Search for: All records

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.