51. Distribution of Pelagic Biogenic Carbonates in the Southern Ocean south of Australia: a Baseline for Ocean Acidification Impact Assessment

Abraham Passmore (1), Thomas W. Trull (1)*, Diana M. Davies (1), Tim Smit (1)

1. Antarctic Climate and Ecosystems CRC, CSIRO Oceans and Atmosphere, and University of Tasmania Institute of Marine and Antarctic Studies, Hobart, Tasmania, 7001, Australia

The Southern Ocean provides a vital service by absorbing about one sixth of humankind’s annual emissions of CO2. This comes with a cost – an increase in ocean acidity that is expected to have negative impacts on ocean ecosystems. The reduced ability of phytoplankton and zooplankton to precipitate carbonate shells is a clearly identified risk. The impact depends on the significance of these organisms in Southern Ocean ecosystems, but there is very little information on their abundance or distribution. To quantify their presence, we used coulometric measurement of particulate inorganic carbonate (PIC) on particles filtered from surface seawater into two size fractions: 50-200 um to represent foraminifera (the most important biogenic carbonate forming zooplankton) and 1-50 um to represent coccolithophores (the most important biogenic carbonate forming phytoplankton). Ancillary measurements of biogenic silica (bSi) and particulate organic carbon (POC) provided context as estimates of the abundance of diatoms (the most abundant phytoplankton in polar waters), and total microbial biomass, respectively. Results along 9 transects from Australia to Antarctica in 2008-2015 showed low levels of all biogenic carbonate fractions compared to northern hemisphere polar waters. Levels were also ~5-fold lower than suggested by MODIS Aqua satellite remote sensing images. Coccolithophores exceeded the biomass of diatoms in Subantarctic waters, but their abundance decreased more than 10-fold southwards into Antarctic waters where diatoms dominated. Foram PIC contents were much lower, ~1/10 of coccolithophore levels, and also decreased southward. These decreases parallel the southward decrease in the saturation state of calcium carbonate in seawater, consistent with the theory that decreasing saturation restricts abundance and thus that advancing acidification will impact distributions. But other controls are also likely, and responses are likely to differ among different species. Expanded and more detailed surveys are required to better determine probable impacts.