Temperature-controlled culture experiments with primary polyps of coral Acropora digitifera: Calcification rate variations and skeletal Sr/Ca, Mg/Ca, and Na/Ca ratios

Coral skeletons preserve useful archival indicators from which to reconstruct past environments. We conducted temperature-controlled culture experiments (20, 22, 27, and 31 °C) with primary polyps of Acropora digitifera to accurately assess the impact of a wide range of temperatures on the calcification rate and to methodically evaluate the skeletal trace elements (Sr/Ca, Mg/Ca, and Na/Ca ratios). Water temperatures positively affected the calcification rate up to 31 °C, which exceeds the temperature threshold for this species. The calcification rates also varied widely (> 20%) during each of the four temperature treatments. The skeletal Sr/Ca ratio was most strongly correlated with water temperature (R2 = 0.68, p < 0.001), whereas the Mg/Ca and Na/Ca ratios showed lower correlations (R2 = 0.53, p < 0.001; R2 = 0.34, p = 0.011, respectively). The variations in the skeletal Sr/Ca, Mg/Ca, and Na/Ca ratios at the four different temperatures were 0.36%-1.20%, 0.83%-3.76%, and 0.52%-3.24%, respectively. Thus, the Sr/Ca ratio showed the least variation, despite the wide variations in the calcification rate. We confirm that the Sr/Ca ratio of A. digitifera juveniles is a robust proxy of temperature, regardless of variations in the calcification rate and the wide range of ambient temperatures. In addition, our results regarding skeletal Mg/Ca and Na/Ca ratios further our understanding of coral calcification processes.