The impact of ocean acidification caused by the increasing global concentration of atmospheric CO2 has been studied in marine calcifiers, including hermatypic corals, in which calcification is expected to decrease significantly. However, the effect of elevated pCO2 on the early developmental stage of the coral life cycle has been little studied, even though corals at this stage seem to be susceptible to stresses including ocean acidification. In this study, we reared polyps of Acropora digitifera, one of the dominant species around the Ryukyu Islands, Japan, in seawater of various pH (6.55, 7.31, 7.64, 7.77, 8.03), controlled by CO2 bubbling. We measured the dry weight of the polyp skeletons after the 40 days experiment to investigate the relationship between the aragonite saturation state and polyp growth. In addition, we measured the Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca ratios of their skeletons to estimate the pH dependence of these ratios. Although skeletons of polyps reared at pH 6.55 dissolved during the experiment after settlement, the other skeletal weight of coral polyps increased with the aragonite saturation state. Then, skeletal weight apparently reached a saturation plateau above pH 7.77, a finding in agreement with previous results for adult corals. Ba/Ca and U/Ca were significantly related to pH, whereas Sr/Ca showed a moderate positive correlation with pH. On the other hand, there was no clear relationship between Mg/Ca and pH. Although both the Ba/Ca and U/Ca ratios showed a strong dependency on pH, coral Ba/Ca is reported to be affected by multiple factors such as terrestrial runoff and upwelling. But, U/Ca ratios have been reported a relatively simple dependency on temperature. Therefore, coral U/Ca may have a potential for pH proxy.
Inoue, M., Suwa, R., Iguchi, A., Suzuki, A., Sakai, K. & Kawahata, H., 2010. Ocean acidification impact on growth and the pH dependence of trace elements in skeleton of juvenile corals (Acropora digitifera). American Geophysical Union, Fall Meeting 2010, abstract #OS21D-1543. Abstract.
