What was done
In a study designed to explore what controls calcification in corals, the author suspended - by means of a torsion microbalance (as per Kesling and Crafts, 1962) - small pieces of coral that he carefully removed from the edges of thin plates of Agaricia agaricites corals and lowered into gently-stirred temperature-controlled seawater, after which he used the microbalance to measure coral net calcification rates over a range of seawater temperature and pH.

What was learned
First of all, Sandeman found that calcification rates of live A. agaricites coral increased by 15-17.7% per °C as seawater temperature rose from 27 to 29.5°C; and in his experiments in which the pH of the seawater was reduced from an average of 8.2 to 7.6, he observed that calcification in living corals increased significantly. On the other hand, similar experiments conducted with small portions of dead coral skeleton revealed that "when the average pH was reduced from 8.2 to 7.5, calcification rate decreased." More specifically, he determined that the difference between calcification rates in going from seawater of pH 8.2 to seawater of pH 7.8 ranged from +30% for coral with no dead areas to -21.5% for coral with 30% dead exposed surface area.

What it means
The Trent University researcher from Peterborough, Ontario (Canada) says his findings suggest that lower seawater pH due to atmospheric CO2 enrichment and increased temperature (but short of reaching the bleaching level) "will both enhance active biotic calcification." And he therefore states that the wide range of results between his and other scientists' studies of calcification rate and carbon dioxide "may be explainable in terms of the ratio of 'live' to 'dead' areas of coral," as is also suggested by the work of Rodolfo-Metalpa et al. (2011) and Ries (2011), all of which information leads him to conclude that coral species that typically have smaller areas of exposed dead surface "may have a better chance of survival as pH levels drop."

References
Kesling, R.V. and Crafts, F.C. 1962. Ontogenetic increase in archimedian weight of the ostracod Clamidotheca unispinosa (Baird). American Midland Naturalist 68: 149-153.

Ries, J. 2011. Acid ocean cover up. Nature Climate Change 1: 294-295.

Rodolfo-Metalpa, R., Houlbreque, F., Tambutte, E., Boisson, F., Baggini, C., Patti, F.P., Jeffree, R., Fine, M., Foggo, A., Gattuso, J.P. and Hall-Spencer, J.M. 2011. Coral and mollusk resistance to ocean acidification adversely affected by warming. Nature Climate Change 1: 308-312.