Biogeosciences, 15, 1843–1862, 2018 https://doi.org/10.5194/bg-15-1843-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Coccolithophore populations and their contribution to carbonate export during an annual cycle in the Australian sector of the Antarctic zone Andrés S. Rigual Hernández1, José A. Flores1, Francisco J. Sierro1, Miguel A. Fuertes1, Lluïsa Cros2, and Thomas W. Trull3,4 1Área de Paleontología, Departamento de Geología, Universidad de Salamanca, 37008 Salamanca, Spain 2Institut de Ciències del Mar, CSIC, Passeig Marítim 37-49, 08003 Barcelona, Spain 3Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Tasmania 7001, Australia 4CSIRO Oceans and Atmosphere Flagship, Hobart, Tasmania 7001, Australia Correspondence: Andrés S. Rigual Hernández ([email protected]) Received: 5 December 2017 – Discussion started: 13 December 2017 Revised: 25 February 2018 – Accepted: 27 February 2018 – Published: 29 March 2018 Abstract. The Southern Ocean is experiencing rapid and re- We report here on seasonal variations in the abundance lentless change in its physical and biogeochemical proper- and composition of coccolithophore assemblages collected ties. The rate of warming of the Antarctic Circumpolar Cur- by two moored sediment traps deployed at the Antarctic zone rent exceeds that of the global ocean, and the enhanced up- south of Australia (2000 and 3700 m of depth) for 1 year take of carbon dioxide is causing basin-wide ocean acidifi- in 2001–2002. Additionally, seasonal changes in coccolith cation. Observational data suggest that these changes are in- weights of E. huxleyi populations were estimated using cir- fluencing the distribution and composition of pelagic plank- cularly polarised micrographs analysed with C-Calcita soft- ton communities. Long-term and annual field observations ware. Our findings indicate that (1) coccolithophore sink- on key environmental variables and organisms are a criti- ing assemblages were nearly monospecific for E. huxleyi cal basis for predicting changes in Southern Ocean ecosys- morphotype B/C in the Antarctic zone waters in 2001– tems. These observations are particularly needed, since high- 2002; (2) coccoliths captured by the traps experienced weight latitude systems have been projected to experience the most and length reduction during summer (December–February); severe impacts of ocean acidification and invasions of al- (3) the estimated annual coccolith weight of E. huxleyi at lochthonous species. both sediment traps (2.11 ± 0.96 and 2.13 ± 0.91 pg at 2000 Coccolithophores are the most prolific calcium-carbonate- and 3700 m) was consistent with previous studies for mor- producing phytoplankton group playing an important role photype B/C in other Southern Ocean settings (Scotia Sea in Southern Ocean biogeochemical cycles. Satellite imagery and Patagonian shelf); and (4) coccolithophores accounted has revealed elevated particulate inorganic carbon concen- for approximately 2–5 % of the annual deep-ocean CaCO3 trations near the major circumpolar fronts of the Southern flux. Our results are the first annual record of coccolithophore Ocean that can be attributed to the coccolithophore Emilia- abundance, composition and degree of calcification in the nia huxleyi. Recent studies have suggested changes during Antarctic zone. They provide a baseline against which to the last decades in the distribution and abundance of South- monitor coccolithophore responses to changes in the en- ern Ocean coccolithophores. However, due to limited field vironmental conditions expected for this region in coming observations, the distribution, diversity and state of coccol- decades. ithophore populations in the Southern Ocean remain poorly characterised. Published by Copernicus Publications on behalf of the European Geosciences Union. 1844 A. S. Rigual Hernández et al.: Coccolithophore populations and their contribution to carbonate export 1 Introduction anthropogenic CO2 (Khatiwala et al., 2009; Takahashi et al., 2009; Frölicher et al., 2015), and it exports nutrients to more 1.1 Background and objectives northern latitudes, ultimately supporting ∼ 75 % of the ocean primary production north of 30◦ S (Sarmiento et al., 2004a). The rapid increase in atmospheric CO2 levels since the onset Model projections suggest that the reduction in the saturation of the industrial revolution is modifying the environmental state of CaCO3 will reach critical thresholds sooner in cold, conditions of marine ecosystems in a variety of ways. The en- high-latitude ecosystems such as the Southern Ocean (Orr hanced greenhouse effect, mainly driven by increased atmo- et al., 2005; McNeil and Matear, 2008; Feely et al., 2009). spheric CO2 levels, is causing ocean warming (Barnett et