| Summary: | Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Coral Reefs 30 (2011): 321-328, doi:10.1007/s00338-010-0697-z. Rising concentrations of atmospheric CO2 are changing the carbonate chemistry of the oceans, a process known as ocean acidification (OA). Absorption of this CO2 by the surface oceans is increasing the amount of total dissolved inorganic carbon (DIC) and bicarbonate ion (HCO3 -) available for marine calcification, yet is simultaneously lowering the seawater pH and carbonate ion concentration ([CO3 2-]), and thus the saturation state of seawater with respect to aragonite (Ωar). We investigated the relative importance of [HCO3 -] versus [CO3 2-] for early calcification by new recruits (primary polyps settled from zooxanthellate larvae) of two tropical coral species, Favia fragum and Porites astreoides. The polyps were reared over a range of Ωar values, which were manipulated by both acid-addition at constant pCO2 (decreased total [HCO3 -] and [CO3 2-]) and by pCO2 elevation at constant alkalinity (increased [HCO3 -], decreased [CO3 2-]). Calcification after two weeks was quantified by weighing the complete skeleton (corallite) accreted by each polyp over the course of the experiment. Both species exhibited the same negative response to decreasing [CO3 2-] whether Ωar was lowered by acid-addition or by pCO2 elevation - calcification did not follow total DIC or [HCO3 -]. Nevertheless, the calcification response to decreasing [CO3 2-] was non-linear. A statistically significant decrease in calcification was only detected between Ωar = < 2.5 and Ωar = 1.1 – 1.5, where calcification of new recruits was reduced by 22 – 37 % per 1.0 decrease in Ωar. Our results differ from many previous studies that report a linear coral calcification response to OA, and from those showing that calcification increases with increasing [HCO3 -]. Clearly, the coral ...
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