Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef

The concentration of CO2 in the atmosphere is projected to reach twice the preindustrial level by the middle of the 21st century. This increase will r educe the concentration of CO32- of the surface ocean by 30% relative to th e preindustrial level and will reduce the calcium carbonate saturation stat e of the surface ocean by an equal percentage. Using the large 2650 m(3) co ral reef mesocosm at the BIOSPHERE-2 facility near Tucson, Arizona, we inve stigated the effect of the projected changes in seawater carbonate chemistr y on the calcification of coral reef organisms at the community scale. Our experimental design was to obtain a long (3.8 years) time series of the net calcification of the complete system and all relevant physical and chemica l variables (temperature, salinity, light, nutrients, Ca2+, pCO(2), TCO2, a nd total alkalinity). Periodic additions of NaHCO3, Na2CO3, and/or CaCl2 we re made to change the calcium carbonate saturation state of the water. We f ound that there were consistent and reproducible changes in the rate of cal cification in response to our manipulations of the saturation state. We sho w that the net community calcification rate responds to manipulations in th e concentrations of both Ca2+ and CO32- and that the rate is well described as a linear function of the ion concentration product, [Ca2+](0.69)[CO32-] . This suggests that saturation state or a closely related quantity is a pr imary environmental factor that influences calcification on coral reefs at the ecosystem level. We compare the sensitivity of calcification to short-t erm (days) and long-term (months to years) changes in saturation state and found that the response was not significantly different. This indicates tha t coral reef organisms do not seem to be able to acclimate to changing satu ration state. The predicted decrease in coral reef calcification between th e years 1880 and 2065 A.D. based on our long-term results is 40%. Previous small-scale, short-term organismal studies predicted a calcification reduct ion of 14-30%. This much longer, community-scale study suggests that the im pact on coral reefs may be greater than previously suspected In the next ce ntury coral reefs will be less able to cope with rising sea level and other anthropogenic stresses.