09:15 〜 09:30
[AOS16-02] 西部北太平洋中緯度域における溶存有機態窒素・リンの南北分布
Nutrient-rich North Pacific Intermediate Water (NPIW) flows southward in the western North Pacific Ocean. The NPIW potentially influences the nutrient environment and primary production in the upper layer of the subtropical gyre through diffusion and mixing between the NPIW and mode waters. Since the nitrate +nitrite (N+N)/ phosphate (PO4) ratio in the NPIW is lower than the Redfield N/P ratio of phytoplankton (16), its upward supply and subsequent phytoplankton utilization would result in a surplus of PO4 in the subtropical upper layer. However, N+N and PO4 are both depleted in the subtropical upper layer, while dissolved organic nitrogen (DON) and phosphorus (DOP) are abundant and potentially utilized by phytoplankton there. The DON and DOP would be produced through biological metabolism using the N+N and PO4 originated from the NPIW, but their distributions and N/P ratios have not been clarified in the western North Pacific. In this study, we investigated the distribution of DON and DOP in addition to N+N and PO4 by a meridional observation of the western North Pacific and discussed spatial variability of their N/P ratios.
Water sampling was performed using the training vessel Shioji-maru at seven stations from 27.1°N to 36.5°N along 141.5°E in August 2022. Total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations were measured using an autoanalyzer with wet-oxidation colorimetry. N+N and PO4 concentrations were measured using a colorimetric autoanalyzer. DON and DOP concentrations were determined by subtracting N+N and PO4 concentrations from the TDN and TDP concentrations, respectively.
The N+N and PO4 concentrations were high in the NPIW (~43.45 μM and, ~3.17 μM, respectively), while those were near the detection limit (3 nM) in the upper layer of the subtropical gyre. The N+N/PO4 ratios in the study area were generally lower than the Redfield N/P ratio (16). The DON and DOP concentrations ranged from 1.31 μM to 6.03 μM and from 0.00 μM to 0.22 μM, respectively, and were higher in the subtropical upper layer than in the NPIW. The TDN and TDP concentrations ranged from 3.61 μM to 46.86 μM and from 0.11 μM to 3.25 μM, and the TDN/TDP ratios were higher than 16 in the subtropical upper layer. The high TDN/TDP ratios (>16) mostly reflected the DON/DOP ratio there, probably resulting from active dinitrogen fixation which could release N-rich dissolved organic matter to the surface water. Furthermore, in the deep layer between 500 m and 1000 m, the DOP concentrations were near the detection limit (0.04 μM), while DON concentrations remained at approximately 2 μM, which could be regarded as refractory DON. When bioavailable TDN (BTDN) was estimated by subtracting 2 μM from TDN, the BTDN/TDP ratios in the subtropical upper layer (14.7±3.3) were close to the Redfield N/P ratio (16), suggesting that the N/P ratios of biological utilization and regeneration were in near-stoichiometric balance.
Water sampling was performed using the training vessel Shioji-maru at seven stations from 27.1°N to 36.5°N along 141.5°E in August 2022. Total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) concentrations were measured using an autoanalyzer with wet-oxidation colorimetry. N+N and PO4 concentrations were measured using a colorimetric autoanalyzer. DON and DOP concentrations were determined by subtracting N+N and PO4 concentrations from the TDN and TDP concentrations, respectively.
The N+N and PO4 concentrations were high in the NPIW (~43.45 μM and, ~3.17 μM, respectively), while those were near the detection limit (3 nM) in the upper layer of the subtropical gyre. The N+N/PO4 ratios in the study area were generally lower than the Redfield N/P ratio (16). The DON and DOP concentrations ranged from 1.31 μM to 6.03 μM and from 0.00 μM to 0.22 μM, respectively, and were higher in the subtropical upper layer than in the NPIW. The TDN and TDP concentrations ranged from 3.61 μM to 46.86 μM and from 0.11 μM to 3.25 μM, and the TDN/TDP ratios were higher than 16 in the subtropical upper layer. The high TDN/TDP ratios (>16) mostly reflected the DON/DOP ratio there, probably resulting from active dinitrogen fixation which could release N-rich dissolved organic matter to the surface water. Furthermore, in the deep layer between 500 m and 1000 m, the DOP concentrations were near the detection limit (0.04 μM), while DON concentrations remained at approximately 2 μM, which could be regarded as refractory DON. When bioavailable TDN (BTDN) was estimated by subtracting 2 μM from TDN, the BTDN/TDP ratios in the subtropical upper layer (14.7±3.3) were close to the Redfield N/P ratio (16), suggesting that the N/P ratios of biological utilization and regeneration were in near-stoichiometric balance.