Dataset: nitrogen fixation rates

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.

15N2 fixation rate measurements: Short-term N2 fixation rate measurements incubated with 15N2 gas from Sigma Aldrich (lot #s SZ1670V and MBBB0968V) were carried out in acid-washed, sample-rinsed light transparent 4 L polycarbonate bottles amended with 1.5 mL of 99% 15N2  and 1.5 mL of 0.5M NaH13CO3. Incubations were performed under simulated in situ conditions of temperature and light and run for staggered periods (e.g. 0, 12, 24 and 48 hrs). Incubations were terminated by filtration of the 4 L sample onto a precombusted 25 mm Whatman GF/F, which was then analyzed by mass spectrometry at the University of Southern California for d15N and d13C. During ESTP 1, periodic time zero incubations were measured, with samples filtered within 5 to 10 minutes of injection of 15N2. Zero time enrichments were typically near natural abundance levels with the greatest observed enrichment in 8 experiments at 23‰, essentially at natural abundance level for our tracer experiments. We assumed natural abundance (0.3663 atom %) for calculations. Limits of detection are about 10-14 µg for N and 6-10 µg for C. Reproducibility as the standard deviation for isotopic analysis is about ±0.3‰ for d15N and ±0.2‰ for d13C, and about ±0.1-0.2 µg for mass determinations.