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Venti, A, Andersson A, Langdon C.  2014.  Multiple driving factors explain spatial and temporal variability in coral calcification rates on the Bermuda platform. Coral Reefs. 33:979-997.   10.1007/s00338-014-1191-9   AbstractWebsite

Experimental studies have shown that coral calcification rates are dependent on light, nutrients, food availability, temperature, and seawater aragonite saturation (Omega (arag)), but the relative importance of each parameter in natural settings remains uncertain. In this study, we applied Calcein fluorescent dyes as time indicators within the skeleton of coral colonies (n = 3) of Porites astreoides and Diploria strigosa at three study sites distributed across the northern Bermuda coral reef platform. We evaluated the correlation between seasonal average growth rates based on coral density and extension rates with average temperature, light, and seawater Omega (arag) in an effort to decipher the relative importance of each parameter. The results show significant seasonal differences among coral calcification rates ranging from summer maximums of 243 +/- A 58 and 274 +/- A 57 mmol CaCO3 m(-2) d(-1) to winter minimums of 135 +/- A 39 and 101 +/- A 34 mmol CaCO3 m(-2) d(-1) for P. astreoides and D. strigosa, respectively. We also placed small coral colonies (n = 10) in transparent chambers and measured the instantaneous rate of calcification under light and dark treatments at the same study sites. The results showed that the skeletal growth of D. strigosa and P. astreoides, whether hourly or seasonal, was highly sensitive to Omega (arag). We believe this high sensitivity, however, is misleading, due to covariance between light and Omega (arag), with the former being the strongest driver of calcification variability. For the seasonal data, we assessed the impact that the observed seasonal differences in temperature (4.0 A degrees C), light (5.1 mol photons m(-2) d(-1)), and Omega (arag) (0.16 units) would have on coral growth rates based on established relationships derived from laboratory studies and found that they could account for approximately 44, 52, and 5 %, respectively, of the observed seasonal change of 81 +/- A 14 mmol CaCO3 m(-2) d(-1). Using short-term light and dark incubations, we show how the covariance of light and Omega (arag) can lead to the false conclusion that calcification is more sensitive to Omega (arag) than it really is.

Kuffner, IB, Jokiel PL, Rodgers KS, Andersson AJ, Mackenzie FT.  2012.  An apparent "vital effect" of calcification rate on the Sr/Ca temperature proxy in the reef coral Montipora capitata. Geochemistry Geophysics Geosystems. 13   10.1029/2012gc004128   AbstractWebsite

Measuring the strontium to calcium ratio in coral skeletons reveals information on seawater temperatures during skeletal deposition, but studies have shown additional variables may affect the ratio. Here we measured Sr/Ca in the reef coral Montipora capitata grown in six mesocosms continuously supplied with seawater from the adjacent reef flat. Three mesocosms were ambient controls, and three had seawater chemistry simulating "ocean acidification" (OA). We found that Sr/Ca was not affected by the OA treatment and neither was coral calcification for these small colonies (larger colonies did show an OA effect). The lack of OA effects allowed us to test the hypothesis that coral growth rate can affect Sr/Ca using the natural range in calcification rates of the corals grown at the same temperature. We found that Sr/Ca was inversely related to calcification rate (Sr/Ca = 9.385 - 0.0040 (calcification rate)). Using a previously published calibration curve for this species, a 22 mg d(-1) colony(-1) increase in calcification rate introduced a 1 degrees C warmer temperature estimate, with the 27 corals reporting "temperatures" ranging from 24.9 to 28.9 degrees C, with mean 26.6 +/- 0.9 degrees C standard deviation. Our results lend support to hypotheses invoking kinetic processes and growth rate to explain vital effects on Sr/Ca. However, uncertainty in the slope of the regression of Sr/Ca on calcification and a low R-squared value lead us to conclude that Sr/Ca could still be a useful proxy in this species given sufficient replication or by including growth rate in the calibration.