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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, the spatial distribution of DON production and consumption in the surface ocean remains poorly resolved due to the relatively narrow range in euphotic zone DON concentrations. Recently, the stable isotopic composition (d15N) of DON has been used to identify DON production and consumption in the surface ocean, making isotopic measurements a more sensitive indicator of DON cycling than concentration measurements alone. Here we report DON concentration and d15N measurements in the upper ~300 m from a zonal transect along ~30˚S in the South Pacific (GO-SHIP P06-2017), including samples in the Western South Pacific (154˚E-170˚W), in the oligotrophic South Pacific Subtropical Gyre (110˚W -170˚W), and overlying the Oxygen Deficient Zone (ODZ) in the east (78˚W-110˚W). We observed small variations in surface DON concentrations. Surface DON in Western South Pacific, oligotrophic South Pacific Subtropical Gyre and above the ODZ are 4.6±1.0 µM, 4.3±0.7 µM, and 4.8±0.5 µM, respectively. d15N of DON in the euphotic zone is lower in the west and higher in the east, consistent with distributions of nitrogen fixation and denitrification, respectively, in the South Pacific. Similar decreasing trend in DON d15N in the euphotic zone and subsurface nitrate d15N was observed from the east to the west in the South Pacific, suggesting the d15N in subsurface nitrate could be imprinted in the DON d15N in the euphotic zone. Low surface ocean DON d15N in the Western South Pacific (2.4±1.8 ‰) and oligotrophic South Pacific Subtropical Gyre (2.6±1.6 ‰) compared with surface ocean DON d15N above ODZ (5.4±2.3 ‰) infer significant low-d15N nitrogen is added to the western South Pacific and oligotrophic South Pacific Subtropical Gyre, potentially from N2 fixation. Additionally, high DON d15N at ~180˚ was consistent with entrainment of subsurface NO3- into surface waters due to shallow bathymetry. Together, these observations suggest that DON production and consumption processes operate on timescales adequately fast to produce isotopic gradients across the South Pacific. Comparisons of surface ocean DON d15N with subsurface nitrate d15N constrain the locations and timescales of these processes. 

Significant rates of export production and nitrogen fixation occur in oligotrophic gyres in spite of low inorganic nutrient concentrations in surface waters. Prior work suggests that dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) are important nutrient sources when inorganic nutrients are scarce. In particular, DOP has been shown to be an important P source for diazotrophs, which may be better suited to using low concentrations of organic vs. inorganic P. Prior modeling work has also suggested that DOP is important for supporting export production in oligotrophic gyres. However, validation of such models is limited by the number of upper ocean DOP concentration measurements, especially in the South Pacific and Indian Oceans. Here, we present measurements of DOP concentration from the 2016 GO-SHIP I08S and I09N meridional transect in Eastern Indian Ocean, and DON and DOP concentration measurements from the 2017 GO-SHIP P06 zonal transect in the subtropical South Pacific Ocean. Together with DOC and DON concentration measurements from prior occupations of the same GO-SHIP lines we evaluated changes in euphotic zone DOC:DON:DOP stoichiometry. Stoichiometry changes across these two transects are used to infer regions of preferential DON and/or DOP production and consumption. Specifically, north of 36 S in the Indian Ocean an increase in DOC:DON and DOC:DOP concentration ratios, from 11:1 to 14:1 and 118:1 to 190:1, respectively, are observed. Similarly, west of 136 W in the South Pacific Ocean significant increases in DOC:DOP and DON:DOP concentration ratios are observed, from 224:1 to 398:1 and 21:1 to 39:1, respectively. These stoichiometric shifts in upper ocean DOC:DON:DOP concentration ratios are considered in the context of ocean circulation, especially upwelling patterns in the Indian and eastern Pacific Oceans, as well as prior observations of the distribution of nitrogen fixation, especially in the western tropical South Pacific.