The stable isotopic composition of dissolved organic nitrogen and nitrate in the subtropical ocean.

摘要

A method for the analysis of the stable isotopic ratio (15N/ 14N) of dissolved organic nitrogen (DON) was developed and applied in combination with 15N/14N and 18O/ 16O measurements of nitrate (NO3-) to study nitrogen (N) fluxes in the nutrient-poor tropical and subtropical ocean. Two topics addressed by this work are (1) rates of N2 fixation, the dominant process that adds N to the ocean and (2) the relationship between surface ocean DON and N2 fixation. The traditional view holds that N2 fixation occurs primarily in the low-latitude, oligotrophic ocean and has been found to augment ambient bulk DON in field, mesocosm, and culture studies. In the Sargasso Sea, both the concentration and 15 N/14N of DON were invariant in space and time (∼ 4-5 muM and delta15N ∼ 4‰ relative to air, respectively), and, with analyses of (NO3-) 15N/14N, were used to revise the N isotope budget near Bermuda, confirming that N2 fixation is a negligible component of the new N supply on an annual basis there. The constancy of DON concentration and DON 15N/14N over the course of a year also requires minimal fluxes to or from the surface ocean DON pool on this time scale. A similar study in the North Pacific found no correlation between DON concentration and DON 15N/14N, or between N 2 fixation rates and either DON concentration or DON 15N/ 14N, in spite of an eight-fold range in N2 fixation rates. The DON data from the North Atlantic and North Pacific Oceans demonstrate that DON is not sensitive to N2 fixation in oligotrophic regions on time scales of months or less.; (NO3-) 15N/14N measurements between Bermuda and Puerto Rico suggest that in the Sargasso Sea, ∼ 2 muM of thermocline (NO3-) originates from remineralization of newly fixed N. Preliminary calculations imply that attributing this amount of (NO3-) to N2 fixation is consistent with previous regional geochemical N2 fixation rate estimates. These findings also suggest that the geochemical signal of N2 fixation accumulates in the thermocline on a time scale longer than water mass formation and circulation through the subtropical gyre.