An observational case study for inorganic nitrogen dry deposition potential on sea-surface primary production in the subtropical, western North Pacific

The deposition of nutrients from the atmosphere into the ocean has been recognized as an important pathway for marine ecosystems. We measured the photosynthetic rates of phytoplankton for surface seawater sampled at three sites with nitrogen-nutrient-depleted condition in the subtropical, western North Pacific in March 2021 during the R/V Mirai cruise using a photosynthesis-irradiance experiment, to investigate the impact of atmospheric inorganic nitrogen deposition. The maximum photosynthetic rates at the Sites 1 (25-00°N, 145-00°E), 2 (26-30°N, 135-00°E), and 3 (28-00°N, 135-17°E) were 5.56±0.15, 6.91±0.17 and 7.38±0.15 mg-C (mg-Chl-a)^-1 h^-1, respectively. We estimated the atmospheric deposition fluxes of inorganic nitrogen to be 6.3±0.9, 12.7±2.2 and 21.2±3.3 µmol-N m^-2 during the 24 h before the seawater sampling at Sites 1, 2, and 3, respectively, using the regional chemical transport model. The primary production fluxes estimated from the simulated inorganic nitrogen deposition flux assuming the all bioavailable with the Redfield ratio corresponded well the changes in the potential production of the surface seawater based on the measured maximum photosynthetic rates and Chlorophyll-a concentration at each site. These findings highlight the role of atmospheric inorganic nitrogen deposition in sustaining primary production in nutrient-depleted subtropical western North Pacific.