What are coral reefs? Where are coral reefs found in the world? Zones of a coral reef.

What are coral reefs?

Coral reefs are the skeletons of stony coral polyps cemented together. Corals grow very slowly - some grow only about 3-20mm per year. Therefore, some reefs form over several

million years (Veron, 2000).

As these corals grow and die, they leave behind their calcium carbonate skeletons. On these skeletons, other corals grow. As the years pass, walls of coral begin to form: massive walls of rock. As the waves and currents beat upon these reefs, nooks, crannies, ledges and caverns form in these walls.

Just as there are different types of corals, there are different types of coral reefs. The three main types of reefs are fringing reefs, barrier reefs and atolls (Veron, 2000).

Fringing reefs

Fringing reefs are coral reefs that grow in shallow waters. They closely border the coastline or are separated from it by a narrow stretch of water. Many of the reefs round Sri Lanka and Thailand are fringing reefs.

 

Fringing reef

Barrier reefs

Barrier reefs grow parallel to the coast, but are separated from land by a lagoon. They are found sometimes many kilometers from shore (10–100km). Barrier reefs can grow in fairly deep water, because, often, the living coral builds upon remains of corals that grew in the same area when sea level was lower, during the last ice age. The Great Barrier Reef of Australia extends about 2,010km parallel to the east coast.

Barrier reef

Atolls

Atolls grow to surround (or partly surrounding) an island which then sinks relative to sea level (usually because volcanic activity forming the island stops), or was flooded as sea level rose after the last ice age. Atolls surround (or partly surround) a central lagoon. The Maldives consists of 26 atolls.

Atoll

Although these are the three main types of reefs, there are many reefs that do not fit these models.

Other reef types or variants are:

Patch reef – common, isolated, comparatively small reef outcrop, usually within a lagoon or embayment, often circular and surrounded by sand or seagrass

Apron reef – short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore

Bank reef – linear or semicircular shaped-outline, larger than a patch reef

Ribbon reef – long, narrow, possibly winding reef, usually associated with an atoll lagoon

Table reef – isolated reef, approaching an atoll type, but without a lagoon

Habili – reef specific to the Red Sea; does not reach the surface near

enough to cause visible surf; may be a hazard to ships (from the Arabic for "unborn")

Microatoll – community of species of corals; vertical growth limited by average tidal height; growth morphologies offer a low-resolution record of patterns of sea level change; fossilized remains can be dated using radioactive carbon dating and have been used to reconstruct Holocene sea levels

Cays – small, low-elevation, sandy islands formed on the surface of coral reefs from eroded material that piles up, forming an area above sea level; can be stabilized by plants to become habitable; occur in tropical environments throughout the Pacific, Atlantic and Indian Oceans (including the Caribbean and on the Great Barrier Reef and Belize Barrier Reef), where they provide habitable and agricultural land

Seamount or guyot – formed when a coral reef on a volcanic island subsides; tops of seamounts are rounded and guyots are flat; flat tops of guyots, or tablemounts, are due to erosion by waves, winds, and atmospheric processes.

Where are coral reefs found in the world?

Coral reefs are found

• where the sea is shallow (less than 100m);

• where the sea is warm (usually between 25° and 29°C);

• and therefore, are located within the latitude of 30°N to 30°S i.e., only in tropical seas.

Although corals exist both in temperate and tropical waters, shallow-water reefs form only in a zone extending from approximately 30° N to 30° S of the equator. Tropical corals do not grow at depths of over 50 meters (160 ft). The optimum temperature for most coral reefs is 26–27 °C (79–81 °F), and few reefs exist in waters below 18 °C (64 °F). However, reefs in the Persian Gulf have adapted to temperatures of 13 °C (55 °F) in winter and 38 °C (100 °F) in summer.[29] There are 37 species of scleractinian corals identified in such harsh environment around Larak Island.

Zones of a coral reef

Coral reef ecosystems contain distinct zones that represent different kinds of habitats. Usually, three major zones are recognized: the fore reef, reef crest, and the back reef (frequently referred to as the reef lagoon).

All three zones are physically and ecologically interconnected. Reef life and oceanic processes create opportunities for exchange of seawater, sediments, nutrients, and marine life

among one another.

Thus, they are integrated components of the coral reef ecosystem, each playing a role in the support of the reefs' abundant and diverse fish assemblages.

Most coral reefs exist in shallow waters less than 50 m deep. Some inhabit tropical continental shelves where cool, nutrient rich upwelling does not occur, such as Great Barrier Reef. Others are found in the deep ocean surrounding islands or as atolls, such as in the Maldives. The reefs surrounding islands form when islands subside into the ocean, and atolls form when an island subsides below the surface of the sea.

Alternatively, Moyle and Cech distinguish six zones, though most reefs possess only some of the zones.

The reef surface is the shallowest part of the reef. It is subject to the surge and the rise and fall of tides. When waves pass over shallow areas, they shoal, as shown in the diagram at the right. This means the water is often agitated. These are the precise condition under which corals flourish. Shallowness means there is plenty of light for photosynthesis by the symbiotic zooxanthellae, and agitated water promotes the ability of coral to feed on

plankton. However, other organisms must be able to withstand the robust conditions to flourish in this zone.

The off-reef floor is the shallow sea floor surrounding a reef. This zone occurs by reefs on continental shelves. Reefs around tropical islands and atolls drop abruptly to great depths, and do not have a floor. Usually sandy, the floor often supports seagrass meadows which are important foraging areas for reef fish.

The reef drop-off is, for its first 50 m, habitat for many reef fish who find shelter on the cliff face and plankton in the water nearby. The drop-off zone applies mainly to the reefs surrounding oceanic islands and atolls.

The reef face is the zone above the reef floor or the reef drop-off. This zone is often the most diverse area of the reef. Coral and calcareous algae growths provide complex habitats and areas which offer protection, such as cracks and crevices. Invertebrates and epiphytic algae provide much of the food for other organisms. A common feature on this forereef zone is

spur and groove formations which serve to transport sediment downslope.

The reef flat is the sandy-bottomed flat, which can be behind the main reef, containing chunks of coral. This zone may border a lagoon and serve as a protective area, or it may lie between the reef and the shore, and in this case is a flat, rocky area. Fishes tend to prefer living in that flat, rocky area, compared to any other zone, when it is present.

The reef lagoon is an entirely enclosed region, which creates an area less affected by wave action that often contains small reef patches.

However, the "topography of coral reefs is constantly changing. Each reef is made up of irregular patches of algae, sessile invertebrates, and bare rock and sand. The size, shape and relative abundance of these patches changes from year to year in response to the various factors that favor one type of patch over another. Growing coral, for example, produces constant change in the fine structure of reefs. On a larger scale, tropical storms may knock out large sections of reef and cause boulders on sandy areas to move."