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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.
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Quantifying N2 fixation and its contribution to export production near the Tonga-Kermadec Arc using nitrogen isotope budgets
The spatial distribution of marine di-nitrogen (N2) fixation informs our understanding of the sensitivities of this process as well as the potential for this new nitrogen (N) source to drive export production, influencing the global carbon (C) cycle and climate. Using geochemically-derived δ15N budgets, we quantified rates of N2fixation and its importance for supporting export production at stations sampled near the southwest Pacific Tonga-Kermadec Arc. Recent observations indicate that shallow (<300 m) hydrothermal vents located along the arc provide significant dissolved iron to the euphotic zone, stimulating N2fixation. Here we compare measurements of water column δ15NNO3+NO2with sinking particulate δ15N collected by short-term sediment traps deployed at 170 m and 270 m at stations in close proximity to subsurface hydrothermal activity, and the δ15N of N2fixation. Results from the δ15N budgets yield high geochemically-based N2fixation rates (282 to 638 µmol N m-2d-1) at stations impacted by hydrothermal activity, supporting 64 to 92% of export production in late spring. These results are consistent with contemporaneous15N2uptake rate estimates and molecular work describing highTrichodesmiumspp. and other diazotroph abundances associated with elevated N2fixation rates. Further, the δ15N of sinking particulate N collected at 1000 m over an annual cycle revealed sinking fluxes peaked in the summer and coincided with the lowest δ15N, while lower winter sinking fluxes had the highest δ15N, indicating isotopically distinct N sources supporting export seasonally, and aligning with observations from most other δ15N budgets in oligotrophic regions. Consequently, the significant regional N2fixation input to the late spring/summer Western Tropical South Pacific results in the accumulation of low-δ15NNO3+NO2in the upper thermocline that works to lower the elevated δ15NNO3+NO2generated in the oxygen deficient zones in the Eastern Tropical South Pacific.
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- Award ID(s):
- 1829797
- PAR ID:
- 10473479
- Publisher / Repository:
- Frontiers
- Date Published:
- Journal Name:
- Frontiers in Marine Science
- Volume:
- 10
- ISSN:
- 2296-7745
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3389/fmars.2023.1249115
