Your Online Guide to the World of the Coral Reef


Coral Reef Biome

The coral reef biome is one of the most important and distinctive types of biological communities found on Planet Earth.

In terms of biodiversity, coral reefs are the richest of all marine biomes.

Although accounting for only a tiny fraction of the total surface area of the sea, the coral reef biome is nonetheless home to nearly 25% of all known marine species.

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.

WHAT IS A "BIOME"? - A BIOME is best defined as a major biological community-type that recurs over a broad geographic scale under a particular set of environmental conditions.

Geography of the Coral Reef Biome

Examples of the coral reef biome are mainly found in shallow tropical portions of the Western Pacific, Indian, and Atlantic Oceans.

geography of coral reef biome
Geography of the coral reef biome

These are the places on Earth most ideally suited for the settlement, growth, and survival of reef-building hard corals.

For more information on where coral reefs are found, visit our web page on coral reef distribution.

Dominant Species of the Coral Reef Biome

Coral reefs are home to thousands of different species of plants,fishes, and other types of animals, but the coral reef biome is dominated by just a few main groups.

These "dominant groups" include one type of unusual animals called the stony or Scleractinian corals, and a type of marine algae called crustose coralline red algae.

Stony (Scleractinian) Corals

Most of the accumulated calcium carbonate that forms the foundation of modern coral reefs is derived from the hard, protective skeletons of countless numbers of small individual coral animals called polyps.

But coral polyps do not lead solitary lives. Instead, they live as parts of large cohesive groups called coral colonies that generally exhibit one of three basic growth forms, called massive, branching, or plate-like (pictured below).

coral growth forms
Massive (left), branching (center) and plate-like (right) are the most common growth forms of scleractinian coral colonies.

Each coral colony is composed of a great number of genetically identical individuals. Within each colony, only the outermost layers are composed of living polyps. Succeeding generations are built upon the skeletal remains of their forbearers.

The exact shape, size, and color of these colonies are subject to modification by local environmental conditions such as wave action, currents, prevailing winds, etc., often leading to substantial variability in the appearance of colonies of the same species.

Occasionally isolated colonies may be found, but coral colonies typically grow in larger multi-species assemblages that we call coral reefs.

Crustose Coralline Algae (CCA)

Unlike terrestrial biomes - whose physical structure and complexity is primarily determined by large plants - large-scale aspects of the physical structure of the coral reef biome is largely determined by animals.

crustose coralline algae
Crustose coralline algae

Still, corals are not the only dominant species found in the coral reef biome.

One group of red algae (Phy. Rhodophyta) called the crustose coralline algae or "CCA" (Subclass Corallinophycidae) are ubiquitous on coral reefs, and play an important structural role in the formation of coral reefs.

Structure of the Coral Reef Biome: Reef Zonation

One of the defining features of the coral reef biome is its characteristic large-scale structure, which consists of a series of distinct "zones" of differing species composition and topographic structure.

There are numerous ways to classify this zonation pattern, but perhaps the simplest and most practical scheme recognizes three primary, easily recognizable kinds of areas found in most.

coral reef zonation
Coral reef zonation, Moorea French Polynesia.

These are (going from shoreward to seaward) the:

Back Reef (Lagoon)

The back reef (aka "lagoon", "back reef", "reef flat") of a coral reef ecosystem is, for linear reefs (barrier and fringing reefs), that portion that lies between the reef crest and the main shoreline that the reef parallels.

In the case of circular reef systems (atolls), the lagoon is centrally located and nearly completely surrounded by the reef.

lagoon habitats

A number of distinctive habitat-types are commonly found in coral reef lagoons (see photo, above). These may include:

lagoonal patch reef
A small patch reef on a lagoonal sand plain. © Istockphoto.com

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.

Compared to other coral reef zones, this area also experiences comparatively large temperature and salinity variations, reduced water circulation, and considerable sediment accumulation.

Reef Crest

The reef crest is the shallowest part of the reef, and the narrowest of the 3 major zones that make up coral reef ecosystems. It lies between the shoreward, protected lagoon and the outer reef face.

aerial view of the reef crest
Reef crest. © Fotolia.com

The reef crest stands between the open sea and the shore - a massive wall that absorbs and dissipates the energy of incoming waves. This formidable barrier results in the calm waters of the lagoon, and is instrumental in protecting many tropical shorelines.

When large waves are present, plumes of spray splash skyward as the waves break over the leading edge of the crest. When this occurs, the crest is easily located from higher altitudes by a bright line of breaking surf along its outer margin.

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 environmentt.

Fore Reef

The fore reef zone (also often called the "reef face" or "reef front") is the outermost (seaward) of the three main zones. It begins at the seaward base of the reef crest and extends to the lower limits of coral growth.

This zone contains the greatest mass and diversity of hard corals and reef fishes found in coral reef ecosystems. For these reasons - along with the zone's extensive depth range and ease of access from the open sea - this is where most recreational coral reef scuba diving occurs.

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

Two distinctly different segments (sub-zones) of the fore reef are generally recognized. These are respectively known as the "upper fore reef" 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, often into waters too poorly lit to permit coral growth. This descent sometimes creates nearly vertical "walls" of solid coral.

Zonation Summary

The extent and development of each zone often 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.




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