hidden beneath the sand during the hours of daylight.
The life dwelling within the expanses of sand represents a major food source for a number of fishes that reside in, or regularly visit, the back reef zone. Some of these forage and feed here by day, using specialized appendages and techniques to locate and capture hidden prey. Many others feed in the sand plains only under the relatively safe cover of darkness, when hidden multitudes of tiny invertebrate animals emerge from the sand to feed on the rich plankton of the darkened water column above.
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