Coral islands

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It is recommended to read this article together with the article Coral reefs.


Coral island development

Fig. 1. Maldives archipelago, a chain of 26 atolls in the Indian Ocean southwest of Sri Lanka and India. In the period 2000-2017, the archipelago has gained 37.50 km2 of land area by natural accretion[1]. Photo credit Fabio Di Lupo, Flickr Creative Commons.

Charles Darwin assumed that coral islands (reef islands) formed through coral colonization of slowly sinking volcanoes in mid-ocean basins. Observations a century later confirmed that this assumption holds for the majority of coral islands. Most coral islands are situated in the tropical zones of the Pacific and Indian oceans, e.g. Kiribati, Maldives (Fig. 1), 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 and physical processes, whereby waves mechanically (e.g. abrasion of the reef edge) break the skeletal structure of carbonate material[2]. Bioerosion results from the action of grazing organisms (e.g. scraping by parrotfish (Fig. 2) and urchins), mechanical boring (macroborers, including sponges, bivalves, worms and microborers, including algae, fungi, cyanobacteria and foraminifera), and chemical dissolution, mainly by microborers[3].

Fig. 2. Bumphead parrotfish. Parrotfish contribute to bioerosion by rasping algae from coral and other rocky substrates with their teeth tightly packed on the external surface of their jaw bones. For example, parrotfish grazing accounted for the production of 85% of island sand in the Maldives because of their high biomass and feeding intensity[4]. Photo credit Jenny Huang. Flickr creative commons licence.

Vertical reef building dominates when the reef has to catch up with sea level rise and lateral expansion when sea level is stabilizing or falling. 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 vertical reef growth rate was several times larger too[5][6]. In contrast, lagoon infill rates are increasing and are currently in the range of 0.5-4 mm/year. Coral islands result from the accumulation of detrital sediment (coral sand/gravel, shell fragments) derived from the reef flat by waves and currents. This started when the rate of sea level rise was slowing, about 5-6 thousand years ago. The highest sea level in the central Pacific occurred about three thousand years ago during the mid-Holocene high-stand (current sea level plus 1-2 m). Detrital sand accumulations, bound and stabilized by precipitation of calcium carbonate, reached a few meters above present sea level. This favored the development of reef islands when the sea level began to fall[7]. 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[2][8].

Coral reef island protection

Fig. 3. Example of a typical reef shape.

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[9] (Fig. 3). 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[9]. 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[10]. 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 and climate change (e.g. high temperatures, lower pH, lower light, more frequent and intense cyclones). Reef-fronted shorelines (e.g. beaches and islands) are some of the most at risk landforms to climate change due to their low-lying nature and reliance on reef-derived sediment[3]. 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[11][12][1]. Observations and models suggest that this can be explained by the influx of sediment eroded from the surrounding reefs by storm waves[13]. Under higher sea levels, reef erosion by storm waves can increase, thus providing sediments for island accretion[11][14]. 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, as sediment supply depends on live corals in the adjacent reef communities, the impact of climate change and ocean acidification on coral growth poses a serious threat. The long-term fate of many coral islands is therefore difficult to predict but probably less reassuring if the increase of the rate of sea level rise persists[11][8].


Related articles

Coral reefs


References

  1. ↑ 1.0 1.1 Holdaway, A., Ford, M. and Owen, S. 2021. Global-scale changes in the area of atoll islands during the 21st century. Owen Anthropocene 33, 100282
  2. ↑ 2.0 2.1 Kench, P.S. and Owen, S.D. 2022. Coral Systems. Ch. 8.22 of Treatise on Geomorphology (ed. Shroder, J.F.), Elsevier
  3. ↑ 3.0 3.1 Browne, N.K., Cuttler, M.V.W., Moon, K., Morgan, K.M., Ross, C.L., Castro-Sanguino, C., Kennedy, E.V., Harris, D.L., Barnes, P., Bauman, A.G., Beetham, E.P., Bonesso, J., Bozec, Y., Cornwall, C.E., Dee, S., Decarlo, T.M., D’olivo, J.P., Doropoulos, C., Evans, R.D., Eyre, Bradley D., Gatenby, P., Gonzalez, M., Hamylton, S., Hansen, J.E., Lowe, R.J., Mallela, J., O’Leary, M.J., Roff, George, Saunders, B.J. and Zweifler, A. 2021. Predicting responses of geological carbonate reef systems to climate change: A conceptual model and review. Oceanography and Marine Biology, pp. 229–370
  4. ↑ Morgan, K.M. and Kench, P.S. 2016. Parrotfish erosion underpins reef growth, sand talus development and island building in the Maldives. Sedimentary Geology 341: 50–57
  5. ↑ Montaggioni, L.F. 2005. History of Indo-Pacific coral reef systems since the last glaciation: Development patterns and controlling factors. Earth-Science Reviews 71: 1-75
  6. ↑ Gischler, E. and Hudson, J.H. 2019. Holocene tropical reef accretion and lagoon sedimentation: A quantitative approach to the influence of sea-level rise, climate and subsidence (Belize, Maldives, French Polynesia). Depositional Rec. 5: 515–539
  7. ↑ Dickinson, W. R. 2003. Impact of mid-Holocene hydro-isostatic highstand in regional sea level on habitability of islands in Pacific Oceania. J. Coastal Res. 19: 489–502
  8. ↑ 8.0 8.1 East, H.K., Perry, C.T., Kench, P.S., Liang, Y. and Gulliver, P. 2018. Coral reef island initiation and development under higher than present sea levels. Geophys. Res. Lett. 45: 11265–11274
  9. ↑ 9.0 9.1 Ferrario, F., Beck, M.W., Storlazzi, C.D., Micheli, F., Shepard, C.C, and Airoldi, L. 2014. The effectiveness of coral reefs for coastal hazard risk reduction and adaptation. Nature Communications 5: 3794 , DOI: 10.1038/ncomms4794
  10. ↑ Lirman, D. and Schopmeyer, S. 2016. Ecological solutions to reef degradation: optimizing coral reef restoration in the Caribbean and Western Atlantic. PeerJ, 4, e2597. https://doi.org/10.7717/peerj.2597
  11. ↑ 11.0 11.1 11.2 Kench, P.S., Ford, M.R. and Owen, S.D. 2018. Patterns of island change and persistence offer alternate adaptation pathways for atoll nations. Nature Communications 9: 605
  12. ↑ Duvat, V. K. E. 2018. A global assessment of atoll island planform changes over the past decades. Wiley Interdisc. Rev. Clim. Change 10, e557
  13. ↑ Tuck, M.E, Ford, M.R., Kench, P.S. and Masselink, G. 2021. Sediment supply dampens the erosive effects of sea‑level rise on reef islands. Nature Scientific Reports 11: 5523
  14. ↑ Masselink, G., Beetham, E. and Kench, P. 2020. Coral reef islands can accrete vertically in response to sea level rise. Sci. Adv. 6 : eaay3656


The main author of this article is Job Dronkers
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