Controls on dissolved organic carbon and nitrogen distributions in the upper South Pacific Ocean

We used data from the GO-SHIP latitudinal P18 transect across the South Pacific Ocean (P18/1994, 2007 and 2017) to investigate controls on dissolved organic carbon and nitrogen (DOC and DON, respectively) concentrations in the upper layer. While DOC distribution is mainly controlled by physical processes (i.e., vertical stability favors surface accumulation), DON can be biologically consumed far from its production zone once advected to oligotrophic regions, likely fueling biological production. We investigated the study region between 5˚S and 40˚S along 108˚W, passing through the biologically productive Equatorial Pacific, the center of the N-limited South Pacific Subtropical Gyre, and the high-nutrient Subtropical Front (STF). In order to identify DON concentration variations associated with biological activity, DON as well as total dissolved nitrogen (TDN; the sum of dissolved inorganic nitrogen and DON) were analyzed using the persulfate oxidation and denitrifier method. Preliminary results show higher DOC concentrations (~80 µM) in the northernmost portion of the gyre, and lower concentrations (~60 µM) at the equator and the STF. There was a strong correlation between DOC and potential temperature distributions with little interannual variability, highlighting strong physical controls. As for DON, lower isotopic signatures were associated with high concentrations in the productive regions (i.e. DON ~7 µM and 𝛿¹⁵N ~3.5‰ at the equator). As the equatorial waters advected toward the northern portion of the subtropical gyre, DON gradually decreased to ~5 µM while 𝛿¹⁵N increased to 6-8‰, indicating biological consumption far from its production site.In the southern portion of the gyre, DON had the lowest concentrations (~3 µM) and 𝛿¹⁵N ~4‰ associated with waters advected from the west. In the STF, DON had concentrations ~4 µM and 𝛿¹⁵N ~2-4‰ signaling local production.