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

Abstract

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 μM and δ¹⁵N ∼ 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 μM 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.