Effects of calcium ion concentration on calcification rates of six stony corals: A mesocosm study

A 1.5-ton indoor artificial coral reef mesocosm was established in order to evaluate the effects of changes in calcium ion (Ca2+) concentration on calcification of six stony corals, including two species of Acroporiidae (Acropora millepora and A. echinata), three species of Pocilloporidae (Pocillopora damicorins, Stylophora pistillata and Seriatopora caliendrum) and one species of Agariciidae (Pavona decussata). Periodic addition of CaCl2 was conducted to obtain a long-term series (23 weeks) of six reef coral calcification rates. Species-specific calcification rates were found among six stony corals, which shows linear increase in S. pistillata and A. millepora and exponential increase in S. caliendrum with elevated ambient seawater Ca2+ concentrations (ranging from 157.4 mg/L to 456.7 mg/L). Calcification rates tended to plateau with increased ambient seawater Ca2+ concentration (300-400 mg/L) for P. damicornis, while displaying a reverse parabolic pattern for A. echinata and P. decussata. We hypothesized that Ca2+ deposition by most stony corals would follow Michaelis-Menten kinetics, but the plateau would be reached at different Ca2+ concentrations for different coral species. Linear or exponential increases with Ca2+ concentrations observed in S. pistillata, A. millepora and S. caliendrum may be still at the initial stages of Michaelis-Menten curve, and they did not reach peak calcification rates at <460 mg/L of Ca2+ concentration. This work is valuable in providing instruction for the methodology and timing for periodical Ca2+ supplementation in closed recirculating aquacultural systems for stony corals.