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

Constraining the rates and spatial distribution of dinitrogen (N₂/ fixation fluxes to the ocean informs our understanding of the environmental sensitivities of N₂ 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 N₂ fixation as well as its contribution to export production along a zonal transect in the western tropical South Pacific (WTSP) Ocean using N isotope ("δ¹⁵N") budgets. Comparing measurements of water column nitrateCnitrite δ¹⁵N with the δ¹⁵N of sinking particulate N at a western, central, and eastern station, these δ¹⁵N budgets indicate high, modest, and low rates of N₂ fixation at the respective stations. The results also imply that N₂ fixation supports exceptionally high, i.e. ⩾50 %, of export production at the western and central stations, which are also proximal to the largest iron sources. These geochemically based rates of N₂ 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 N₂ fixation, which may compensate for N removal in the oxygen-deficient zones of the eastern tropical Pacific. [ABSTRACT FROM AUTHOR]