Biological calcification rate and species-specific contributions of coccolithophores to total calcite inventory in the eastern Indian Ocean

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.