Pelagic coccolithophore production and dissolution and their impacts on particulate inorganic carbon cycling in the western North Pacific

Abstract. Coccolithophores, a type of single-celled phytoplankton that is abundant in global oceans, are closely associated with the carbonate pump and thus play a crucial role in the marine carbon cycle. Here we investigated coccolithophore abundances, species compositions, coccolithophore calcium carbonate (CaCO₃ as calcite) and particulate inorganic carbon (PIC) concentrations in the upper water column of the western North Pacific Ocean, along a meridional transect spanning the oligotrophic subtropical gyre and the nutrient-richer Kuroshio-Oyashio transition region. Our results revealed that Umbellosphaera tenuis was the dominant coccolithophore species in the former, while Emiliania huxleyi and Syracosphaera spp. dominated in the latter. Coccolithophore calcite contributed a major fraction of the PIC standing stocks above a depth of 150 m, among which E. huxleyi was the most important producer while less abundant and larger species also played a role. The coccolithophore CaCO₃ production rate in the subtropical gyre (0.62 mol m⁻² yr⁻¹) was ~5-fold higher than that in the Kuroshio-Oyashio transition region (0.14 mol m⁻² yr⁻¹), indicating that inorganic carbon metabolism driven by coccolithophores is relatively strong in oligotrophic ocean waters. Using a box model including coccolithophore CaCO₃ production and metabolic calcite saturation state, we demonstrated that CaCO₃ dissolution associated with organic carbon metabolism can generate excess alkalinity in the oversaturated upper water column of the western North Pacific Ocean. Results of our study highlight the critical role of coccolithophores in CaCO₃ production and dissolution; knowledge of these processes is important to assess PIC cycling and carbonate pump efficiency in the pelagic ocean.