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

• Published in: Biogeosciences
• Main Authors: Hernandez, ASR, Flores, JA, Sierro, FJ, Fuertes, MA, Cros, L, Trull, TW
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus GmbH 2018
• Subjects: coccolithophore; carbonate export; Southern Ocean; Antarctic zone; phytoplankton; Antarctic; The Antarctic; Scotia Sea; Antarc*; Ocean acidification
• Online Access: https://eprints.utas.edu.au/29982/; https://eprints.utas.edu.au/29982/1/131878%20-%20Coccolithophore%20populations%20and%20their%20contribution%20to%20carbonate.pdf

The Southern Ocean is experiencing rapid and relentless change in itsphysical and biogeochemical properties. The rate of warming of the AntarcticCircumpolar Current exceeds that of the global ocean, and the enhanceduptake of carbon dioxide is causing basin-wide ocean acidification.Observational data suggest that these changes are influencing thedistribution and composition of pelagic plankton communities. Long-term andannual field observations on key environmental variables and organisms are acritical basis for predicting changes in Southern Ocean ecosystems. Theseobservations are particularly needed, since high-latitude systems have beenprojected to experience the most severe impacts of ocean acidification andinvasions of allochthonous species.Coccolithophores are the most prolific calcium-carbonate-producingphytoplankton group playing an important role in Southern Oceanbiogeochemical cycles. Satellite imagery has revealed elevated particulateinorganic carbon concentrations near the major circumpolar fronts of theSouthern Ocean that can be attributed to the coccolithophore Emiliania huxleyi. Recentstudies have suggested changes during the last decades in the distributionand abundance of Southern Ocean coccolithophores. However, due to limitedfield observations, the distribution, diversity and state of coccolithophorepopulations in the Southern Ocean remain poorly characterised.We report here on seasonal variations in the abundance and composition ofcoccolithophore assemblages collected by two moored sediment traps deployedat the Antarctic zone south of Australia (2000 and 3700 m of depth) for 1 year in 2001–2002. Additionally, seasonal changes in coccolith weights ofE. huxleyi populations were estimated using circularly polarisedmicrographs analysed with C-Calcita software. Our findings indicatethat (1) coccolithophore sinking assemblages were nearly monospecific forE. huxleyi morphotype B/C in the Antarctic zone waters in2001–2002; (2) coccoliths captured by the traps experienced weight andlength reduction during summer (December–February); (3) the estimated annualcoccolith weight of E. huxleyi at both sediment traps(2.11 ± 0.96 and 2.13 ± 0.91 pg at 2000 and 3700 m) wasconsistent with previous studies for morphotype B/C in other Southern Oceansettings (Scotia Sea and Patagonian shelf); and (4) coccolithophores accountedfor approximately 2–5 % of the annual deep-ocean CaCO3 flux. Ourresults are the first annual record of coccolithophore abundance, compositionand degree of calcification in the Antarctic zone. They provide a baselineagainst which to monitor coccolithophore responses to changes inthe environmental conditions expected for this region in coming decades.