ORIGINS OF ISOTOPICALLY LIGHT NITRATE IN THE WATER COLUMN OF THE EASTERN TROPICAL NORTH PACIFIC

The nitrogen isotopic composition of nitrate, concentrations of inorganic nitrogen and phosphorus, dissolved oxygen, chlorophyll fluorescence and nitrification rates were determined in water column samples at six stations in a transect from the North Pacific subtropical gyre (NPSG) to the eastern tropical North Pacific (ETNP) (Station ALOHA: 22.7^oN, 158^oW; Station 2: 16.0^oN, 150^oW; Station 3: 16.0^oN, 136^oW; Station 4: 15.8^oN, 119.1^oW; Station 5: 16.3^oN, 107.2^oW; Station 6: 15.6^oN, 98^oW). Isotopically light nitrate (1.1 and 3.2 ‰) was observed in the upper 200 m of the water column in the ETNP that prompted us to examine if nitrogen fixation and/or nitrification contributed to its origin. This is important because nitrogen fixation may serve to balance the loss of fixed nitrogen in the ETNP and nitrification does not increase the pool size of available nitrogen. Ammonium (¹⁵NH₄) oxidation 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. The minima in d¹⁵N-NO₃^- are coincident with sampling depths where nitrification was active. However, the maxima in nitrification rates did not always correlate with minima in d¹⁵N-NO₃^- because of the influence of other microbial processes. Nitrification rates and biogeochemical indicators of nitrogen fixation indicated that nitrification was the most likely explanation for isotopically light nitrate in the euphotic zone of the ETNP during the period of our study. The results of this study indicate that euphotic zone nitrification must be accounted for in models of primary productivity and the interpretation of nitrogen isotope signals in modern and ancient environments.