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

Abstract

<p><strong>Abstract.</strong> The Southern Ocean provides a vital service by absorbing about one sixth of humankind's annual emissions of CO<sub>2</sub>. 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 abundance of diatoms (the most abundant phytoplankton in polar waters), and total microbial biomass, respectively. Results for 9 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 (though somewhat higher than) the shipboard results in Subantarctic waters, but greatly over-estimated 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. 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.</p>

A niche comparison of <i>Emiliania huxleyi</i> and <i>Gephyrocapsa oceanica</i> and potential effects of climate change

“…Similar low levels of coccolithophores have been projected in the Southern Ocean in Gregg and Casey (2007) (very low coccolithophore chlorophyll a), Krumhardt et al (2017) (growth rates at or close to zero which equates to low to zero CCPP) and Sinha et al (2010) (high nutrients resulting in coccolithophores being dominated by diatoms). For the Southern Ocean, it has been suggested that satellite PIC concentrations in subantarctic waters are overestimated by a factor of 2-3 while those in Antarctic waters may be even more so (Holligan et al,10 2010; Balch et al, 2011;Trull et al, 2018). The fact that three other global estimates, based on different sets of environmental parameters, all estimate very little productivity in the Southern Ocean seems to support this theory.…”

Species distribution modelling of the Southern Ocean benthos: a review on methods, cautions and solutions

“…When the limits of species environmental ranges are not fully captured, this uncertainty can significantly impact the accuracy of SDM predictions (Hortal et al 2007, 2008, Rocchini et al 2011, Sánchez-Fernández et al 2011, Titeux et al 2017, El-Gabbas & Dormann 2018. It reduces the applicability of models for predictive purposes (Thuiller et al 2004), induces model overfitting (Tsoar et al 2007, Barve et al 2011, Guillaumot et al 2018b) and can lead to overestimating the extent of suitable areas (Anderson & Raza 2010). This bias can be partly overcome by reducing the extent of the projection area to the known distribution of the available occurrence records (Anderson & Raza 2010) and by increasing knowledge regarding species ecology and physiology in order to identify the environmental conditions that are unsuitable for their survival or development (Byrne et al 2016).…”

“…Whenever possible, it is recommended to validate environmental data derived from satellite products at regional and local scales by comparing pixels on a satellite image with ‘real’ field observation data (Henson et al 2015, Trull et al 2018). Simple correlation analyses or more complex ground-truth processes are available to compare satellite and in situ data and to secure the interpretation of satellite-derived products (White-Newsome et al 2013, Allan 2014).…”

Regional Characteristics of the Temporal Variability in the Global Particulate Inorganic Carbon Inventory

“…These waters would include the Austral Polar province (54) and to some extent, the Antarctic province (53). It has been reported that E. huxleyi abundance is typically low in the high latitude Southern Ocean (Charalampopoulou et al, 2016;Holligan et al, 2010) and other phenomena such as highly-reflective glacial flour or reflective loose ice could produce sufficient reflectance to adversely overestimate satellite PIC retrievals in this specific region (Balch et al, 2011;Balch, 2018;Trull et al, 2018). High latitude Phaeocystis blooms might also abnormally elevate the reflectance (Alvain et al, 2008).…”