Your Online Guide to the World of the Coral Reef

How Are Coral Reefs Formed?

Coral reef formation involves a variety of processes operating at different spatial scales, and our discussion of how coral reefs are formed reflects that reality.

At smaller spatial scales we can examine reef building process at the level of the individual reef.

At broader spatial scales, we can consider the processes affecting the structure and formation of entire coral reef ecosystems.

How Are Individual Reefs Built?

The structural foundation of individual coral reefs is formed by a multitude of marine animals and plants through the processes of slow accumulation and deposition of calcium carbonate (limestone) extracted from seawater.

The Main Reef Builders

While a wide variety of marine life ultimately contributes to the structural complexities of coral reefs, most of the reef's underlying solid framework is constructed by just a few types of marine organisms.

These are the hard (Scleractinian) corals, and the coralline red algae that grows upon and amidst the coral colonies.

encrusting coralline algae grows upon a reef surface
Coralline algae. Courtesy NOAA

Shallow marine waters are rich in calcium (Ca++) and bicarbonate (HCO3-) ions.

Hard corals and coralline algae are able to extract these raw materials from sea water, and both types of marine life have the ability to combine them to form calcium carbonate (CaCO3).

This substance, also known as limestone, is used to fashion the protectecive outer skeletons of coral polyps as well as to harden the fleshy parts of the coralline algae.

After the individual organisms die, they leave behind their limestone skeletons. Over time, the accumulated and compacted minerals contained in the multitudes of these skeletal remains become the large, solid structures we call coral reefs.

The overall reef-building process is slow; coral reefs are built over decades and centuries - not weeks or months.

Other Reef Builders

A great variety of other organisms - including many types of invertebrate animals - assist the two primary types of reef builders (discussed above) in the reef-building process by depositing lesser amounts of carbonate particles.

These minor contributors include sponges, octocorals ("soft" corals), fire corals, and many other types of invertebrates. Individually, the contribution of each of these groups may be comparatively small, but collectively their contribution can sometimes be substantial.

How Are Coral Reef Ecosystems Formed?

At the broader spatial scale of the entire reef ecosystem, structure and formation is shaped not only by the dominant animals and plants.

WHAT IS AN ECOSYSTEM? - Here, we define "coral reef ecosystems" as large, interactive areas of coral reef development containing many individual reefs and associated habitats (e.g., seagrass meadows). These ecosystem extends from the shorelines of tropical islands and coasts to the deep blue waters of the open sea, and includes all the marine life within the area as well as the non-living parts of their environment.

Rather, interactions among a host of other abiotic ("physical") factors AND the underlying geology of the area also come into play. In fact, these physical factors usualy represent THE dominant influences on large-scale reef structure.

Most notably, such factors include sea floor depth profile, substrate composition, water movement, light penetration, and other variables that affect hard coral species distribution, growth forms, and abundance.

These varied and complex interactions typically impose a characterstic "zonation" pattern upon the overall reef ecosystem. This is true whether the reef is of the atoll, barrier, or fringing type.

Coral Reef Ecosystem Zonation

There are numerous ways to classify the zonation patterns typical of coral reef ecosystems, 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. ©

The habitat composition and extent of coral reef lagoons varies substantially with reef typelocal 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. ©

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 environment.

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. ©

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.

Types of Coral Reefs ►