[A-CG45] Biogeochemical linkages between the surface ocean and atmosphere

[ACG45-02] Distribution and determinants of nutrients and primary production in the eastern Indian sector of the Southern Ocean

*Manami Tozawa¹, Daiki Nomura^1,2,3, Kaihe Yamazaki⁴, Masaaki Kiuchi¹, Daisuke Hirano^4,5, Shigeru Aoki⁶, Hiroko Sasaki⁷, Hiroto Murase⁸ (1.Faculty/Graduate School of Fisheries Sciences, Hokkaido University, 2.Field Science Center for Northern Biosphere, Hokkaido University, 3.Arctic Research Center, Hokkaido University, 4.National Institute of Polar Research, 5.The Graduate University for Advanced Studies, SOKENDAI, 6.Institute of Low Temperature Science, Hokkaido University, 7.Japan Fisheries Research and Education Agency, Yokohama, 8.Tokyo University of Marine Science and Technology)

Keywords:Nutrients, net community production, Southern Ocean Indian Ocean sector, seasonal sea ice zone

To investigate the spatial distributions and determinants of nutrient concentrations, we measured NO₃⁻+NO₂⁻, PO₄³⁻, and Si(OH)₄concentrations in the eastern Indian Ocean sector of the Antarctic Ocean (80−150°E, south of 60°S) between December 2018 and February 2019. In the Antarctic Circumpolar Current (ACC) domain, nutrient concentrations were increased by nutrients supplied from the deep layer and by organic matter decomposition and remineralization within the seasonal pycnocline after the development of strong stratification. Strong stratification also enhanced phytoplankton growth and nutrient consumption by photosynthesis. In contrast, in the subpolar region, nutrient concentrations were increased by nutrients supplied by brine discharged during sea ice formation and decreased by dilution with sea ice meltwater. Although high salinity in the surface and subsurface layers corresponded well to upwelling areas around subpolar subgyres, high salinity was not necessarily correlated with nutrient concentrations. We estimated primary production both from in situ nutrient data and from satellite-acquired chlorophyll-a data. According to both estimation methods, primary production was high in the subpolar region, especially around 120−130°E. However, nutrient-based estimation also showed high production in coastal areas where, because of sea ice and cloud cover, estimation based on satellite data was not possible. To understand primary production in seasonal ice areas, the best estimation method should be selected for the research goals or multiple methods should be used in combination.