ORCID

0000-0002-5721-3061 (Selden), 0000-0001-8819-189X (Mulholland), 0000-0003-3848-7013 (Bernhardt), 0000-0002-1527-2717 (Widner)

Document Type

Article

Publication Date

2019

DOI

10.1029/2019GB006242

Publication Title

Global Biogeochemical Cycles

Volume

33

Issue

9

Pages

1187-1202

Abstract

The Eastern Tropical North Pacific Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hot spots for reactive nitrogen (Nr) removal processes, ODZs generate conditions proposed to promote Nr inputs via dinitrogen (N2) fixation. In this study, we quantified N2 fixation rates by 15N tracer bioassay across oxygen, nutrient, and light gradients within and adjacent to the ODZ. Within subeuphotic oxygen‐deplete waters, N2 fixation was largely undetectable; however, addition of dissolved organic carbon stimulated N2 fixation in suboxic (μmol/kg O2) waters, suggesting that diazotroph communities are likely energy limited or carbon limited and able to fix N2 despite high ambient concentrations of dissolved inorganic nitrogen. Elevated rates (>9 nmol N·L−1·day−1) were also observed in suboxic waters near volcanic islands where N2 fixation was quantifiable to 3,000 m. Within the overlying euphotic waters, N2 fixation rates were highest near the continent, exceeding 500 μmol N·m−2·day−1 at one third of inshore stations. These findings support the expansion of the known range of diazotrophs to deep, cold, and dissolved inorganic nitrogen‐replete waters. Additionally, this work bolsters calls for the reconsideration of ocean margins as important sources of Nr. Despite high rates at some inshore stations, regional N2 fixation appears insufficient to compensate for Nr loss locally as observed previously in the Eastern Tropical South Pacific ODZ.

Rights

© 2019. The Authors

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Data Availability

Article states: "Data will be made available at the website (https://www.bco-dmo.org/project/472492)."

Original Publication Citation

Selden, C. R., Mulholland, M. R., Bernhardt, P. W., Widner, B., Macías-Tapia, A., Ji, Q., & Jayakumar, A. (2019). Dinitrogen fixation across physico-chemical gradients of the Eastern Tropical North Pacific oxygen deficient zone. Global Biogeochemical Cycles, 33(9), 1187-1202. doi:10.1029/2019GB006242

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