al., Therefore, calcifying organisms living in these regions will 2005), shallowing of mixed layer depths (Levitus et al., 2000) be the first to face the most severe impacts of ocean acidifi- and changes in light penetration and nutrient supply (Bopp et cation. al., 2001; Rost and Riebesell, 2004; Sarmiento et al., 2004b; In view of the rapid changes in climate and other en- Deppeler and Davidson, 2017). Moreover, the enhanced ac- vironmental stressors presently occurring in the Southern cumulation of CO2 in the ocean is giving rise to changes in Ocean, a major challenge facing the scientific community the ocean carbonate system, including reduction of carbon- is to predict how phytoplankton communities will reorgan- ate ion concentrations and lowering of seawater pH. Most ise in response to global change. In this regard, two main evidence suggests that the ability of many marine calcifying aspects of the distributions of coccolithophores are emerg- organisms to form carbonate skeletons and shells may be re- ing. Firstly, coccolithophores exhibit high concentrations in duced with increasing seawater acidification including some the Subantarctic Southern Ocean, a feature termed by Balch (but not all) species of coccolithophores, corals, pteropods et al. (2011) as the “Great Calcite Belt” based on satellite and foraminifera (e.g. Orr et al., 2005; Moy et al., 2009; reflectance estimates of PIC abundances. However, the PIC Lombard et al., 2010; Beaufort et al., 2011; Andersson and accumulations are significantly less than those that arise in Gledhill, 2013). Since phytoplankton are extremely sensitive the North Atlantic, and the satellite algorithm is not reli- to global environmental change (Litchman et al., 2012) all able in Antarctic waters where it badly overestimates PIC predicted changes in marine environmental conditions are abundances (Balch et al., 2016; Trull et al., 2018). Secondly, likely to modify the abundance, composition and distribution recent studies suggest that the magnitude and geographical of phytoplankton communities. distribution of E. huxleyi blooms may be experiencing sig- Changes in the relative abundances of major phytoplank- nificant and rapid changes. Cubillos et al. (2007) and Win- ton functional groups are likely to influence ocean biogeo- ter et al. (2014) postulated that E. huxleyi has expanded its chemistry and ocean carbon storage, with feedbacks to the ecological niche south of the Polar Front in recent decades. rate of climate change (e.g. Boyd and Newton, 1995; Boyd Contrastingly, Freeman and Lovenduski (2015) suggested an et al., 1999; Falkowski et al., 2004; Cermeño et al., 2008). overall decline in Southern Ocean PIC concentrations using The precipitation and sinking of CaCO3 by coccolithophores satellite records between 1998 and 2014. The explanation of has the potential for complex contributions to carbon cy- these contrasting results may lie in the methodologies ap- cling. Carbonate precipitation removes more alkalinity than plied. While shipboard surface water observations provide a dissolved inorganic carbon from surface waters, thereby act- highly detailed picture of a given ecosystem, they are very ing to increase pCO2 in surface waters (the so-called car- sparse, only represent a snapshot in time and can easily miss bonate counter pump; e.g. Zeebe, 2012). On the other hand, blooms of any given species. The satellite PIC signal has the ballasting by carbonates appears to increase the transfer of great advantage of large-scale and repeated coverage, but can organic carbon to the ocean interior (Armstrong et al., 2002; miss subsurface populations (e.g. Winter et al., 2014) and Klaas and Archer, 2002). On seasonal timescales the counter be mimicked by the spectral characteristics of other scatter- pump contribution dominates (Boyd and Trull, 2007), but ing sources. The most important among them are probably more complex interactions can occur over longer timescales microbubbles (Zhang et al., 2002), glacial flour (Balch et as a result of changing extents of carbonate dissolution in al., 2011) and non-calcifying organisms such as Phaeocystis sediments, including the possibility that enhanced calcite antarctica (Winter et al., 2014), a colonial prymnesiophyte dissolution in the Southern Ocean contributed to lower at- algae very abundant in high-latitude systems of the Southern mospheric CO2 levels during glacial maxima (Archer and Ocean (e.g. Arrigo et al., 1999, 2000). Notably, the PIC algo- Maier-Reimer, 1994; Sigman and Boyle, 2000; Ridgwell and rithm performs particularly poorly in Antarctic waters (Balch Zeebe, 2005). et al., 2016; Trull et al., 2018). The Southern Ocean is a critical component of the Earth’s For these reasons, year-round field