Barrier reefs, like fringing reefs, are associated with land masses, but are separated from land by a lagoon. These reefs grow parallel to the coast and are large and continuous. They develop at a much greater distance from the shore than fringing reefs, and are usually found 1-5 km from shore, but can also be found farther out than that: for instance, the Australian Great Barrier Reef may be as far as 100 km off the mainland. They can also be mid-ocean reefs, such as in the case of Bora Bora.
Reef-building corals are located between 30°N and 30°S, where the environmental conditions favour their growth. Therefore, barrier reefs can be expected to be found within these latitudes, where temperature, sunlight, wave action and salinity concentration are ideal. Furthermore, the warm waters that favour the growth of reef-building corals are often found along the eastern shores of major landmasses. Thus, the two largest barrier reefs, Australia’s Great Barrier Reef and the Meso-American Reef in the Caribbean, are both located on eastern coastlines.
Barrier reefs are the biggest type of reef. They may be 300-1000 m wide, and sometimes consist of a few parallel ‘ribbon reefs’. They may develop in the same way as atolls, building up around the edge of an oceanic volcanic island that is subsiding to form a barrier around the island, until the island disappears completely, forming an atoll. Alternatively, they might form as a result of changing sea levels that create suitably high underwater relief to allow reef-building corals to grow.
Growth of a barrier reef occurs in five stages; the first is a primary reef framework growth of coral and coralline algae that provide a reef structure of live and dead animals. This is followed by secondary reef framework growth, which essentially consists of binding and encrusting over the primary framework by coralline algae, corals and forams. The third stage is formed by physical (waves) and bio-erosion (e.g. animal grazing) of this framework to produce void-filling sediments, which leads to the fourth stage, known as sedimentation. This stage occurs when the existing voids are filled and the structure is strengthened. Finally, the whole structure is cemented and consolidated by further growth. The coral growth of a barrier reef continues outward.
Barrier reefs also include reef flats (the area of the reef not exposed), the reef crest - which runs parallel to the coast and is protected from waves - and a coral terrace (a slope of sand with isolated coral peaks). These features are followed by another coral terrace and a vertical drop into deeper waters. The zoning of barrier reefs, created mainly by patterns of wave energy, current direction and intensity and underwater light, will have an impact on the species that live in each zone. Areas of highest wave action (i.e. the reef front) will be dominated by encrusting forms, followed by branching and massive forms as the wave energy dissipates.
Barrier reefs can be classified according to their age and stage of development. The younger reefs are juveniles and show little evidence of lateral expansion. As they mature, they start to expand laterally and fill in the adjacent lagoon with sediment. Finally, in the senile stage, they completely fill in the lagoon, creating a coral cay. They may also be classified according to their morphology, and six types of reefs can be distinguished: submerged, patches, crescentic, lagoonal, planar and ribbon.
The best-known and most studied barrier reef is Australia’s Great Barrier Reef, which incorporates almost one-fifth of the world’s reef area. This reef stretches more than 2000 km, from Rockhampton in the south to past Cape York in the north, and is the longest barrier reef that exists on Earth. It is not one single reef, as often believed, but instead consists of more than 3000 reefs, of which 760 are fringing reefs. It contains all six types of reef, of which the most common are submerged (566) and planar (544), followed by patches (446). Lagoonal (270) and crescentic (254) reefs are less common and ribbon reefs (66) are limited to the northern reaches of the Great Barrier Reef.
The area, covering 345,000 km2, was declared a marine park in 1975, with the purpose of preserving the area’s outstanding biodiversity whilst providing for reasonable use of the Park. It is probably the best-known and the largest marine park in the world and was declared a World Heritage Area in 1981 in recognition of: (i) its outstanding natural universal values representing the major stages in the earth’s evolutionary history; (ii) an outstanding example representing significant ongoing ecological and biological processes; (iii) an example of superlative natural phenomena; and (iv) containing important and significant habitats for in situ conservation of biological diversity.
The second largest barrier reef is the Meso-American reef. This barrier reef stretches from Mexico’s Yucatan peninsula to Honduras.
Barrier reefs are highly valuable for the communities that live near them. One of their most important roles is to shelter the land from harsh ocean storms and floods, allowing in many regions coastal development in areas that might otherwise be unsuitable. Reefs also provide important resources for fisheries and the tourism industry. More than 100 million tourists visit coral reefs around the world each year, and its tourism income is estimated at over US$140 billion/year worldwide. Finally, reefs are becoming increasingly important to biotechnology companies that are discovering important medical compounds, including anti-cancer treatments and sunscreen compounds, that exist within the defensive chemicals produced by corals.
Reefs also face some major threats however, from human activities and natural causes. Some of the major human threats include: (i) increasing coastal development; (ii) water pollution; (iii) tourism; (iv) damage caused by boats and ships; (v) increasing sea temperatures; (vi) destructive fishing practices, including poison and dynamite fishing for reef fishes; and (vii) the souvenir trade. Significant natural threats include rising sea temperatures that cause coral bleaching and the impact of the crown of thorns starfish, which is a major coral predator. While classed as natural, both of these threats may be human induced.
Many of these risks are highlighted in the Reefs at Risk report (Burke et al., 1998). The significance of this report is that it encourages strategic management to be implemented with the help of concerned communities and not-for profit organizations that are able to monitor the health of the reef and involve communities in programmes to help safeguard the health of barrier reefs around the world.
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