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

• Published in: Biogeosciences
• Main Authors: Knapp, Angela N., McCabe, Kelly M., Grosso, Olivier, Leblond, Nathalie, Moutin, Thierry, Bonnet, Sophie
• Format: Text
• Language: English
• Published: 2019
• Subjects: Pacific; North Atlantic
• Online Access: https://doi.org/10.5194/bg-15-2619-2018; https://www.biogeosciences.net/15/2619/2018/

Constraining the rates and spatial distribution of dinitrogen (N 2 ) fixation fluxes to the ocean informs our understanding of the environmental sensitivities of N 2 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 N 2 fixation as well as its contribution to export production along a zonal transect in the western tropical South Pacific (WTSP) Ocean using N isotope (“ δ 15 N”) budgets. Comparing measurements of water column nitrate + nitrite δ 15 N with the δ 15 N of sinking particulate N at a western, central, and eastern station, these δ 15 N budgets indicate high, modest, and low rates of N 2 fixation at the respective stations. The results also imply that N 2 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 N 2 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 N 2 fixation, which may compensate for N removal in the oxygen-deficient zones of the eastern tropical Pacific.