Multiple driving factors explain spatial and temporal variability in coral calcification rates on the Bermuda platform

Experimental studies have shown that coral calcification rates are dependent on light, nutrients, food availability, temperature, and seawater aragonite saturation (Ω_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 Ω_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 ± 58 and 274 ± 57 mmol CaCO₃ m⁻² d⁻¹ to winter minimums of 135 ± 39 and 101 ± 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 Ω_arag. We believe this high sensitivity, however, is misleading, due to covariance between light and Ω_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 °C), light (5.1 mol photons m⁻² d⁻¹), and Ω_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 ± 14 mmol CaCO₃ m⁻² d⁻¹. Using short-term light and dark incubations, we show how the covariance of light and Ω_arag can lead to the false conclusion that calcification is more sensitive to Ω_arag than it really is.