Your Online Guide to the World of the Coral Reef

Coral Reef Biome

by William Alevizon Last Modified

The coral reef biome is one of the most charismatic and distinctive of Earth's major ecosystems.

This colorful biome consists of a localized concentration of tropical limestone (calcium carbonate) reefs of biogenic origin, along with a characteristic array of associated marine life and habitats (see below).

Together, these features comprise tropical marine ecosystems of unparalleled biological diversity. Although accounting for only a tiny fraction of the total surface area of the sea floor, the coral reef biome is nonetheless home to nearly 25% of all known marine species.

coral reef biome distribution
Geography of the coral reef biome

Examples of the coral reef biome are numerous and widespread; they can be found throughout broad reaches of tropical portions of the Pacific, Indian, and Atlantic oceans.

Unfortunately, among all of the Earth's marine biomes, the world of the coral reef is also the most threatened by the ever-expanding impacts of human activities. In 2014 alone, another 20 species of corals have been afforded "threatened" status under provisions of the U.S. Environmental Protection Act.

Biome Marine Life

The coral reef biome is occupied by a unique assemblage of living things. Notably, the large-scale physical structure of this biome is primarily determined by animals. This is quite unlike the situation found in most terrestrial biomes, in which large plants are the main architects of habitat structure.

coral colony
Stony coral colony

As the name implies, the dominant organisms of the coral reef biome are a group of small animals called corals - more specifically, the stony (Scleractinian) corals that are the chief reef-builders. Most of the limestone in a coral reef is derived from the activities of countless numbers of these animals.

Also present (and abundant on some reefs) are another type of colonial coral - the octocorals (aka "soft corals"). These have flexible skeletons, and seem somewhat plant-like as the colorful colonies gently bend and sway with water movement over the reef

While primary, corals are not the only dominant species found in the coral reef biome. A specialized type of red algae (Phylum Rhodophyta) called the crustose coralline algae ("CCA") are also ubiquitous on coral reefs, and play an important role in reef formation.

crustose coralline algae
Coralline algae

Besides corals and CCA, the characteristic biota of the coral reef biome includes a great variety of other invertebrate animals, along with diverse fishes and a few other types of vertebrates. Common plant groups include seagrasses and mangroves. Many varieties of algae and micro-organisms are also usually present.

Structure of the Coral Reef Biome

A distinguishing feature of the coral reef biome is a characteristic ecosystem structure, which consists of a series of distinct "zones" of differing species and habitat composition.

zones of coral reef biome
Zones of the coral reef biome

There are numerous ways to classify this zonation pattern, but perhaps the simplest and most practical scheme recognizes the three distinctive areas (i.e., "zones") found in most.

From shoreward to seaward these are the: (1) Back Reef (Lagoon), (2) Reef Crest, and (3) Fore Reef ("Reef Face").

The extent and development of these zones varies considerably at different geographic locations, and even within different parts of single reef systems. In some places, one zone may abruptly transition to another, while in other places such transitions are more gradual. At some locations an entire zone may be poorly developed or entirely missing.

Back Reef (Lagoon)

For linear reefs (fringing and barrier), the back reef (aka "lagoon") lies between the reef crest and the shore. The lagoons of atolls are centrally located and nearly completely surrounded by the reef.

A number of distinctive habitat-types are commonly found in coral reef lagoons, including:

The habitat composition and extent of coral reef lagoons varies substantially with reef type, local sea floor topography, and a host of other variables. Some parts of the lagoon may be sufficiently shallow so as to be regularly exposed at low tide.

lagoon habitats
Habitats of the coral reef lagoon (backreef)

Compared to the other two zones, the lagoon experiences comparatively large temperature and salinity variations, reduced water circulation, and considerable sediment accumulation.

Reef Crest

The reef crest is the shallowest and narrowest of the 3 zones. It lies between the shoreward lagoon and the seaward fore reef.

reef crest
Waves break on reef crest

The crest stands between the open sea and the shore - a massive wall that absorbs and dissipates the energy of incoming waves, resulting in the calm waters of the shoreward lagoon. The crest thus serves as a natural seawall, and is instrumental in protecting tropical shorelines.

The reef crest is easily located from above the surface by a bright line of breaking surf along its seaward margin. The crest rises so near the sea surface that at low tide some of its corals may be exposed to the air for a few hours.

reef crest corals
Reef crest from underwater

The composition and structure of the reef crest varies considerably with prevailing wind direction, severity of wave action, type of coral reef, and geographic location. The crest is generally best developed to windward, where regular exposure to powerful waves creates a high-energy environment.

Fore Reef

The fore reef zone (also often called the "reef face") is the outermost (most seaward) of the three zones. It begins at the outer margin of the reef crest and extends to the lower depth limits of coral growth. This zone contains the greatest mass and diversity of hard corals and reef fishes found in the entire coral reef biome.

scuba divers on the lower reef face
Scuba Divers exploring the lower fore reef. ©

Two distinctly different segments (sub-zones) of the fore reef are generally recognized. These are respectively known as the "upper" and "lower" fore reef.

The upper fore reef begins immediately seaward of the reef crest. It is characterized by a comparatively gentle downward slope and very high coral diversity. This sub-zone usually extends to depths somewhere between about 15 to 20 meters.

Then - where conditions permit - the sea floor begins to slope ever more steeply. It is here that the "upper" fore reef gives way to the "lower" segment, characterized by different dominant corals. The lower fore reef continues to plunge downward, sometimes creating nearly vertical "walls" of solid coral.

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