Ocean acidification impacts on Southern Ocean calcareous zooplankton
Howard, W and Moy, AD and Roberts, D and Trull, T and Bray, SG and Hopcroft, RR and Roberts, JL and Roden, N, Ocean acidification impacts on Southern Ocean calcareous zooplankton, EOS Transactions, Ocean Sciences Meeting Supplement, 22-26 February 2010, Portland, Oregon, pp. Abstract PO21C-05. (2010) [Conference Extract]
Laboratory experiments suggest that decreased ocean carbonate-ion concentrations due to anthropogenic CO2 will result in reduced calcification rates in marine calcareous organisms. However, in situ observations of calcification in marine organisms are limited. Here we combine sediment trap data at 47°S, Holocene sediments, and glacial-interglacial variability to show reduced calcification in Southern Ocean planktonic foraminifera since the industrial revolution. Our sediment trap results provide the first field observations for reduced calcification (inferred from planktonic foraminiferal shell weights) in Southern Ocean carbonate organisms and indicate there has been a 30-34% reduction in calcification in the planktonic foraminifer Globigerina bulloides (Moy et al., 2009) and a smaller reduction in calcification in Globorotalia inflata. These changes are consistent with the impact due to altered ocean chemistry resulting from anthropogenic CO2 storage. Down-core shell weight variations show reduced calcification during glacial-interglacial CO2 increases that are similar to the post-industrial increase. Planktonic foraminifera contribute ~23 - 50 % of the total open-marine carbonate flux in the modern ocean, thus a reduction in the calcification of planktonic foraminifera may imply a corresponding decrease in calcium carbonate export from the surface ocean. The same sediment trap mooring site provides data on interannual variability of pteropod export and calcification from 1997-2006 at 2,000 meters depth. These aragonitic molluscs likely to be more sensitive to changes in carbonate chemistry than calcite producers, however they are rarely preserved in Southern Ocean sediments so we do not have a preindustrial baseline, and we rely on detection of decadal-scale trends. Our data show few clear interannual trends over the ten years of sample collection. However, they do suggest a slight decadal decline in mean shell weight and contributions to total pteropod flux of Limacina helicina antarctica forma antarctica. Attribution of this trend to acidification or any environmental variable is unclear. These small though discernible interannual decreases may represent an emerging response to changing carbonate-mineral saturation in the Southern Ocean, which does have a clear, if slow, decadal decline.