The Impact of ocean acidification on calcification of mediterranean cold-water corals

The atmospheric CO2 concentration (pCO2) has increased from approximately 280 μatm to 390 μatm since the beginning of the industrial revolution. A futher increase to about 750 μatm is projected by 2100. As the atmospheric CO2 concentration is increasing, CO2 is taken up by the ocean inducing a decrease in pH and a change of seawater carbonate chemistry. The marine environment has been relatively stable for thousands of years and may thus respond dramatically to the relatively fast changes projected until the end of the century. Organisms building calcareous skeletons and shells may be particular affected. Cold-water corals are thought to be specifically threatened as they live close to the ragonite saturation horizon (ASH), the depth that distinguishes between calcium carbonate dissolution and precipitation (Ωa= l). Cold-water corals are able to build large reef like structures (bioherms) providing a suitable habitat for many species and thus contribute to deep-sea diversity. In this study, 3 common Mediterranean cold-water coral species (Lophelia pertusa, Madrepora oculata, and Desmophyllum sp.) were exposed to four different pCO2 levels over a period of 4 month in the laboratory. CO2 concentrations were 390, 500, 700, and 950 μatm respectively, with an Ωa between 1.4 and 3.3 and a pH between 7.74 and 8.17. The coral replicates were incubated independendly in 300 or 700 mL vials with an individual aeration and water supply. Calcification rates were determined using the alkalinity-anomaly and the buoyant weight techniques. Beforehand, it was tested, if the time span between feeding and determination of calcification rates, caused a short-term response in calcification, which seemed not to be the case. Lophelia pertusa showed calcification rates of 124 ± 14 μg CaCO3 g-1d-1 (±S.D.). The calcification rates observed for M. oculata and Desmophyllum sp. were in the same order of magnitude with 234 ± 15 and 223 ± 32 μg CaC03 g-1d-1 (±S.D.), respectively. In general, calcification rates revealed a high variability for repeated measurements of the same coral. Also, the corals retained high calcification rates even at 950 μatm. lt almost seemed as if the cold-water corals responded rather to the abrupt change in seawater carbonate chemistry, especially L. pertusa, than to the high concentration of CO2. A repeated measurement ANOVA revealed no significant difference in calcification rates between treatments. lt appears, that cold-water corals are already adapted to low aragonite saturation and may be able to deal with the conditions projected for 2100. lt is still unclear if cold-water corals will be able to live in undersaturated water in respect to aragonite. Furthermore, it is not known how the projected CO2 increase will impact the associated environment and thus indirectly affect cold-water corals.