Lagoon (Back Reef)

The lagoon (aka back reef zone) of a coral reef ecosystem is, for linear reefs (barrier and fringing reefs), that portion that lies between the reef crest and the shoreline. In the case of circular reef systems (atolls), it is centrally located.

A coral reef lagoon is a comparatively sheltered environment that contains a number of productive and unique habitats.

The extent, depth, and habitat complexity of coral reef lagoons varies substantially with reef type and local sea floor topography.

Some parts of the lagoon 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 “lagoon” or “back reef”, particularly in discussions of fringing reefs.

Habitats Of The Lagoon

Four distinctive habitat-types are commonly found in coral reef lagoons. These include:

Patch Reefs
Sand Plains
Seagrass Meadows
Mangrove Forests

Not all of these habitats are always present, and their comparative extent and developemnt often differs substantially at different reefs.

Patch Reefs

Patch reefs are key habitat elements of coral reef lagoons, 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.

patch reef with many fish — Patch reef. © Istockphoto.com

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 lagoonal habitat-types (see photo, left).

For larger fishes and invertebrate animals, most lagoon 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, lagoonal 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

Sand plains (aka “sand flats“) are expanses of nearly featureless coralline sand. In many cases sand flats occupy most of the area of the lagoon, 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, coral lagoons. 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.

Seagrass Meadows

When seagrasses occur within the protected lagoonal waters of coral reef ecosystems, these plants often form dense and extensive meadows. Typically, seagrass meadows are interspersed with large areas of sand plain habitat.

Seagrass meadows represent a distinctive habitat type within coral reef ecosystems. They play a key role in the lives of many coral reef animals, including fishes, invertebrate animals, and visiting marine reptiles and mammals.

Sponges and other animals provide shelter for small reef fishes in seagrass meadows — Sponges provide refuge for small fishes in a seagrass meadow. © Fotolia.com

These meadows may serve as primary home, foraging ground, nursery area and/or breeding ground for many species of coral reef lagoon residents and visitors.

Seagrass meadows also provide food and shelter for some of the most highly valued (commercially) species of invertebrates, including most notably conch and spiny lobster. Today, 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 are mainly small invertebrates. These seagrass dwellers rely heavily on concealment during daylight hours, either through camouflage or burrowing. A few, like the once-ubiquitous conch, have large stout shells to protect them and can safely roam about in the open.

At night however, many of the larger reef fishes that shelter by day on nearby patch reefs or in the outer reef zones move into the seagrass meadows to forage under the cover of darkness.

Mangrove Forests

When present within the coral reef biome, mangroves occupy the shoreward margins of the ecosystem. For this reason, except in fringing reef systems, mangroves are usually located quite distant from the main biomass of hard corals that form the massive reef crest and fore reef.

Nonetheless, when present these plants have the capacity to considerably enhance the abundance and diversity of reef fishes and other types of marine life throughout the coral reef ecosystem. 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 animals attach.

In the Caribbean region, research has demonstrated that these plants have a strong influence on nearby coral reef fish community structure.

Falling leaves and nesting birds add nutrients to the water below, thereby enhancing opportunities for the growth of other nearby marine life. Also, 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.

Despite these documented ecosystem benefits, mangrove forests are clearly not essential to the formation or maintenance of healthy coral reef ecosystems. Many extensive and well-developed coral reefs exist without them.

What Are Coral Reefs? | Where Are Coral Reefs Located? | How Are Coral Reefs Formed? | Types of Coral Reefs | Coral Reef Zones

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