It is recommended to read this article together with the article Coral reefs.
Coral island development
Charles Darwin assumed that coral islands (reef islands) formed through coral colonization of slowly sinking volcanoes in mid-ocean basins, a theory confirmed by observations a century later. Most coral islands are situated in the tropical zones of the Pacific and Indian oceans, e.g. Kiribati, Maldives, Marshall Islands, Tokelau and Tuvalu. Coral island development results from the interplay of constructive and destructive processes, all of which are important in reef construction. Constructive processes include carbonate production by reef building corals (highly variable, up to about 4 kg CaCO3 m-2year-1) and secondary framework builders (e.g., crustose coralline algae) and benthic organisms (foraminifera, bryozoans, calcareous algae, and mollusks) and precipitation of cements that bind and stabilize sediments. Destructive processes include bioerosion, the action of organisms in destroying reef framework through mechanical boring, etching and chemical dissolution, and physical processes, whereby waves mechanically break the skeletal structure of carbonate material. Vertical reef building dominates when the reef has to catch up with sea level rise and lateral expansion when sea level has stabilized. During vertical reef growth, carbonate sediment is retained in the reef framework. However, once reefs attain sea level, excess carbonate is shed from the reef system. In the past millennium, the average rate of vertical reef growth was below 5 mm/year, while before 6000 years ago, when sea level rise was several times faster, the accretion rate was several times larger too. In contrast, lagoon infill rates are increasing and are currently in the range of 0.5-4 mm/year. Present coral islands are accumulations of detrital sediment (sand and gravel) deposited by waves and currents; these accumulations can reach several meters above sea level. Wave interaction with coral reef platforms is recognized as the principal process mechanism activating geomorphic process on reefs and controlling the formation of reef islands.
Coral reef island protection
Coral reefs provide important protection for coasts and coastal populations against the destructive forces of the sea under storm conditions. Coral reefs are particularly effective wave attenuators. Assessment of a large number of studies shows that a whole coral reef reduces wave height by 76–89%. The most important wave reduction (~64%) is due to the reef crest and a further ~43% reduction is achieved by the reef flat (Fig. 2). Both reef crests and reef flats dissipate disproportionately more wave energy as the incident wave energy increases. In cases where reefs have been degraded, recovery is more beneficial than technical repair with hard structures, in both environmental and cost aspects. Recovery of the reef crest is most effective. However, there is not yet much experience with the design of reef restoration projects. Some success has been achieved with a technique called 'coral gardening', which consists of collecting and selecting coral seed for germination in nurseries, after which the adult corals can be reintroduced on the degraded reef. Coral gardening seems to be a more promising technique than previously experimented coral transplantation. To be effective, recovery techniques require first eliminating the original causes of reef degradation.
Climate change impact on coral islands
Coral islands are highly vulnerable to sea level rise. Evidence from field observations, physical and numerical modelling suggest that the rate of sea level rise is a crucial factor, along with sediment availability. Observations show that coral islands have increased in size during the past decades in spite of an increase in the rate of sea level rise; this holds in particular for the larger sand-gravel islands. Observations and models suggest that this can be explained by the influx of sediment eroded from the surrounding reefs by storm waves. Under higher sea levels, reef erosion by storm waves can increase, thus providing sediments for island accretion. It is therefore unlikely that sea level rise will render coral islands uninhabitable within the next couple of decades as a result of physical destabilization through wave erosion, an increase in the frequency and magnitude of wave-driven flooding and saline intrusion in the groundwater reservoirs. However, the long-term fate of many coral islands is difficult to predict but probably less reassuring when the increase of the rate of sea level rise persists.
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