[A-OS16] Chemical and Biological Oceanography

[AOS16-01] Comparative evaluation of dinitrogen fixation and nitrate supply in the western North Pacific Ocean

*Keitaro Waki¹, Takuhei Shiozaki², Saori Yasui-Tamura¹, Riko Fushimi¹, Michiyo Yamamoto-Kawai¹, Toshiya Katano¹, Gloria Silvana Duran Gomez¹, Takeyoshi Nagai¹, Koji Shimada¹, Fuminori Hashihama¹ (1.Tokyo University of Marine Science and Technology, 2.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Dinitrogen fixation, Nitrate supply, New production, the western North Pacific Ocean

Oceanic primary production consists of new production and regenerated production. As new production is equivalent to the net biological uptake of CO₂ at steady-state, it is important for understanding climate change and biological productivity. Nitrate supply from deep water to the euphotic zone is recognized as a major source of new production, while dinitrogen fixation has recently shed light on an important source of new production in the oligotrophic subtropical ocean. In addition, recent improvements in dinitrogen fixation assay have led the dinitrogen fixation activities to be higher than previous estimates. In this study, we examined dinitrogen fixation activity and nitrate supply along the meridional transect of the western North Pacific to reveal a major source of new production in this region.
Observations were conducted aboard the Shioji-maru of Tokyo University of Marine Science and Technology in August 2022. We occupied five stations along a 141.5°E transect from 27.0°N to 36.5°N. Water samples were collected from three light-intensity depths (100 % (5 m), 10 %, 1 %) using acid-cleaned Niskin-X bottles mounted on a CTD system. Dinitrogen fixation and primary production were measured by ¹⁵N and ¹³C tracer methods, respectively. Nitrate supply from deep water was calculated from vertical diffusivity and concentration gradient of nitrate at the bottom of the euphotic zone (1 % light intensity depth).
The depth-integrated dinitrogen fixation in the euphotic zone varied from 6.2 to 33.8 μmol m^-2 d^-1. High dinitrogen fixation (15.6-33.8 μmol m^-2 d^-1) were observed in the subtropical region (27.0-33.0°N) south of Kuroshio, where low nitrate supplies (0.0-14.6 μmol m^-2 d^-1) were observed. Meanwhile, in the region north of Kuroshio (27.0-36.5°N), low dinitrogen fixation activities (6.2-12.0 μmol m^-2 d^-1) and high nitrate supplies (33.4-1945.4 μmol m^-2 d^-1) were observed. If assuming dinitrogen fixation and nitrate supply as driving sources of new production, the contribution of dinitrogen fixation to new production was higher in the subtropical region (62-100 %) than in the northern region (1-16 %). The depth-integrated primary production in the euphotic zone ranged from 18.5 to 72.5 mmol m^-2 d^-1. Carbon-based new production ([dinitrogen fixation+nitrate supply]*6.6) accounted for ~1 % of primary production, indicating that most primary production was regenerated production. Although new production is less significant in primary production regime of the study area, dinitrogen fixation would play an important role in sustaining new production in the subtropical region of the western North Pacific.