The Xiamen Symposium on Marine Environmental Sciences (XMAS 2027)

Nitrogen fixation, a critical source of new nitrogen, plays a vital role in sustaining primary production in oligotrophic marine environments such as the Tropical Western North Pacific (TWNP). Seawater samples were collected during two R/V Isabu cruises in late summer to early fall of 2023 and 2024 to examine temporal variations in nitrogen fixation and associated environmental drivers. Daily N₂ fixation rates were measured using the ¹⁵N₂ tracer-enriched seawater method. Depth-integrated nitrogen fixation within the euphotic zone was higher in 2023 (60.0 ± 22.7 µmol m⁻² d⁻¹) than in 2024 (44.3 ± 20.7 µmol m⁻² d⁻¹). Dust-derived PM_2.5 concentrations were significantly higher in 2023 (0.18 ± 0.08 µg m⁻³) than in 2024 (0.09 ± 0.02 µg m⁻³) (p < 0.05), likely increasing iron availability and stimulating diazotrophic activity. The contribution of nitrogen fixation to gross primary production was minor (0.2–3.0%), but accounted for 16.0 ± 9.1%, up to approximately 41%, of new production during the study periods. These findings indicate that, despite extremely low nutrient concentrations, dust-borne iron inputs, which may stimulate nitrogen fixation, substantially support new production in the oligotrophic TWNP.