Dataset: Nitrate + nitrite concentration and d15N from the Eastern Tropical South Pacific collected on the R/V Atlantis (AT15-61) and R/V Melville (MV1104) in 2010-2011 (N2 fixation ETSP project, Microbial Nitrification project)

Water column sample collection: Samples were collected on the R/V Atlantis in January through February 2010, and the R/V Melville in March through April 2011 on a zonal transect along 20º S between 80º W and 100º W, with exact station locations and sample depths, nutrient concentrations and isotopic compositions reported below. Water column samples were collected by Niskin bottles deployed on a rosette equipped with conductivity-temperature-depth (CTD) sensors. All samples were collected into acid-washed, sample-rinsed HDPE bottles, and samples from the upper 400 m passed a 0.2 µm filter before collection. All samples were stored at -20º C until analysis on land.

Nitrate plus nitrite (NO3-+NO2-) concentration: The concentration of NO3-+NO2- ([NO3-+NO2-]) was determined using chemiluminescent analysis (in a configuration with a detection limit of 0.05 µM, + 0.1 µM 1 S.D.).

Reference: Braman, R. S. & Hendrix, S. A. Nanogram Nitrite and Nitrate Determination in Environmental and Biological-Materials by Vanadium(Iii) Reduction with Chemi-Luminescence Detection. Analytical Chemistry 61, 2715-2718 (1989).

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NO3+NO2 d15N: The d15N of NO3+NO2 was determined using the denitrified method on samples with >0.3 µM NO3+NO2 with a standard deviation <0.2 per mil.

References:

Sigman, D. M. et al. A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater. Analytical Chemistry 73, 4145-4153 (2001).

Casciotti, K. L., Sigman, D. M., Hastings, M. G., Bohlke, J. K. & Hilkert, A. Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Analytical Chemistry 74, 4905-4912 (2002).

McIlvin, M. R. & Casciotti, K. L. Technical Updates to the Bacterial Method for Nitrate Isotopic Analyses. Analytical Chemistry 83, 1850-1856 (2011).