Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: a baseline for ocean acidification impact assessment
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 an...
Main Authors: | , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2018-01-01
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Series: | Biogeosciences |
Online Access: | https://www.biogeosciences.net/15/31/2018/bg-15-31-2018.pdf |
Summary: | 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
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. |
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ISSN: | 1726-4170 1726-4189 |