ISRS “Coral Reefs and Ocean Acidification” briefing paper

Synopsis

This briefing paper summarizes the current knowledge of ocean acidification and how it will affect coral reefs, identifies future research needs, and addresses how ocean acidification should be included in overall coral reef management strategies.

Coral reef organisms and the structures that they build will be increasingly exposed in the coming decades to progressive decreases in seawater pH, associated with the oceanic uptake of carbon dioxide produced by fossil fuel burning, deforestation, cement production, and other human act ivities. These changes in seawater chemistry, popularly termed “ocean acidification”, have been correlated wit h decreased production of calcium carbonate by organisms, along wit h increased calcium carbonate dissolution rates. The evidence that increased carbon dioxide in the atmosphere can have such direct effects on marine ecosystems is compelling but recent. While the calcification 1 response of some calcifying organisms is well characterized, the overall effects of reduced calcification rates on coral reef ecosystems have been barely invest igated. Nonetheless, the potential negative consequences of ocean acidification on coral reefs argue strongly for measures to mitigate further increases in atmospheric carbon dioxide concentrations.

  • Calcification rates in corals, which are the best studied coral reef organisms, will decrease by 30±18% by the time atmospheric carbon dioxide concentrations reach twice the preindustrial level in 30–50 years. Fewer studies have been conducted on coralline algae, Halimeda, and other calcifying algae that contribute to reef sediments, but for reasons both biological and geochemical, their calcification rates are expected also to decrease with ocean acidification. The effects of reduced skeletal growth on the survival and fitness of corals, algae and other calcifying organisms will depend on the function or functions of the skeleton. For example, weaker skeletons will probably afford less protection to coral polyps, while slower growth rates will probably decrease the ability of species to compete for space, and prolong sexual maturity; but these assumptions have not been tested. The effects of reduced skeletal growth on juvenile stages, settlement, and recruitment of calcifying organisms are essentially unknown.
  • Most experiments have not indicated negative impacts on coral tissue growth under elevated carbon dioxide. Indeed, recent experiments have shown that some species cultured under high carbon dioxide concentrations can lose their skeletons altogether without apparent physiological stress or reductions in growth, and then resume skeletal building once carbon dioxide levels are returned to normal.
  • Calcification refers to the process by which organisms secrete calcium carbonate to build their skeletons or shells. Dissolution rates of calcium carbonate rock and sediments on coral reefs will increase in the future. Coupled wit h reduced biological calcification rates, the net calcium carbonate production rates on coral reefs will decrease; that is, reef building will decrease.

ISRS, 2008. Coral reefs and ocean acidification. Briefing Paper 5, International Society for Reef Studies, 9 pp. Article.

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