Biogeosciences, 17, 245–263, 2020 https://doi.org/10.5194/bg-17-245-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Coccolithophore biodiversity controls carbonate export in the Southern Ocean Andrés S. Rigual Hernández1, Thomas W. Trull2,3, Scott D. Nodder4, José A. Flores1, Helen Bostock4,5, Fátima Abrantes6,7, Ruth S. Eriksen2,8, Francisco J. Sierro1, Diana M. Davies2,3, Anne-Marie Ballegeer9, Miguel A. Fuertes9, and Lisa C. Northcote4 1Área de Paleontología, Departamento de Geología, Universidad de Salamanca, 37008 Salamanca, Spain 2CSIRO Oceans and Atmosphere Flagship, Hobart, Tasmania 7001, Australia 3Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Tasmania 7001, Australia 4National Institute of Water and Atmospheric Research, Wellington 6021, New Zealand 5School of Earth and Environmental Sciences, University of Queensland, Brisbane, Queensland 4072, Australia 6Portuguese Institute for Sea and Atmosphere (IPMA), Divisão de Geologia Marinha (DivGM), Rua Alferedo Magalhães Ramalho 6, Lisbon, Portugal 7CCMAR, Centro de Ciências do Mar, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal 8Institute for Marine and Antarctic Studies, University of Tasmania, Private Bag 129, Hobart, Tasmania 7001, Australia 9Departamento de Didáctica de las Matemáticas y de las Ciencias Experimentales, Universidad de Salamanca, 37008 Salamanca, Spain Correspondence: Andrés S. Rigual Hernández ([email protected]) Received: 2 September 2019 – Discussion started: 9 September 2019 Revised: 2 December 2019 – Accepted: 4 December 2019 – Published: 17 January 2020 Abstract. Southern Ocean waters are projected to undergo is an important vector for CaCO3 export to the deep sea, profound changes in their physical and chemical properties in less abundant but larger species account for most of the an- the coming decades. Coccolithophore blooms in the South- nual coccolithophore CaCO3 flux. This observation contrasts ern Ocean are thought to account for a major fraction of with the generally accepted notion that high particulate inor- the global marine calcium carbonate (CaCO3) production ganic carbon accumulations during the austral summer in the and export to the deep sea. Therefore, changes in the com- subantarctic Southern Ocean are mainly caused by E. hux- position and abundance of Southern Ocean coccolithophore leyi blooms. It appears likely that the climate-induced migra- populations are likely to alter the marine carbon cycle, with tion of oceanic fronts will initially result in the poleward ex- feedbacks to the rate of global climate change. However, pansion of large coccolithophore species increasing CaCO3 the contribution of coccolithophores to CaCO3 export in the production. However, subantarctic coccolithophore popula- Southern Ocean is uncertain, particularly in the circumpo- tions will eventually diminish as acidification overwhelms lar subantarctic zone that represents about half of the areal those changes. Overall, our analysis emphasizes the need for extent of the Southern Ocean and where coccolithophores species-centred studies to improve our ability to project fu- are most abundant. Here, we present measurements of an- ture changes in phytoplankton communities and their influ- nual CaCO3 flux and quantitatively partition them amongst ence on marine biogeochemical cycles. coccolithophore species and heterotrophic calcifiers at two sites representative of a large portion of the subantarctic zone. We find that coccolithophores account for a major frac- tion of the annual CaCO3 export, with the highest contribu- tions in waters with low algal biomass accumulations. No- tably, our analysis reveals that although Emiliania huxleyi Published by Copernicus Publications on behalf of the European Geosciences Union. 246 A. S. Rigual Hernández et al.: Coccolithophore biodiversity controls carbonate export 1 Introduction The remoteness and vastness of the Southern Ocean, to- gether with the inherent temporal and spatial variability of The emissions of carbon dioxide (CO2) into the atmosphere pelagic ecosystems, hampers accurate characterization and by anthropogenic industrial activities over the past 200 years quantification of Southern Ocean phytoplankton communi- are inducing a wide range of alterations in the marine envi- ties. Advances in satellite technology and modelling algo- ronment (Pachauri et al., 2014). These include ocean warm- rithms have allowed a circumpolar and year-round coverage ing, shallowing of mixed layer depths, changes in nutrient of the seasonal evolution of major phytoplankton functional supply to the photic zone and decreasing carbonate ion con- groups within the Southern Ocean (e.g. Alvain et al., 2013; centrations and pH of the surface ocean, a process