Production and Consumption of Dissolved Organic Nitrogen Across the South Pacific: An Isotopic Perspective From a Zonal Transect

Dissolved organic nitrogen (DON) is the dominant form of bioavailable nitrogen in the euphotic zone of subtropical gyres, where nitrate (NO₃⁻) concentrations are low. However, identifying regions where DON consumption may support surface ocean productivity remains challenging due to the relatively narrow range in euphotic zone DON concentrations. The stable isotopic composition (δ¹⁵N) of DON has recently emerged as a sensitive tool for identifying regions of DON production and consumption in the surface ocean. Here, we report DON concentration and δ¹⁵N measurements in the upper ∼300 m from a >10, 000 km zonal GO-SHIP transect along ∼30°S in the South Pacific (P06-2017 transect) from the Chilean to the Australian coasts. We observed higher upper 50 m DON concentrations in the east associated with DON production in regions with elevated primary productivity. Upper 50 m DON δ¹⁵N decreases from east to the west, coincident with a gradient in the δ¹⁵N of subsurface (i.e., 100–200 m) NO₃⁻, consistent with subsurface NO₃⁻ fueling DON production. Further, both the partial and complete assimilation of surface NO₃⁻ observed along the transect are imprinted on the δ¹⁵N of surface DON. An inverse relationship between the concentration and δ¹⁵N of surface DON was found in the western portion of the transect, consistent with DON consumption by phytoplankton. Combining concentrations and δ¹⁵N of DON, we identified regions of DON production and consumption across the largest subtropical ocean gyre.