Coccolithophore biodiversity controls carbonate export in the Southern Ocean

Abstract

<p><strong>Abstract.</strong> 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 (CaCO<sub>3</sub>) 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 CaCO<sub>3</sub> 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 CaCO<sub>3</sub> 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 CaCO<sub>3</sub> export with highest contributions in waters with low algal biomass accumulations. Notably, our analysis reveals that although <i>Emiliania huxleyi</i> is an important vector for CaCO<sub>3</sub> export to the deep sea, less abundant but larger species account for most of the annual coccolithophore CaCO<sub>3</sub> flux. This observation contrasts with satellite remote sensing images that mostly reflect <i>E. huxleyi</i> blooms as a result of its higher cell abundance and detachment of its relatively small liths. It appears likely that the climate-induced migration of oceanic fronts will initially result in the poleward expansion of large coccolithophore species increasing CaCO<sub>3</sub> 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.</p>

Coccolith volume of the Southern Ocean coccolithophore Emiliania huxleyi as a possible indicator for palaeo‐cell volume

“…Calcium carbonate production in iron‐limited regions of the ocean, such as the Southern Ocean, is low and contributes a minor part to the global budget (Milliman, 1993) and, in case of coccolithophores assemblages, is mostly associated with larger species than E. huxleyi (e.g. Calcidiscus leptoporus , Helicosphaera carteri and Coccolithus pelagicus ; Rigual Hernández et al (2020)).…”

Southern Ocean Calcification Controls the Global Distribution of Alkalinity

“…Coccolithophore blooms in the Southern Ocean are extensive, covering ∼52 × 10 6 km 2 and accounting for roughly 26% of oceanic suspended PIC (Balch et al, 2005). Indeed, coccolithophores are a large driver of CaCO 3 fluxes in the SAZ, with contributions ranging from ∼10% to >85% of total CaCO 3 (Manno et al, 2018; Rembauville et al, 2016; Rigual Hernández et al, 2020; Wilks et al, 2017). Zooplankton calcifiers, pteropods and foraminifera, also constitute significant portions of CaCO 3 fluxes in the Southern Ocean (Hunt et al, 2008; Rigual Hernández et al, 2018).…”

The haplo-diplontic life cycle expands niche space of coccolithophores

“…Although quantitative interpretation of niche space is difficult since niche space will vary depending on the number of environmental axes included (Blonder et al, 2014), these results highlight that holococcolithophores contribute significantly to the niche space of coccolithophores, in some instances contributing more to total niche space than the heterocococlithophore phase. In this context C. pelagicus is particularly relevant as this species contributes significantly to the global carbonate flux (Ziveri et al (2007); Rigual Hernández et al (2019), and is one of the key calcifiers in the Arctic Ocean (Daniels et al, 2016).…”

“…2e), Coccolithus pelagicus and Calcidiscus leptoporus ( Fig. 2i) which contribute more to the CaC0 3 flux to the deep ocean than E. huxelyi due to their larger coccolith and coccosphere size (Ziveri et al, 2007;Rigual Hernández et al, 2019).…”