known as Hopkins et al., 2015; Rousseaux and Gregg, 2015). In par- ocean acidification (Rost and Riebesell, 2004; IPCC, 2013). ticular, ocean-colour satellite reflectance observations have Substantial evidence from CO2 manipulation experiments in- been used to quantitatively estimate coccolithophore partic- dicates that many species of corals, pteropods, planktonic ulate inorganic carbon (PIC) concentrations throughout the foraminifera and coccolithophores will reduce their calcifi- Southern Ocean (Gordon et al., 2001; Balch et al., 2005b). cation rates under future ocean acidification scenarios (Bi- These satellite estimates suggest apparent high PIC values jma et al., 2002; Langdon and Atkinson, 2005 among oth- during summer near the major Southern Ocean fronts at- ers; Orr et al., 2005; Bach et al., 2015; Meyer and Riebesell, tributed to coccolithophores (Balch et al., 2011, 2016). This 2015). Owing to their moderate alkalinity and cold temper- band of elevated reflectance and PIC that encircles the entire atures, Southern Ocean waters are projected to become un- Southern Ocean was termed the “Great Calcite Belt” by these dersaturated with respect to aragonite no later than 2040 and authors. However, comparison of satellite remote-sensing to calcite by the end of the century (Cao and Caldeira, 2008; data with ship-based observations (Holligan et al., 2010; McNeil and Matear, 2008; Shadwick et al., 2013). This de- Trull et al., 2018) indicates that satellite ocean-colour-based cline in the saturation state of carbonate, together with other PIC estimates could be unreliable, particularly in Antarctic changes in carbonate chemistry speciation, will enhance dis- waters where they erroneously suggest high PIC abundances. solution of both aragonite and calcite shells and will make the Shipboard observations, on the other hand, provide a detailed biological precipitation of carbonate difficult in some marine picture of phytoplankton community composition and struc- calcifying organisms (Fabry et al., 2008; Gattuso and Hans- ture but are dispersed, both temporally and geographically, son, 2011). Since such thresholds will be reached sooner in and provide rather heterogenous data in terms of taxonomic polar regions, Southern Ocean ecosystems have been pro- groups investigated and the sampling scales and methodolo- posed as bellwethers for prospective impacts of ocean acidifi- gies used (e.g. Kopczynska et al., 2001; de Salas et al., 2011; cation on marine organisms at mid- and low latitudes (Fabry Poulton et al., 2013; Patil et al., 2017, among others). In et al., 2009). situ year-round monitoring of key strategic regions is crit- Coccolithophores are a major component of phytoplank- ically needed to establish baselines of phytoplankton com- ton communities in the Southern Ocean, particularly in its munity composition and abundance and to validate and im- northernmost province, the subantarctic zone, where they of- prove ocean biogeochemical models (Rintoul et al., 2012). ten exhibit maximum abundances and diversity (e.g. Graval- This information is also essential if we are to detect possi- osa et al., 2008; Saavedra-Pellitero et al., 2014; Malinverno et ble climate-driven changes in the structure of phytoplankton al., 2015; Charalampopoulou et al., 2016). Coccolithophores communities that could influence the efficiency of the biolog- play an important and complex role in the Southern Ocean ical carbon pump, with consequent feedbacks to the rate of carbon cycle (Salter et al., 2014). On the one hand, the deep-water carbon sequestration and global climate change production of calcite platelets (termed coccoliths) decreases (Le Quéré et al., 2007; Deppeler and Davidson, 2017). the alkalinity of surface waters, thereby reducing the atmo- Here, we document coccolithophore and carbonate parti- spheric uptake of CO2 from the atmosphere into the surface cle fluxes collected over a year by four sediment trap records ocean. On the other hand, the production of organic mat- deployed at two strategic locations of the Australian and ter through photosynthesis, and its subsequent transport to New Zealand sectors of the Southern Ocean considered rep- depth in settling particles, enhances carbon sequestration via resentative of a large portion of the SAZ (see Sect. 2.2 for the biological carbon pump (Volk and Hoffert, 1985). Addi- further details). Our measurements provide coccolith mass tionally, due to their high density and slow dissolution, coc- estimates of the main coccolithophore species and quanti- coliths act as an effective ballast for organic matter, increas- tatively partition annual carbonate fluxes amongst coccol- ing organic carbon sequestration depths (Buitenhuis et al., ithophore