Stable nitrogen isotope dynamics of dissolved nitrate in a transect from the north Pacific subtropical gyre to the eastern tropical north Pacific

Abstract

The stable nitrogen isotopic composition of nitrate, concentrations of inorganic nitrogen and phosphorus, dissolved oxygen and nitrification rates were determined at six stations ranging from the oligotrophic North Pacific Subtropical Gyre (NPSG) to the more productive Eastern Tropical North Pacific (ETNP). Nitrification rates increased along the transect from a maximum rate of 1 nmol L −1 d −1 at station ALOHA to 23.7 nmol L −1 d −1 at station 6. In oxic surface waters, nitrate isotopically enriched in 15N (maximum δ 15N-NO 3 − value of 12.5‰) was most likely the result of assimilatory nitrate reduction. In contrast, high δ 15N-NO 3 − values (maximum of 12.3‰) in association with high nitrate deficits and anoxic conditions supported the interpretation of isotopic fractionation due to denitrification. A one-dimensional vertical advection and diffusion model was used to estimate the fractionation factor for denitrification at two stations in the ETNP. A comparison of modeled to observed δ 15N-NO 3 − data indicated an isotopic enrichment factor (ε) of 30‰ at station 4 and 30 to 35‰ at station 5. Isotopically light nitrate (1.1 and 3.2‰) was observed in the upper 200 m of the water column at stations in the ETNP. Tracer studies of 15NH 4 and biogeochemical indicators of nitrogen fixation supported the interpretation of nitrification as the most plausible explanation for low δ 15N-NO 3 − values observed in water column samples. Our results are consistent with the occurrence of nitrification within the euphotic zone and for the first time provide corroborating stable nitrogen isotopic evidence for this process.