Dissolved Organic Nitrogen Production and Consumption in Eastern Tropical South Pacific Surface Waters

Abstract

AbstractModels of dissolved organic nitrogen (DON) production and consumption indicate that marine DON is produced in the euphotic zone, especially in upwelling environments, and is consumed as it transits into more oligotrophic regions where it may support export production. However, evaluation of such models is data limited, and in particular, few measurements of DON concentration and/or nitrogen isotopic composition (“δ15N”) exist from the South Pacific. Here we present measurements of the concentration and δ15N of DON collected in the eastern tropical South Pacific during the austral summers of 2010 (El Niño) and 2011 (La Niña). The concentration and δ15N of DON from the eastern tropical South Pacific are broadly consistent with prior measurements elsewhere, that is, DON concentrations of 5 ± 1 μM and DON δ15N of 5 ± 1‰, but variations were associated with DON production and consumption. Specifically, surface ocean chlorophyll a concentrations were highly correlated with upper 50‐m DON stocks (r = 0.96), consistent with a photosynthetic source for DON. Additionally, upper 50‐m DON stocks and their δ15N were significantly negatively correlated (r = −0.84, p = 0.0006), indicating that DON was consumed in surface waters with an isotope effect of 5.5 ± 1.2‰. These results inform our understanding of the controls of DON distributions in the surface ocean and indicate that DON may support export production in nitrate‐poor surface waters, which may decouple new from gross production.