Biological dinitrogen (N2) fixation is an important new nitrogen source in oligotrophic subtropical oceans. In numerical model studies, the east-west gradient of iron deposition as atmospheric Asian dust strongly affects the zonal distribution of N2 fixation activity in the North Pacific, but the in-situ relationship at a basin-scale is not well examined. We examined the trans-Pacific longitudinal variation in N2 fixation activity from 120°W to 137°E at 23°N in summer with environmental parameters that potentially influence diazotrophy. The dissolved inorganic iron concentration in surface water was consistently low (<0.4 nM) throughout the study area. The modelled deposition flux of iron as atmospheric dust (dust-Fe) largely increased westward, whereas labile phosphorus (phosphate and labile phosphoric monoesters) in the surface water decreased westward. N2 fixation varied between 34.6–298 µmol N m-2 day-1 and was high (>200 µmol m-2 day-1) in the central area (150–180°W), where both dust-Fe input and the phosphorus stock were in intermediate ranges. The rates of N2 fixation showed an increasing trend with dust-Fe input in the eastern and western parts of 180°, indicating that increasing dust input enhanced N2 fixation activity. However, compared with that of the eastern region, the effect of enhancement on activity was smaller in the western region, where phosphate concentration in the euphotic zone was low (<50 nM), presumably due to the higher iron requirement to utilize organic phosphorus. Our data show that phosphorus availability substantially controls the longitudinal distribution of N2 fixation through co-limitation with iron in the subtropical North Pacific.

Stable isotope ratios of carbon and nitrogen (δ13C and δ15N) in the particulate organic matter (POM) of the euphotic layer were seasonally investigated in the vicinity of the Kuroshio from 2008 to 2019 (n = 490). Generalized linear models (GLMs) showed significant seasonality of δ13C and δ15N of POM in the coastal (between Japan Main Island and the northern edge of the Kuroshio) and offshore (Kuroshio and more southern parts) areas. Seasonal climatological δ13C estimated based on the GLMs and seasonal median values of the environmental parameters was the highest in summer (-22.4 ± 0.2‰ and -22.9 ± 0.2‰ in the coastal and offshore areas, respectively) and lowest in winter in both areas (-23.9 ± 0.2‰ and -24.3 ± 0.2‰ in the coastal and offshore areas, respectively). Seasonal climatological δ15N showed different spatial variations from spring to summer. The δ15N value was the lowest during winter (0.8 ± 0.4‰), and increased to a similar level during the other three seasons in the coastal area (~3‰), suggesting that nitrate originating in the deep-sea water was the main source of new production from spring to autumn. In contrast, δ15N in the offshore areas decreased from spring (2.6 ± 0.4‰) to summer (0.9 ± 0.4‰), suggesting that the significant contributions of atmospheric deposition and nitrogen fixation in the summer were similar to those around Hawaii. Therefore, the nitrogen sources for biological production were different between the areas and seasons in the vicinity of the Kuroshio.