[A-OS16] Coastal ocean circulation and material cycle

[AOS16-P06] Lateral Nitrogen Input During Summer to Sagami Bay, Japan: A Deep Bay and Carbon Dioxide Sink

*Yoshihisa Mino¹, Chiho Sukigara², Daisuke Takahashi³, Akihiko Morimoto⁴ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.Japan Agency for Marine-Earth Science and Technology, 3.Tokai University, 4.Ehime University)

Keywords:Sagami Bay, Surface water suspended particle, Stable nitrogen isotope, Nutrients, Biological carbon pump

Sagami Bay is a deep bay with a depth of 1,500 m at the center, connecting the deep water of the Pacific Ocean through the Sagami Trough. Our previous study revealed that the central Sagami Bay absorbs carbon dioxide (CO₂) at an annual flux (30 gC m^–2 y^–1), more than six times the global average, which was attributed to biological drawdown of the seawater CO₂ partial pressure (i.e., CO₂ fixation by phytoplankton) during spring and summer, followed by accelerated downward transports of particulate organic carbon. This study investigated variations in surface water suspended particulate nitrogen (PN) isotope delta (δ¹⁵N_PN) to examine upper layer PN and nutrient dynamics in Sagami Bay. The δ¹⁵N_PN exhibited variation corresponding to seasonal nitrate concentration changes, and its horizontal distribution in summer closely correlated with a significant salinity gradient, alongside PN concentration. Analysis of the negative δ¹⁵N_PN-salinity relationship, using a conservative mixing model with Kuroshio offshore water and low-salinity coastal water as endmembers, revealed that N with a high δ¹⁵N was supplied to Sagami Bay through freshwater inflow from Tokyo Bay,connected to the bay. The transport efficiency of this N input increased with heavy rain events, significantly contributing to the bay's highest productivity of the year. This summer N input plays a vital role in exporting particulate organic carbon to the mesopelagic zone and absorbing atmospheric CO₂.