[A-OS20] Chemical and Biological Oceanography

[AOS20-02] Impact of Subtropical Mode Water on primary production and biological nitrogen fixation in the western North Pacific Ocean

*Rie Ikeda¹, Keitaro Waki¹, Ikkan Kamiyama¹, Sayaka Tamura¹, Riko Fushimi¹, Saori Yasui-Tamura¹, Michiyo Yamamoto-Kawai¹, Fumiaki Kobashi¹, Takuhei Shiozaki², Fuminori Hashihama¹ (1.Tokyo University of Marine Science and Technology, 2.Atmosphere and Ocean Research Institute,The University of Tokyo )

Keywords:Biogeochemistry, Primary production, Biological nitrogen fixation, Subtropical mode water, Western North Pacific

Subtropical Mode Water (STMW) is characterized by vertically uniform potential temperature and salinity. In the western North Pacific Ocean, the STMW forms through winter vertical mixing south of the Kuroshio Extension region. It is suggested that vertical mixing during STMW formation brings nutrients from the deeper layer to the upper euphotic zone, potentially increasing primary production. However, field studies examining the relationship between STMW and biogeochemical processes have rarely been conducted in the western North Pacific, and the impact of STMW on primary production remains unclear. In this study, we reveal the relationship between STMW and biogeochemical parameters by seasonal shipboard observations in the western North Pacific.
Observations were conducted on the training vessel "Shioji-maru" of Tokyo University of Marine Science and Technology during winter (March 2023, March 2024) and summer (August 2022, July 2023, July 2024) along a transect at 141.5°E between 27°N and 33°N. The STMW thickness, surface mixed layer depth, and euphotic zone depth were determined based on CTD and light observation data. Water samples for primary production, biological nitrogen fixation, and nutrients (nitrate and phosphate) were collected by Niskin bottles attached to the CTD. Primary production and nitrogen fixation were measured with ¹³C and ¹⁵N tracer methods, respectively. Nitrate and phosphate concentrations at a micromolar level were measured using a conventional autoanalyzer, and those at a nanomolar level were determined by a liquid waveguide spectrophotometry.
STMW was generally detected as the surface mixed layer in winter, while it subducted below the euphotic zone in summer. Primary production was generally higher in winter than in summer. Nitrogen fixation activity was detected at all stations in summer, while it was only detected at 27°N and 29°N in winter. Surface nitrate and phosphate concentrations tended to be higher in winter than in summer. We found a significant positive relationship between the STMW thickness in the upper 150 dbar of the water column (approximate euphotic zone) and the depth-integrated primary production in the euphotic zone, surface nutrient concentrations, and depth-integrated nutrient stocks in the upper 150 dbar of the water column. These results suggest that the formation of thick STMW increased nutrient concentrations and primary production in winter. In summer, the impact of STMW on primary production was small because the subduction of STMW below the euphotic zone prevented nutrient supply to the surface layer. We also found a significant negative relationship between the STMW thickness in the upper 150 dbar of the water column and the depth-integrated nitrogen fixation in the euphotic zone. This result suggests that, in summer, nitrogen fixation contributes to primary production, as surface nitrate is more depleted than in winter due to the smaller impact of STMW to the euphotic zone.