Coccolithophore biodiversity controls carbonate export in the Southern Ocean

Abstract

Abstract. Southern Ocean waters are projected to undergo profound changes in their physical and chemical properties in the coming decades. Coccolithophore blooms in the Southern Ocean are thought to account for a major fraction of the global marine calcium carbonate (CaCO3) production and export to the deep sea. Therefore, changes in the composition and abundance of Southern Ocean coccolithophore populations are likely to alter the marine carbon cycle, with feedbacks to the rate of global climate change. However, the contribution of coccolithophores to CaCO3 export in the Southern Ocean is uncertain, particularly in the circumpolar subantarctic zone that represents about half of the areal extent of the Southern Ocean and where coccolithophores are most abundant. Here, we present measurements of annual CaCO3 flux and quantitatively partition them amongst 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 fraction of the annual CaCO3 export, with the highest contributions in waters with low algal biomass accumulations. Notably, our analysis reveals that although Emiliania huxleyi is an important vector for CaCO3 export to the deep sea, less abundant but larger species account for most of the annual coccolithophore CaCO3 flux. This observation contrasts with the generally accepted notion that high particulate inorganic carbon accumulations during the austral summer in the subantarctic Southern Ocean are mainly caused by E. huxleyi blooms. It appears likely that the climate-induced migration of oceanic fronts will initially result in the poleward expansion of large coccolithophore species increasing CaCO3 production. However, subantarctic coccolithophore populations will eventually diminish as acidification overwhelms those changes. Overall, our analysis emphasizes the need for species-centred studies to improve our ability to project future changes in phytoplankton communities and their influence on marine biogeochemical cycles.

Factors controlling the competition between <i>Phaeocystis</i> and diatoms in the Southern Ocean and implications for carbon export fluxes

“…However, calcifying coccolithophores and dimethylsulfide-producing (DMS) Phaeocystis have been found to contribute in a significant way to total phytoplankton biomass in summer/fall in the subantarctic (Balch et al, 2016;Nissen et al, 2018) and in spring/summer at high latitudes, respectively (Smith and Gordon, 1997;Arrigo et al, 1999Arrigo et al, , 2017DiTullio et al, 2000;Poulton et al, 2007), thus suggesting that the succession and competition of different plankton groups govern biogeochemical cycles at the (sub)regional scale. As climate change is expected to differentially impact the competitive fitness of different phytoplankton groups and ultimately their contribution to total net primary production (NPP; IPCC, 2014; Constable et al, 2014;Deppeler and Davidson, 2017) with a likely increase in the relative importance of coccolithophores and Phaeocystis in a warming world at the expense of diatoms (Bopp et al, 2005;Winter et al, 2013;Rivero-Calle et al, 2015), the resulting change in SO phytoplankton community structure is likely to affect global nutrient and carbon distributions, ocean carbon uptake, and marine food web structure (Smetacek et al, 2004). While a number of recent studies have elucidated the importance of coccolithophores for subantarctic carbon cycling (e.g., Rosengard et al, 2015;Balch et al, 2016;Nissen et al, 2018;Rigual Hernández et al, 2020), few estimates quantify the role of present and future highlatitude SO phytoplankton community structure for ecosystem services such as NPP and carbon export (e.g., Wang and Moore, 2011;Yager et al, 2016).…”

“…As climate change is expected to differentially impact the competitive fitness of different phytoplankton groups and ultimately their contribution to total net primary production (NPP; IPCC, 2014; Constable et al, 2014;Deppeler and Davidson, 2017) with a likely increase in the relative importance of coccolithophores and Phaeocystis in a warming world at the expense of diatoms (Bopp et al, 2005;Winter et al, 2013;Rivero-Calle et al, 2015), the resulting change in SO phytoplankton community structure is likely to affect global nutrient and carbon distributions, ocean carbon uptake, and marine food web structure (Smetacek et al, 2004). While a number of recent studies have elucidated the importance of coccolithophores for subantarctic carbon cycling (e.g., Rosengard et al, 2015;Balch et al, 2016;Nissen et al, 2018;Rigual Hernández et al, 2020), few estimates quantify the role of present and future highlatitude SO phytoplankton community structure for ecosystem services such as NPP and carbon export (e.g., Wang and Moore, 2011;Yager et al, 2016).…”

Particle Fluxes at the Australian Southern Ocean Time Series (SOTS) Achieve Organic Carbon Sequestration at Rates Close to the Global Median, Are Dominated by Biogenic Carbonates, and Show No Temporal Trends Over 20-Years

“…However, the sum of the masses of the measured component fluxes is on average only ∼90% of the total measured mass fluxes, leaving open the possibility of lithogenic fluxes contributing up to 10% of the total flux. Comparing estimates of coccolithophore calcite(Rigual Hernández et al, 2019;Rigual-Hernández et al, 2020) and foraminifera shell weights…”

Haplo-diplontic life cycle expands coccolithophore niche

“…It should, however, be noted that the response of heterococcolithophores to ocean acidification is both strain and species dependent (Langer et al, 2006(Langer et al, , 2009Meyer and Riebesell, 2015), and global calcification rates might be more impacted by shifts in species composition rather than individual responses (Ridgwell et al, 2009). Furthermore, contradicting evidence suggesting increased coccolithophore abundance in response to higher CO 2 has been noted in situ (Rivero-Calle et al, 2015) Finally, additional experiments on the numerical response of heterococcolithophores and holococcolithophores to various environmental drivers such as those performed on E. huxleyi would allow a better understanding of individual environmental pressures and will furthermore be highly valuable for future modelling approaches. In this context a better understanding of the triggers of phase transition would ad-J.…”