Dataset: ETSP 15N rates

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.