Sources and Transformation of Dissolved and Particulate Organic Nitrogen in the North Pacific Subtropical Gyre Indicated by Compound-Specific Nitrogen Isotope Analysis of Amino Acids

Yasuhiko T Yamaguchi, University of California Santa Cruz, Santa Cruz, CA, United States; The University of Tokyo, Atmosphere and Ocean Research Institute, Kashiwa, Chiba, Japan and Matthew D. McCarthy, University of California Santa Cruz, Ocean Sciences Department, Santa Cruz, CA, United States
Dissolved and particulate organic nitrogen (DON, PON) together play the roles in N cycling in the upper ocean. This study explores the use of compound-specific nitrogen isotope of amino acids (δ15N-AA) of coupled DON and PON samples as a new approach to examine relative sources, transformation processes, and potential coupling of the main detrital organic nitrogen form in the ocean water column. We measured δ15N-AA distributions in high-molecular-weight dissolved organic nitrogen (HMW DON) and suspended PON in the North Pacific Subtropical Gyre (NPSG) from surface to mesopelagic depths. A new analytical approach achieved far greater δ15N-AA measurement precision for DON than earlier work, allowing us to resolve previously obscured differences in δ15N-AA signatures, both with depth and between ON pools.

First, the δ15N-AA signatures of both surface and mesopelagic HMW DON suggest mainly heterotrophic bacterial sources, with the mesopelagic HMW DON bearing signatures of far more degraded material. These results contrast with a previous proposal that DON δ15N-AA patterns are essentially “pre-formed” in the surface ocean, undergoing little further change with depth. Second, different δ15N-AA values and patterns of HMW DON vs. suspended PON in the surface NPSG suggest that their sources and cycling are largely decoupled. We propose a new idea for production of surface HMW DON, in which ON is ultimately derived from subsurface nitrate, while PON in the mixed layer is strongly linked to N2 fixation and N recycling. In contrast, the comparative δ15N-AA values of HMW DON and suspended PON in the mesopelagic depth suggest a potentially important role of PON in the production of HMW DON at mesopelagic depths. Overall, these new δ15N-AA data have provided an unprecedented window into the sources, cycling, and likely biogeochemical linkage of DON and PON pools in the oligotrophic ocean.