DONUT

The goal of the proposed work is to field test hypotheses concerning the biological production and consumption of marine dissolved organic nitrogen (DON) related to its role as a potential ‘organic nutrient’ source sustaining net community production within the ocean’s subtropical gyres. A North Pacific cruise will use observations of DON concentration ([DON]), chlorophyll a concentration ([chl a ]), DON d 15 N, Dd 15 N(DON-NO 3 - ), and DON molecular composition to identify regions of net DON production and consumption in the context of biological productivity evaluated using measurements of net (O 2 /Ar) and gross O 2 (Triple O isotopes) production. Novel organic geochemistry tools will be used to identify the molecular composition of the allochthonous DON sustaining biological N demand in the subtropical North Pacific. Testing previously proposed hypotheses, t he depth of DON consumption will be investigated using ship-board incubations that expose surface DON collected from three stations with gradients in surface nitrate concentration ([NO 3 - ]) to microbial communities from the mixed layer and the sub-euphotic zone. Finally, a new semi-labile DON tracer will be encoded in a global ocean biogeochemistry model, with the organic matter and isotopic data sets serving to validate the model for semi-labile DON cycling using data assimilation techniques.