Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Supplemental Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Seawater samples were collected on R/V Atlantis (AT15-61) cruise in Jan-Feb 2010 and on R/V Melville (MV1104) cruise in Mar-Apr 2011.  Water samples were collected at discrete depths using Niskin bottle type rosette samplers equipped with either 24 bottles (10L), or 12 bottles (20L), and an SBE9plus conductivity-temperature-depth (CTD) sensor package (SeaBird Electronics, Bellevue, WA).  Water for rate incubations was collected into 160 mL glass serum bottles (AT15-61) or 500 mL Tedlar sampling bags (MV1104).  See below in Processing Description for further details.

Ammonia and nitrite oxidation rates were determined using 15N tracer additions. Full methods can be found in the manuscript Santoro et al. (submitted). As described below, incubation methods varied slightly between the first and second cruises.

In Year 1 (2010), rates were determined at four depths at all six stations. Incubations were conducted in 160 mL glass serum vials. Six serum bottles were filled and sealed from each incubation depth, spiked with 15N tracer (100-200 nM 15NH4Cl or Na15NO2-), and incubated in flowing seawater incubators screened to mimic the in situ light environment (euphotic zone samples) or temperature controlled chambers (sub-euphotic zone). Duplicate bottles were sacrificed at timepoints of 0, 12, and 24 h from each incubation depth, 0.2 µm syringe-filtered into a 60 mL HDPE bottle, and frozen at -20ºC.

In Year 2 (2011), rates were determined at six depths at five stations. No rates were determined at Stn 5 in Year 2. Incubations were conducted in 500 mL Tedlar bags. Duplicate incubation bags per treatment were filled from the Niskin bottles using silicone tubing, and 15N tracer (200 nM 15NH4Cl or Na15NO2) was added via the septum injection port. As in the previous year, bags were incubated at as close to in situ light and temperature as possible. At timepoints of 0, 12, 24, and 36 h, 50 mL samples were drawn from each bag through the three-way sampling port using a 60 mL syringe. At each timepoint, incubation water was 0.2 µm syringe filtered into a 60 mL HDPE bottle tripled rinsed with sample, and frozen at -20ºC.

Frozen samples were transported frozen to the laboratory, thawed, and prepared for 15NNO2+NO3 analysis using the ‘denitrifier method’ (Sigman et al., 2001). For nitrite oxidation rate samples, the added 15NO2- tracer was removed using sulfamic acid addition and subsequent neutralization with NaOH (Granger et al., 2006) prior to sample preparation and analysis. For 2010 samples, where only three timepoints were taken, rates were calculated using the linear fitting method of Dugdale and Goering (Dugdale and Goering, 1967). For 2011, where four timepoints were taken, rates were calculated using a least squares fitting approach that accounts for changes in 15NNO2+NO3 from co-occurring nitrate uptake (Santoro et al., 2010 and attached analysis files).

Nitrate reduction rates to nitrite were determined in Year 2 using 15NO3- tracer additions (>98 atm% Na15NO3). 15NO3- incubations were only conducted in the euphotic zone (three depths). Tedlar incubation bags were prepared and filled as above, and 200 or 400 nM (final concentration) of Na15NO3 was added to each bag using a plastic syringe. Timepoints were sampled and preserved as for the nitrification rate incubations above. In the laboratory, samples were prepared for 15NNO2 determination using the ‘azide method’ (McIlvin and Altabet, 2005). Following azide conversion to N2O, samples and standards (N23, N7373, and N10219; (Casciotti et al., 2007) were analyzed by IRMS and rates were calculated as described above.

Light inhibition experiments were conducted in Year 2 to test the effect of sunlight on ammonia oxidation, nitrite oxidation, and nitrate reduction. These incubations were conducted at the two shallowest incubation depths, approximating the 1% and 10% light depths at Stations 7, 9, and 11. For these experiments, one set of duplicate incubation bottles for each rate type was incubated at ambient light (bottles 1 and 2 in the data file) and the other in the dark (bottles 3 and 4 in the data file). Tracer addition, subsampling, analysis, and rate calculations were as described above.

MATLAB processing:
The MATLAB script 'etsp_rates_2011_indvT0.m' loads two types of files:  initial conditions ('initials...') and isotope  data ('data...').  Compiled rates were deposited in a tab-delimited text file. For more information, see Supplemental Files section.  

Parameters for Initial Conditions file(s) called "initials"

• AP15N = Starting atom percent of 15N in the substrate pool; [15N/(15N+14N)], expressed as a fraction (e.g. 99atm% is 0.99)
• Init_conc_No_15 = Initial concentration of 15N (micromoles) in the product pool
• Init_conc_No_14 = Initial concentration of 14N (micromoles) in the product pool

​Parameters for the Isotope Data file(s) called "data":

• bottle = bottle (either replicate or treatement)
• time = time duration in hours
• 15R = ratio of 15N to 14N in nitrate

BCO-DMO processing:
- Added a conventional header with dataset name, PI name, version date
- Adjusted parameter names to comply with database requirements. 
- Combined year, month, day fields into one date field.  
- Units removed and added to Parameter Description metadata section.
- Default missing identifier of 'NaN' replaced with 'nd' ('nd' is BCO-DMO system default missing data identifier), but BDL (below detection limit) was preserved. 

Description from NSF award abstract:
Closing the marine budgets of nitrate and nitrous oxide are central goals for researchers interested in nutrient-driven changes in primary productivity and climate change. With the implementation of new methods for oxygen isotopic analysis of seawater nitrate, it will be possible to construct a budget for nitrate based on its oxygen isotopic distribution that is complementary to nitrogen isotope budgets. Before we can effectively use oxygen isotopes in nitrate to inform the current understanding of the marine nitrogen cycle, we must first understand how different processes that produce (nitrification) and consume (assimilation, denitrification) nitrate affect its oxygen isotopic signature.

In this study, researchers at the Woods Hole Oceanographic Institution will provide a quantitative assessment of the oxygen isotopic systematics of nitrification in the field and thus fill a key gap in our understanding of 18O variations in nitrate, nitrite, and nitrous oxide. The primary goal is to develop a quantitative prediction of the oxygen isotopic signatures of nitrite and nitrate produced during nitrification in the sea. The researchers hypothesize that oxygen isotopic fractionation during nitrification is the primary factor setting the 18O values of newly produced nitrate and nitrite. Secondly, they hypothesize that oxygen atom exchange is low where ammonia oxidation and nitrite oxidation are tightly coupled, but may increase in regions with nitrite accumulation, such as in the primary and secondary nitrite maxima. They will test these hypotheses with a series of targeted laboratory and field experiments, as well as with measurements of nitrite and nitrate isotopic distributions extending through the euphotic zone, primary nitrite maximum, and secondary nitrite maximum of the Eastern Tropical South Pacific. The results of these experiments are expected to provide fundamental information required for the interpretation of 18O isotopic signatures in nitrite, nitrate, and N2O in the context of underlying microbial processes. A better understanding of these features and the processes involved is important for quantifying new production, controls on the N budget, and N2O production in the ocean -- which should lead to a better understanding of the direct and indirect interactions among the nitrogen cycle, marine chemistry, and climate.