Coccolithophores are calcifying protists that have significant role in marine biological pump through calcite production (CP). We determined the biological calcification rate and ecologically key species (e.g. Gephyrocapsa oceanica) contribution to total calcite stocks in the eastern Indian Ocean (EIO) during spring intermonsoon, this region was quite under-sampled compared to other oceanic regions. Our results indicated that the numerically dominant species are of great importance to cell calcite even with low cellular calcite. CP rate in EIO ranged from 0.148 to 85.017 μmol C m^-3 d^-1 in the entire dataset, which was lower than those in the global ocean. Coccolithophores contributed 8.5% of total phytoplankton carbon fixation. CP acted as a function of light irradiance in the euphotic zone, while a decoupling between CP and primary productivity (PP) was found in most of the bottom of euphotic zone. To some degree, this decoupling indicated calcification was more nitrate-dependent and less light-dependent than photosynthesis. CP/PP ratios were highly variable in our database and were possibly influenced by photoacclimation across various coccolithophore species. As roughly estimated, the turnover rate of coccolith calcite took values around 0.02-0.05 d^-1. The association between particulate organic carbon (POC) and calcite stocks implied a potential increasing ballast as depth increase. The current profiles of calcification and associated biogeochemical elements obtained from this field study helps our understanding about characteristics of biological pump in the EIO and are valuable for estimating calcification rate in the region using satellite data and numerical modeling.

The eastern Indian Ocean (EIO) cruise took place onboard R/V “SHIYAN 3” during the period 27 February to 22 April 2017 spring inter-monsoon and consisted of 15 stations along three transects. Water samples were collected from a Rosette sampler system with attached Seabird CTD (conductivity-temperature-depth) profiler. Pressure, conductivity, and temperature at all designed stations were collected by means of CTD probes. For coccolithophore sampling, 500 ml seawater was gently filtered through mixed cellulose membrane (25 mm diameter, 0.22 μm) with a vacuum pump lower than 20 mm Hg pressure. For nutrient sampling, 100 ml seawater was filtered through acid-cleaned 0.45 μm pore-size acetate cellulose filters and then chilled at -20°C. In the laboratory, five kinds of nutrients (NO₃⁻, NO₂⁻, PO₄³⁻, and SiO₃²⁻) were analyzed using an Autoanalyzer 3-AA3 (Bran+ Luebbe, Norderstedt, Germany). For Chl a determination, 1000 ml seawater was filtered onto 25 mm Whatman GF/F filters and chilled at -20°C. In the laboratory, Chl a extraction and measurement followed the procedure of Parsons et al. (1984). Radioactive carbon isotope was utilized to trace the inorganic carbon uptake through calcification process. Field seawater was collected according to five light attenuations (100%, 50%, 25%, 10%, and 1% of surface PAR) within euphotic layer. The above all biochemical parameters (section 2.2 and section 2.3) were sampled following the same depth layer. 200 ml seawater was transferred to 250 ml Nalgene polycarbonate bottles. For each light level, triplicate light bottles and one dark bottle were incubated 6 h (from morning 10:00 to afternoon 16:00) on deck spiked with 10-20 μCi ¹⁴C-labelled NaH¹⁴CO₃. After incubation, the samples were filtered gently through GF/F filters (Whatman 25 mm) and then rinsed with GF/F filtered-seawater three times. The filters were then kept in 20 ml scintillation vials under cool storage.

All data were derived from field investigation, and there are some missing values during measurement.