Global warming and acidification threaten coral survival

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Global warming and increasing acidification of marine waters threaten the long-term survival of coral reefs, according a review published in the most recent issue of the journal Science.

The review by Ove Hoegh-Guldberg and coauthors paints a bleak future for coral reefs in the light of increasing carbon dioxide levels in the Earth's atmosphere.

The increased carbon dioxide level increases the acidity of oceanic waters by increasing the amount of carbonic acid (which is formed when carbon dioxide dissolves in water) entering it.

Carbonic acid dissociates to form bicarbonate ions and protons, which in turn react with carbonate ions to produce more bicarbonate ions, reducing the availability of carbonate to biological systems (including coral, which need the carbonate to build reefs).

Decreasing carbonate-ion concentrations thus reduce the rate of reef building corals, and elevated levels of carbonic acid in seawater eventually leads to erosion of existing reefs.

Citing the results of previous studies, the authors show that a doubling of pre-industrial atmospheric carbon dioxide concentration to 560 ppm decreases coral calcification and growth by up to 40% through the inhibition of aragonite formation (the principal crystalline form of calcium carbonate deposited in coral skeletons) as carbonate-ion concentrations decrease.

According to the authors: Reef-building corals may exhibit several responses to reduced calcification, all of which have deleterious consequences for reef ecosystems.

First, the most direct response is a decreased linear extension rate and skeletal density of coral colonies...Second, corals may maintain their physical extension or growth rate by reducing skeletal density.

However, erosion could be promoted by the activities of grazing animals such as parrotfish, which prefer to remove carbonates from lower-density substrates. Increasingly brittle coral skeletons are also at greater risk of storm damage...thus, if rates of erosion outstrip calcification, then the structural complexity of coral reefs will diminish, reducing habitat quality and diversity.

A loss of structural complexity will also affect the ability of reefs to absorb wave energy and thereby impairs coastal protection. Third, corals may maintain both skeletal growth and density under reduced carbonate saturation by investing greater energy in calcification.

A likely side effect of this strategy is the diversion of resources from other essential processes, such as reproduction, as seen in chronic stress...which could ultimately reduce the larval output from reefs and impair the potential for recolonization following disturbances.

Based on analyses of data, the authors write: Increases in the temperature of tropical and subtropical waters over the past 50 years...have already pushed reef-building corals close to their thermal limits.

Projections for ocean acidification include reductions in oceanic pH by as much as 0.4 pH units by the end of this century, with ocean carbonate saturation levels potentially dropping below those required to sustain coral reef accretion by 2050...

Changes in ocean acidity will vary from region to region, with some regions, such as the Great Barrier Reef and Coral Sea, and the Caribbean Sea, attaining risky levels of aragonite saturation more rapidly than others.

It is a sobering thought that serious, if not devastating, consequences for the future of coral reefs are foreseen even though the authors have used the lower limits for projections of global warming and ocean acidification in their analyses.

For more information, see the paper: Hoegh-Guldberg, O, PJ Mumby, AJ Hooten, RS Steneck, P Greenfield, E Gomez, CD Harvell, PF Sale, AJ Edwards, K Caldeira, N Knowlton, CM Eakin, R Iglesias-Prieto, N Muthiga, RH Bradbury, A Dubi and ME Hatziolos (2007) Coral reefs under rapid climate change and ocean acidification. Science 318, pp. 1737"1742.