Dependence of calcification on light and carbonate ion concentration for the hermatypic coral Porites compressa

Abstract

Temperature, light and aragonite saturation state of seawater are important determinants of the global distribution of coral reefs. Saturation state of surface seawater is decreasing in response to increases in atmospheric pCO2, causing concern for a global reduction in the rates of reef accretion. In this study we address the influences of light and saturation state on skeletal growth of the hermatypic coral Porites compressa. We conducted 2 experiments using coral nubbins; one in a controlled laboratory with Biosphere 2 ocean water and the other in the 711 m2 Biosphere 2 ocean itself. The laboratory experiment consisted of 3 light treatments (PAR = 698, 150, 81 μmol photons m-2 s-1) and 2 chemical treatments (pCO2 = 199 μatm, Ω = 5.05, CO32- = 355 μmol kg-1; and pCO2 = 448 μatm, Ω = 2.48, CO32- = 183 μmol kg-1). In the Biosphere 2 ocean, coral nubbins were deployed at 4 depths (6, 4, 2 and 0.5 m) which corresponded to different light conditions (PAR from 2.7 to 29.5 mol photons m-2 d-1) during 3 different chemical states, mimicking the conditions of the Last Glacial Maximum (LGM), the present day and the year 2100 (Y2100). Calcification rate changed with light following the typical hyperbolic tangent function. Calcification rate was positively correlated with saturation state, which ranged from 5 (LGM) to 2.4 (Y2100). From the glacial experimental conditions (Ω = 5.05, pCO2 = 186) to those of the future (Ω = 2.25, pCO2 = 641), calcification dropped by 30%; from present day conditions (Ω = 3.64, pCO2 = 336) to those of the future, calcification dropped by 11%. This decrease in calcification rate occurred at all light levels, indicating that rising CO2 will impact corals living at all depths.