Coralline algae will suffer as oceans acidify

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Corals are not the only reef organisms to suffer directly from increased ocean acidification, according to a study to be published in the journal Nature Geoscience.

Ilsa Kuffner, Andreas Andersson, Paul Jokiel, Ku'ulei Rodgers and Fred Mackenzie carried out a seven-week experiment examining the effects of increased ocean acidification on crustose coralline algae, a group of calcifying algae that is important in many shallow water habitats, including coral reefs.

The authors constructed six fibreglass mesocosm tanks were supplied with flowing sea water pumped from the edge of a coral reef in Hawaii.

Three of the tanks were maintained at ambient carbon dioxide levels to serve as controls, while the other three were subject to elevated carbon dioxide levels approximating the level to be expected by the end of this century (assuming current levels of emission continue).

The authors then placed clear acrylic cylinders in order to study the rate and degree at which they were colonised by algae.

The authors found that at the elevated carbon dioxide levels, recruitment rate and percentage cover of the coralline algae decreased by 78% and 92%, respectively, whereas percentage cover of non-calcifying algae (a mixed assemblage of macroalgal germlings, diatoms and small filamentous algae) increased by 52% relative to controls.

The authors conclude: ur study demonstrates that changes in benthic community structure on coral reefs may occur owing to the impact of ocean acidification on ecological processes such as recruitment and competition for space.

Extrapolation of experiments measuring decreases in calcification rates by various organisms to predict future reef accretion rates may underestimate the impacts of ocean acidification by failing to account for the replacement of calcifying organisms by those that do not produce calcium carbonate.

Predicting changes in community structure resulting from ocean acidification and other stressors (for example high-temperature anomalies) will be important in modeling future rates of carbonate production by coral reefs and associated ecosystems.

For more information, see the paper: Kuffner, IB, AJ Andersson, PL Jokiel, KS Rodgers and FT Mackenzie. Decreased abundance of crustose coralline algae due to ocean acidification. Nature Geoscience advance online publication, 23 December 2007 (doi:10.1038/ngeo100).