Selden, C. R. , Mulholland, M. R. , Bernhardt, P. W. , Widner, B. , Macías‐Tapia, A., Ji, Qixing and Jayakumar, A. (2019) Dinitrogen Fixation Across Physico‐Chemical Gradients of the Eastern Tropical North Pacific Oxygen Deficient Zone. Global Biogeochemical Cycles, 33 (9). pp. 1187-1202. DOI 10.1029/2019GB006242.

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Abstract

The Eastern Tropical North Pacific Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hot spots for reactive nitrogen (N-r) removal processes, ODZs generate conditions proposed to promote N-r inputs via dinitrogen (N-2) fixation. In this study, we quantified N-2 fixation rates by N-15 tracer bioassay across oxygen, nutrient, and light gradients within and adjacent to the ODZ. Within subeuphotic oxygen-deplete waters, N-2 fixation was largely undetectable; however, addition of dissolved organic carbon stimulated N-2 fixation in suboxic (<20 mu mol/kg O-2) waters, suggesting that diazotroph communities are likely energy limited or carbon limited and able to fix N-2 despite high ambient concentrations of dissolved inorganic nitrogen. Elevated rates (>9 nmol N center dot L-1 center dot day(-1)) were also observed in suboxic waters near volcanic islands where N-2 fixation was quantifiable to 3,000 m. Within the overlying euphotic waters, N-2 fixation rates were highest near the continent, exceeding 500 mu mol N center dot m(-2)center dot 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 N-r. Despite high rates at some inshore stations, regional N-2 fixation appears insufficient to compensate for N-r loss locally as observed previously in the Eastern Tropical South Pacific ODZ.

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