Seasonal changes in dissolved organic nitrogen in the western North Pacific Subtropical Gyre

Abstract

The goal of this study is to determine the d¹⁵N values for low concentrations of dissolved organic nitrogen (DON) in surface and subsurface water and to gain new insight into nitrogen cycle in the North Pacific Subtropical Gyre. We analyzed concentrations and d¹⁵N for both DON and nitrate at two sites, KEO (32.3N, 144.5E) and KEOS (25N, 145E), in the North Pacific Subtropical Gyre.The water sampling was performed in December 2021 and April 2022. The results showed that nitrate concentrations were below 0.4 uM from the surface to 150 m depths and the d¹⁵N of them could not be determined, except in April at KEO. DON concentrations were 4.7 +- 0.3 uM (n = 7) and 4.6 +- 0.3 uM (n = 7) in December and April at KEO, respectively, and were 4.5 +- 1.9 uM (n = 10) and 4.5 +- 0.9 uM (n = 8) at KEOS, respectively. The d¹⁵N_DON (permil Air) was +2.2 +- 0.7 and +3.4 +- 0.3 at KEO, and +5.3 +- 1.0 and +1.9 +- 1.8 at KEOS, respectively. These results suggest that the DON sources in the surface layers of KEO and KEOS is the contribution of organic matter derived from nitrogen fixation, which exhibits lower d¹⁵N than that of nitrate in the Mesopelagic zone.