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The Southern Ocean provides a vital service by absorbing about one-sixth of\udhumankind's annual emissions of CO<sub>2</sub>. This comes with a cost – an\udincrease in ocean acidity that is expected to have negative impacts on ocean\udecosystems. The reduced ability of phytoplankton and zooplankton to\udprecipitate carbonate shells is a clearly identified risk. The impact depends\udon the significance of these organisms in Southern Ocean ecosystems, but\udthere is very little information on their abundance or distribution. To\udquantify their presence, we used coulometric measurement of particulate\udinorganic carbonate (PIC) on particles filtered from surface seawater into\udtwo size fractions: 50–1000 µm to capture foraminifera (the most\udimportant biogenic carbonate-forming zooplankton) and 1–50 µm to capture\udcoccolithophores (the most important biogenic carbonate-forming\udphytoplankton). Ancillary measurements of biogenic silica (BSi) and\udparticulate organic carbon (POC) provided context, as estimates of the\udbiomass of diatoms (the highest biomass phytoplankton in polar waters) and\udtotal microbial biomass, respectively. Results for nine transects from Australia\udto Antarctica in 2008–2015 showed low levels of PIC compared to Northern\udHemisphere polar waters. Coccolithophores slightly exceeded the biomass of\uddiatoms in subantarctic waters, but their abundance decreased more than\ud30-fold poleward, while diatom abundances increased, so that on a molar basis\udPIC was only 1 % of BSi in Antarctic waters. This limited importance of\udcoccolithophores in the Southern Ocean is further emphasized in terms of\udtheir associated POC, representing less than 1 % of total POC in Antarctic\udwaters and less than 10 % in subantarctic waters. NASA satellite\udocean-colour-based PIC estimates were in reasonable agreement with the shipboard\udresults in subantarctic waters but greatly overestimated PIC in Antarctic\udwaters. Contrastingly, the NASA Ocean Biogeochemical Model (NOBM) shows\udcoccolithophores as overly restricted to subtropical and northern\udsubantarctic waters. The cause of the strong southward decrease in PIC\udabundance in the Southern Ocean is not yet clear. The poleward decrease in pH is\udsmall, and while calcite saturation decreases strongly southward, it remains\udwell above saturation ( > 2). Nitrate and phosphate variations\udwould predict a poleward increase. Temperature and competition with diatoms\udfor limiting iron appear likely to be important. While the future trajectory\udof coccolithophore distributions remains uncertain, their current low\udabundances suggest small impacts on overall Southern Ocean pelagic ecology

Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: a baseline for ocean acidification impact assessment