Acidified Baltic Sea waters corrode benthic foraminifera

ABSTRACT: Culturing experiments, simulating a projected future rise in atmospheric CO2 concentrations, were performed with the benthic foraminifer Ammonia aomoriensis from Flensburg Fjord, southwestern Baltic Sea. The experiments simulated a future rise in atmospheric CO2. We exposed living specimens to five seawater pCO2 levels ranging from 618 µatm (pH 7.9) to 3130 µatm (pH 7.2) for six weeks. Growth rates and mortality differed significantly between pCO2 treatments. The highest increase of mean test diameter by 19% was observed at 618 µatm. At partial pressures >1829 µatm, the mean test diameter decreased during the experiment by up to 22% at 3130 µatm. At pCO2 levels of 618 and 751 µatm, the tests of A. aomoriensis were found intact after the experiment. The last chambers of specimens incubated at 929 and 1829 µatm were severely damaged by corrosion. Visual inspection of specimens incubated at 3130 µatm revealed wall dissolution of all outer chambers, and only the inner organic lining stayed intact. Our results demonstrate that pCO2 values of 929 µatm and above cause reduced growth of A. aomoriensis and lead to shell dissolution in Baltic Sea waters. The bottom waters in Flensburg Fjord and adjacent areas regularly experience pCO2 levels in this range during summer and fall. Therefore, increasing atmospheric CO2-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.