Biometry and dissolution features of the benthic foraminifer Ammonia aomoriensis at high pCO2

Abstract

Culturing experiments were performed with the benthic foraminifer Ammonia aomoriensis from Flensburg Fjord, western Baltic Sea. The experiments simulated a projected rise in atmospheric CO 2 concentrations. We exposed specimens to 5 seawater pCO 2 levels ranging from 618 atm (pH 7.9) to 3130 atm (pH 7.2) for 6 wk. Growth rates and mortality differed significantly among pCO 2 treatments. The highest increase of mean test diameter (19%) was observed at 618 atm. At partial pressures >1829 atm, the mean test diameter was observed to decrease, by up to 22% at 3130 atm. At pCO 2 levels of 618 and 751 atm, A. aomoriensis tests were found intact after the experiment. The outer chambers of specimens incubated at 929 and 1829 atm were severely damaged by corrosion. Visual inspection of specimens incubated at 3130 atm revealed wall dissolution of all outer chambers, only their inner organic lining stayed intact. Our results demonstrate that pCO 2 values of 929 atm in Baltic Sea waters cause reduced growth of A. aomoriensis and lead to shell dissolution. The bottom waters in Flensburg Fjord and adjacent areas regularly experience pCO 2 levels in this range during summer and fall. Increasing atmospheric CO 2 concentrations are likely to extend and intensify these periods of undersaturation. This may eventually slow down calcification in A. aomoriensis to the extent that net carbonate precipitation terminates. The possible disappearance of this species from the Baltic Sea and other areas prone to seasonal undersaturation would likely cause significant shifts in shallow-water benthic ecosystems in the near future.