Dinitrogen Fixation Across Physico-Chemical Gradients of the Eastern Tropical North Pacific Oxygen Deficient Zone

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.