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

Abstract

Abstract. The Southern Ocean provides a vital service by absorbing about one-sixth of humankind's annual emissions of CO 2 . 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: 50-1000 µm to capture foraminifera (the most important biogenic carbonate-forming zooplankton) and 1-50 µm to capture 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 biomass of diatoms (the highest biomass phytoplankton in polar waters) and total microbial biomass, respectively. Results for nine transects from Australia to Antarctica in 2008-2015 showed low levels of PIC compared to Northern Hemisphere polar waters. Coccolithophores slightly exceeded the biomass of diatoms in subantarctic waters, but their abundance decreased more than 30-fold poleward, while diatom abundances increased, so that on a molar basis PIC was only 1 % of BSi in Antarctic waters. This limited importance of coccolithophores in the Southern Ocean is further emphasized in terms of their associated POC, representing less than 1 % of total POC in Antarctic waters and less than 10 % in subantarctic waters. NASA satellite ocean-colour-based PIC estimates were in reasonable agreement with the shipboard results in subantarctic waters but greatly overestimated PIC in Antarctic waters. Contrastingly, the NASA Ocean Biogeochemical Model (NOBM) shows coccolithophores as overly restricted to subtropical and northern subantarctic waters. The cause of the strong southward decrease in PIC abundance in the Southern Ocean is not yet clear. The poleward decrease in pH is small, and while calcite saturation decreases strongly southward, it remains well above saturation (> 2). Nitrate and phosphate variations would predict a poleward increase. Temperature and competition with diatoms for limiting iron appear likely to be important. While the future trajectory of coccolithophore distributions remains uncertain, their current low abundances suggest small impacts on overall Southern Ocean pelagic ecology.

New Insights Into Processes Controlling the δ30Si of Sinking Diatoms: A Seasonally Resolved Box Model Approach

“…The model successfully produced a spring bloom of diatoms peaking at the end of November in the PFZ and in January in the AZ. This resulted in maximal concentrations of BSi in the ML of 2.9 and 8.5 μmol/L in the PFZ and AZ, respectively (Figures b and b); values that are consistent with extensive surveys south of Australia (Trull et al, ). In the PFZ, the maximal BSi gross production produced by the model (0.52 μmol L −1 d −1 ) fits well with the range of expected spring values (e.g., 0.58 μmol L −1 d −1 in the Indian Sector, Closset et al, , or from 0.17 to 0.80 μmol L −1 d −1 in the Atlantic Sector; Quéguiner & Brzezinski, ).…”

Coccolithophore populations and their contribution to carbonate export during an annual cycle in the Australian sector of the Antarctic zone

“…Coc- It is important to note that satellite PIC concentration estimates have been reported to be biased for high-latitude systems of the Southern Ocean where the satellite algorithm is thought to produce overestimates (Balch et al, 2016;Trull et al, 2018). Therefore, the PIC data presented here should be viewed with caution.…”

“…Firstly, coccolithophores exhibit high concentrations in the Subantarctic Southern Ocean, a feature termed by as the "Great Calcite Belt" based on satellite reflectance estimates of PIC abundances. However, the PIC accumulations are significantly less than those that arise in the North Atlantic, and the satellite algorithm is not reliable in Antarctic waters where it badly overestimates PIC abundances (Balch et al, 2016;Trull et al, 2018). Secondly, recent studies suggest that the magnitude and geographical distribution of E. huxleyi blooms may be experiencing significant and rapid changes.…”

“…Arrigo et al, 1999Arrigo et al, , 2000. Notably, the PIC algorithm performs particularly poorly in Antarctic waters (Balch et al, 2016;Trull et al, 2018).…”

The Observed Seasonal Cycle of Macronutrients in Drake Passage: Relationship to Fronts and Utility as a Model Metric

“…This biogeography is largely determined by the availability of nutrients (including iron) across frontal zones. The silicate-rich waters of the AAZ and SZ south of the PF support a thriving diatom community (Figure 4; Petrou et al, 2016, and references therein;Balch et al, 2016;Nissen et al, 2018;Trull et al, 2018). The seasonally silicate-limited waters observed north of the PF struggle to Journal of Geophysical Research: Oceans 10.1029/2019JC015052 support high diatom productivity, but the availability of nitrate and phosphate is sufficient to support the smaller phytoplankton communities that do not require silicate (Figures 4a and 4b).…”