Sampling and Analytical Methodology:
\nSamples were collected from Niskin bottles, syringe filtered through 0.2 um pore-size capsule filters and frozen immediately in 60 mL HDPE bottles. Samples were stored frozen until analysis. Nitrate and nitrite concentrations were measured on separate aliquots by auto analyzer at the WHOI nutrient facility.
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Samples were initially analyzed for nitrate +\u00a0 nitrite d15N and d18O using the denitrifier method (Sigman et al., 2001; Casciotti et al., 2002), then nitrite d15N and d18O were subtracted out according to Casciotti and McIlvin (2007). Later, the samples were reanalyzed for nitrate d15N and d18O using the denitrifier method after treatment with sulfamic acid using the method of Granger and Sigman (2009). These latter analyses, which we believe to be more accurate and precise, are reported here. Samples were analyzed in duplicate and are reported as the mean and standard deviation of replicate measurements. Typical precision for these analyses is 0.2-0.3 for d15N and 0.3-0.5 for d18O.
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Samples were analyzed for nitrite d15N and d18O using the azide method (McIlvin and Altabet, 2005). Nitrite d18O data are not reported because it was determined that samples may have undergone O atom equilibration with water during storage (Casciotti et al., 2007). Typical precision for these analyses is 0.3-0.5 for d15N and 0.3-0.5 for d18O.
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All isotopic measurements were conducted on a Thermo Finnigan Delta PLUS XP isotope ratio mass spectrometer.
Data Processing:
\nNitrate d15N and d18O analyses were calibrated against aliquots of the nitrate isotope standards USGS32, USGS34, and USGS35 (Bohlke et al., 2003) prepared and analyzed in parallel with each batch of samples, according to McIlvin and Casciotti (2011).
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Nitrite d15N analyses were calibrated against aliquots of the nitrite isotope standards RSIL-N23, RSIL-N7373, and RSIL-N10219 (Casciotti et al., 2007) prepared and analyzed in parallel with each batch of samples.
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BCO-DMO Processing Notes: modified original parameter names to conform with BCO-DMO naming conventions; replaced blank/missing values with 'nd' to indicate \"no data\".