Taxon-specific coccolithophore carbonate stocks and production across a Pacific trophic gradient from oligotrophic gyre to upwelling zone

Abstract. Coccolithophores contribute substantially to marine carbonate production, yet species-resolved estimates of coccosphere-associated CaCO₃ stocks remain scarce across large-scale trophic gradients. Here we quantify multispecific coccosphere carbonate inventories and first-order production estimates along the BIOSOPE transect across the South Pacific Gyre, one of the most strongly stratified regions of the global ocean. Estimates were obtained using an upgraded SYRACO automated recognition workflow combined with bidirectional circular polarization (BCP) imaging, allowing direct reconstruction of coccosphere calcite content at the cellular scale. Coccolithophore assemblages reproduced the major regional and vertical ecological structures previously described across the transect, including the transition from nutrient-rich surface communities near the Marquesas Archipelago and the eastern upwelling-influenced region to vertically structured assemblages in the oligotrophic gyre. At the transect scale, CaCO₃ standing stocks were primarily controlled by these large-scale trophic gradients, with maxima located in surface waters of mesotrophic regions and at depth within the deep chlorophyll maximum in the stratified gyre, indicating a vertical decoupling between coccosphere abundance and carbonate inventories under oligotrophic conditions. Taxon-specific analyses show that carbonate inventories reflect both coccosphere abundance and strong interspecific contrasts in calcite content. Isochrysidales, particularly Emiliania huxleyi, dominated carbonate stocks through their numerical abundance, whereas more heavily calcified taxa contributed disproportionately relative to their cell densities. Despite structuring assemblages in the lower euphotic zone, deep-dwelling taxa represented only a minor fraction of suspended carbonate stocks in the water column, contrasting with their major contribution to sedimentary assemblages in oligotrophic regions. First-order CaCO₃ production estimates followed spatial patterns similar to standing stocks, with enhanced production in mesotrophic regions and a sustained contribution from oligotrophic communities. Although uncertainties remain related to species-specific growth rates, these estimates provide the first multispecific constraints on coccolithophore calcification dynamics in the southeastern Pacific. Overall, this study demonstrates that combining automated coccosphere recognition with BCP-based carbonate measurements provides a robust framework for resolving species-level carbonate inventories across major oceanic trophic gradients and represents an important step toward quantitative assessments of coccolithophore contributions to pelagic carbonate cycling at basin scale.