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The western subtropical South Pacific (WSSP) has recently been found to support high rates of di-nitrogen (N2) fixation in association with shallow hydrothermal iron fluxes. While previous 15N2 uptake and short-term d15N budgets have found that high rates of N2 fixation contribute significantly to export production, no longer-term evaluations of N2 fixation’s role in supporting the regional ecosystem were available. Here we present results of an annual d15N budget using the d15N of sinking particles captured in a moored sediment trap deployed at 1000 m from Nov 2019 - Nov 2020. We compare the d15N of the particles collected over this annual cycle with the d15N of subsurface nitrate to evaluate the seasonal and annual importance of N2 fixation for supporting export production. The results indicate that N2 fixation supported up to ~20% of annual export and that N2 fixation was most important during the summer. Notably, the d15N of subsurface nitrate at the trap station was low, 2 to 3 per mil compared to stations further from the vents. We also present some of the region’s first dissolved organic nitrogen (DON) d15N data. The DON samples collected in Nov 2019 and Nov 2020 show similar DON concentrations and d15N between years. However, while DON concentrations in the WSSP, 5 +/- 1 uM, were similar to the eastern tropical South Pacific (ETSP), the d15N of DON in the upper 100 m in the WSSP was between 2 to 4 per mil, which is lower than the ETSP, where DON d15N was between 4 to 6 per mil. Together, the results of the annual d15N budget as well as the low-d15N DON provide a longer-term perspective on the significance of N2 fixation in the WSSP. Additionally, the results suggest that N2 fixation in the WSSP introduces significant low-d15N N to the ocean, offsetting the elevated d15N generated in the oxygen deficient zones of the eastern tropical Pacific. 

In regions of the surface ocean with significant concentrations of unconsumed nitrate (NO3-), such as the Southern Ocean, phytoplankton preferentially assimilate the 14N-bearing form of NO3- during NO3- assimilation. This discrimination against the heavier, 15N-bearing form of NO3- is quantified by the NO3- assimilation isotope effect (15e). While a 15e of 5 per mil is commonly assumed for phytoplankton NO3- assimilation, previous field-based observations of the 15e have ranged from 4 to 11 per mil, and even wider variations in 15e have been observed in culture studies that have subjected phytoplankton to iron and/or light stress. In spite of this prior work, we lack a mechanistic explanation for variations in 15e, yet this information is required for interpreting modern water column NO3- d15N and d18O measurements as well as paleoceanographic d15N records. Here we report 15e estimates from springtime water column NO3- isotope profiles collected across four major zones (Subantarctic, Polar Frontal, Antarctic, and Marginal Ice Zones) in the Atlantic sector of the Southern Ocean on the SCALE cruise (Southern oCean seAsonal Experiment; Oct.-Nov. 2019). Consistent with prior austral summer observations, we generally find higher values of 15e in the Subantarctic compared to the Antarctic; however, variations exist within each zone. These data are interpreted in the context of seasonal mixing (closed vs. open system models), phytoplankton community composition, and physiological markers of iron and light stress.