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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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A critical review of the 15 N 2 tracer method to measure diazotrophic production in pelagic ecosystems
Abstract Dinitrogen (N2) fixation is an important source of biologically reactive nitrogen (N) to the global ocean. The magnitude of this flux, however, remains uncertain, in part because N2fixation rates have been estimated following divergent protocols and because associated levels of uncertainty are seldom reported—confounding comparison and extrapolation of rate measurements. A growing number of reports of relatively low but potentially significant rates of N2fixation in regions such as oxygen minimum zones, the mesopelagic water column of the tropical and subtropical oceans, and polar waters further highlights the need for standardized methodological protocols for measurements of N2fixation rates and for calculations of detection limits and propagated error terms. To this end, we examine current protocols of the15N2tracer method used for estimating diazotrophic rates, present results of experiments testing the validity of specific practices, and describe established metrics for reporting detection limits. We put forth a set of recommendations for best practices to estimate N2fixation rates using15N2tracer, with the goal of fostering transparency in reporting sources of uncertainty in estimates, and to render N2fixation rate estimates intercomparable among studies.
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- PAR ID:
- 10457929
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography: Methods
- Volume:
- 18
- Issue:
- 4
- ISSN:
- 1541-5856
- Page Range / eLocation ID:
- p. 129-147
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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