The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Nitrogen fixation (NF) is a critical nitrogen source for export production in the oligotrophic ocean, particularly in subtropical gyres like the North Pacific Subtropical Gyre (NPSG). However, the origins of the nutrients supporting NF and the fate of NF-derived nitrogen remain unclear. We conducted seasonal underway measurements of surface dissolved organic nitrogen (DON) and phosphorus (DOP) in the NPSG during the summer and winter of 2020, respectively. Our observations reveal an inverse seasonal variation between DON and DOP concentrations at the NPSG center, with higher DON (summer: 4.59±0.19 µM) and lower DOP (summer: 0.18±0.02 µM) in summer, and lower DON (winter: 4.15±0.16 µM) and higher DOP (winter: 0.25±0.03 µM) in winter. NF rates measured by the 15N2 gas dissolution method were approximately 5 times higher in summer (5.10±3.48 nmol N/L/d) compared to winter (1.09±1.23 nmol N/L/d). Correlations between DON, DOP, and NF rates, alongside δN15 of DON, suggest that the seasonal variation in DOP adequately supports the enhanced NF observed in summer. And approximately 90% of NF-derived nitrogen is estimated to partition into the dissolved DON pool. These findings highlight the role of surface DOP as a vital phosphorus source for NF and provide insights into seasonal nutrient cycling mechanisms supporting export production in oligotrophic oceans.