ISOTOPIC BIOGEOCHEMISTRY OF DISSOLVED ORGANIC NITROGEN: IMPLICATIONS FOR UNDERSTANDING ECOSYSTEM- AND GLOBAL- SCALE CHANGE

Oceanic nitrogen budgets remain a focal point in marine research due to recent changes in nitrogen supplies leading to ecosystem scale changes and concerns regarding nitrous oxide fluxes. Excluding dinitrogen, dissolved organic nitrogen (DON) is often the largest pool of fixed nitrogen in marine system but is isotopically the most difficult to measure. We used a dialysis technique (100 Da MW cutoff) to characterize the nitrogen isotope value of DON from samples taken at Station ALOHA (22o45'N, 158oW) and EPREX-2 (16^oN, 150^oW) within the North Pacific Subtropical Gyre in October 1998, January 1999 and May 2000. DON concentrations are 4-7 mM in surface waters and decrease to ca. 2 mM at 800 m. This decrease is associated with an increase in 15N and is consistent with microbial assimilation and degradation of DON with depth. Samples collected in summer have lower isotope values (1.6-11 o/oo) than those collected in winter (8.3-14 o/oo). We discuss these trends in terms of seasonal changes in N-fixation and the potential for DON to be injected into deeper waters where it could provide a substrate for nitrification and subsequent N2O production. We suggest that the understanding of DON cycling has important implications for understanding ecosystem and global scale changes in marine environments.