The back reef zone (also commonly called the lagoon) of coral reef biomes lies immediately shoreward of the innermost margin of the reef crest zone, and extends all the way to the shore.
Although generally not 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 may be exposed at low tide, and (compared to other coral reef zones) this area experiences comparatively large temperature and salinity variations, reduced water circulation, and considerable sediment accumulation.
NOTE ON TERMINOLOGY: Coral reef zone terminology differs among reef scientists. Thus, you occasionally may see the term “reef flat” used to denote the same zone we refer to here as the “back reef”, particularly in discussions of fringing reefs.
The main habitat types commonly found in the back reef zone include:
Not all of these habitats are always present in back reef zones, and their comparative extent often differs substantially at different reefs.
Patch reefs are a key element of back reefs, providing refuge and food for many species of coral reef invertebrate animals 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 range from the size of a small car to larger than a football field, and may be composed of just a few or a wide variety of corals. The corals found here are relatively tolerant of the high levels of sedimentation characteristic of this zone.to which they are regularly subjected.
By far, they are the largest and most complex structures found amidst the flat featureless sand plains and seagrass meadows that comprise most of the back reef zone habitat types (see photo, left).
For larger fishes and invertebrate animals, most back reef habitats offer little else in the way of protection from the voracious predators such as sharks and barracudas that constantly prowl these areas during the daylight hours in search of an easy meal.
Thus, small as they may be compared to the extensive coral formations of the reef crest and reef face, lagoon patch reefs play vital roles in providing adequate shelter for the larger forms of marine life that dwell in or visit the lagoon.
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 plains (also called “sand flats”) are expanses of nearly featureless coralline sand. In many cases sand flats occupy most of the area of the back reef zone, only occasionally interrupted by smaller patches of the other habitats types found here.
Sand plains 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 invertebrate animals that remain well hidden beneath the sand during the hours of daylight.
However, in some locations an abundance or occasional group of large octocorals may provide substantially greater structural complexity to these otherwise barren seascapes (see photo, left).
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.
Seagrasses are a specialized group of marine plants most often found in shallow, sheltered marine or estuarine waters.
When they occur within the protected back reef zone of coral reef ecosystems, these plants often form dense and extensive “seagrass meadows”, which are interspersed with large areas of sand plain habitat.
Seagrasses are fast growing plants capable of high production rates. Thus, the lush seagrass meadows of the back reef zone are a key element in coral reef food webs.
Also, by trapping suspended sediments and slowing water movement, seagrass meadows benefit nearby coral reefs by reducing sediment loads in the water.
Seagrass meadows form a distinctive habitat type that serves as home, primary foraging ground, nursery area and/or breeding ground for many species of back reef residents and visitors.
They play a key role in the lives of many coral reef animals, including fishes, invertebrate animals, and visiting marine reptiles and mammals. Seagrass meadows are considered critical habitat for some types of sea turtles and (before being hunted nearly to extinction) manatees and dugongs.
Compared to coral reefs, seagrass habitats have little physical complexity. For larger animals, there is scarce shelter here above the grass blades save for the occasional sponge (see photo, left), octocoral, isolated coral colony, or patch reef.
Thus, the animals active here by day in most of the expanses of seagrass meadows are mainly small invertebrates and fishes. Such seagrass dwellers rely heavily on concealment during daylight hours, typically either through camouflage or burrowing. At night however, many of the larger reef fishes that shelter by day on nearby patch reefs or outer reef zones forage here under the cover of darkness.
Seagrass meadows provide food and shelter for some of the most highly valued (commercially) species of invertebrates, including most notably conch and spiny lobster.
Mangroves are a unique group of large shrub-like plants that grow in thick, physically complex “forests” that line many tropical and sub-tropical shores.
One of the rare terrestrial plants able to tolerate direct immersion in sea water, they have adapted to saline conditions where other plant life cannot survive by means of salt-filtering roots and salt-excreting leaves.
These specialized adaptations enable this group of plants to play a key role in creating land from the sea. Sediment mounds accumulated by seagrass meadows within the back reef zone may eventually form sufficient substrate for initial mangrove colonization.
Once established in this manner, these hardy pioneers grow and propagate rapidly, increasingly adding to the accumulation of ever greater amounts of sediments.
In this capacity, they are instrumental in the building of new shorelines and small islands within coral reef lagoons and other protected inshore waters.
When present within coral reef ecosystems, mangrove forests occupy the shoreward margins of the back reef. Mangroves have the capacity to contribute a good deal to coral reef ecosystem productivity and biological diversity by providing food and shelter for a variety of other forms of marine plant and animal life.
In the Caribbean region, research has demonstrated that these plants have a strong influence on nearby coral reef fish community structure.
When associated with adult habitat such as seagrass meadows or coral reefs (see photo, left), they substantially increase the biomass of some commercially valuable species.
The complex prop root system serves as a nursery habitat that increases the survival rates of young reef fishes, as well as a substrate to which all manner of algae, sponges, and other invertebrate coral reef animals may attach.
Falling leaves and nesting birds add nutrients to the water below, thereby enhancing opportunities for the growth of other nearby marine life.
Additionally, by trapping and consolidating sediments that would otherwise be swept away and dissipated by the sea, the presence of these plants reduces the chance of nearby seagrasses and corals being damaged by excessive sedimentation.
Nonetheless, these plants are clearly not essential to the formation or maintenance of healthy coral reef ecosystems; many extensive and well-developed coral reefs exist without them.
Our pages on the other two major zones of the coral reef biome can be accessed through the menu below.
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