Distribution and rates of nitrogen fixation in the western tropical South Pacific Ocean constrained by nitrogen isotope budgets

Knapp, Angela N.; McCabe, Kelly M.; Grosso, Olivier; Leblond, Nathalie; Moutin, Thierry; Bonnet, Sophie

Constraining the rates and spatial distribution of dinitrogen (Ninline-formula2) fixation fluxes to the ocean informs our understanding of the environmental sensitivities of Ninline-formula2 fixation as well as the timescale over which the fluxes of nitrogen (N) to and from the ocean may respond to each other. Here we quantify rates of Ninline-formula2 fixation as well as its contribution to export production along a zonal transect in the western tropical South Pacific (WTSP) Ocean using N isotope (“inline-formulaδ15N”) budgets. Comparing measurements of water column nitrate inline-formula+ nitrite inline-formulaδ15N with the inline-formulaδ15N of sinking particulate N at a western, central, and eastern station, these inline-formulaδ15N budgets indicate high, modest, and low rates of Ninline-formula2 fixation at the respective stations. The results also imply that Ninline-formula2 fixation supports exceptionally high, i.e. inline-formula≥ 50 %, of export production at the western and central stations, which are also proximal to the largest iron sources. These geochemically based rates of Ninline-formula2 fixation are equal to or greater than those previously reported in the tropical North Atlantic, indicating that the WTSP Ocean has the capacity to support globally significant rates of Ninline-formula2 fixation, which may compensate for N removal in the oxygen-deficient zones of the eastern tropical Pacific.

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Knapp, Angela N. / McCabe, Kelly M. / Grosso, Olivier / et al: Distribution and rates of nitrogen fixation in the western tropical South Pacific Ocean constrained by nitrogen isotope budgets. 2018. Copernicus Publications.

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