Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Water was collected during the MESO-SCOPE research cruise aboard R/V Kilo Moana (KM1709) during June-July 2017. Upper ocean biogeochemistry was characterized at 11 stations along the transect traversing the cyclonic and anticyclonic eddies, using a rosette mounted with 10-liter (L) Niskin® bottles. Water samples for nutrient and nitrate isotope analyses were collected at ~25-meter (m) intervals from 5 m to 500 m with higher vertical resolution (~5 m intervals) near the deep chlorophyll maximum (DCM). Samples were frozen at -20 degrees Celsius (°C) after collection pending analysis.

The N isotope ratios of nitrate (15N/14N) in water samples from stations 4 to 13 were measured with the denitrifier method (Casciotti et al., 2002; Sigman et al., 2001) for concentrations exceeding 0.5 micromoles per liter (µmol L-1). Nitrate was converted to nitrous oxide (N2O) by cell concentrates of the denitrifying bacterial strain Pseudomonas chlororaphis (ATCC 43928, Manassas, VA, USA), which lacks the terminal N2O reductase. The N2O gas was extracted and purified using a custom-modified Thermo Fisher Scientific Gas Bench II fronted by dual cold traps and a GC Pal autosampler, and analyzed with a Thermo Delta V Advantage continuous flow gas chromatograph isotope ratio mass spectrometer (Casciotti et al., 2002; McIlvin & Casciotti, 2011). Working solutions were diluted from primary stocks into nutrient-free seawater to concentrations bracketing sample concentrations to account for potential matrix effects (Weigand et al., 2016; Zhou et al., 2021). Individual samples were measured 3 to 9 times to achieve an analytical uncertainty to ≤ 0.3 ‰. The oxygen isotope ratios of nitrate (ẟ18ONO3) were not measured concurrently as we did not secure sufficient sample volumes to estimate these reliably (see Zhou et al., 2021).

The N isotope ratios are expressed in delta (ẟ) notation in units of per mil (‰) vs. a standard material (N2 gas in the air): ẟ15Nsample = [(15N/14N)sample/(15N/14N)standard – 1] × 1000. Nitrate isotopic analyses were calibrated to internationally recognized nitrate reference materials IAEA-NO3 (International Atomic Energy Agency, Vienna, Austria) and USGS-34 (National Institute of Standards and Technology, Gaithersburg, MD, USA), with reported δ15N values of 4.7 ‰ and −1.8 ‰ (vs. air). Individual samples were measured 3 - 9 times to achieve an analytical uncertainty to ≤ 0.3 ‰.

- Imported original file "Zhou_etal-BCO-DMO.xlsx" into the BCO-DMO submission.
- Renamed fields to comply with BCO-DMO naming conventions.
- Created ISO-DateTime fields in local (HST) and UTC format.
- Removed the original, separate date and time columns.
- Converted longitude values from positive to negative to indicate the direction is West.
- Saved the final file as "948358_v1_nitrate_15n_14n_meso-scope.csv".

NSF Award Abstract:
The nitrogen (N) cycle in the marine environment is controlled by biological processes. Unfortunately, quantifying these processes and assessing their effect on the N cycle is difficult by direct measurements because of large spatial and temporal differences. Isotopic composition measurements of N provide a means to constrain these processes indirectly; however, there is still a great deal to be understood about isotope fractionation of recycled nitrogen through biological processes, which has made interpretation of novel nitrogen isotope data difficult. A researcher from the University of Connecticut plans to determine the influence of biological consumption and production on the isotope fractionation in ammonium. By helping to understand the processes surrounding fractionation of recycled ammonium at the organism level, this research will create a basis for which future researchers can better interpret isotope composition data to infer nitrogen cycle dynamics. A graduate student, a postdoctoral fellow, and two or more undergraduate students will be involved in the research. The researcher plans to integrate science with community-engaged learning by developing an undergraduate field and laboratory course that will require the students to present their research to stakeholders in the community. There will be a manual created for this course that will be disseminated in open-access forums for teachers hoping to develop similar courses.

Biological nitrogen isotope fractionation associated with nitrogen recycling remains poorly constrained despite the advent of a variety of new techniques to analyze nitrogen isotopes in recent years. The use of isotopic composition data can be incredibly useful to interpreting nitrogen cycle processes in the ocean that are difficult to measure directly, which makes it crucial to further understand the processes behind fractionation to catch up with the advancement of the datasets available to researchers. This research will characterize the isotope fractionation dynamics of ammonium during biological consumption and production. The researchers will investigate whether the characteristic low concentrations of ammonium in the surface ocean affect isotope fractionation when the ammonium is recycled and whether there is a trophic isotope effect associated with ammonium recycling by protozoan grazers. With this research, there will be a baseline from which researchers can interpret recycled nitrogen dynamics from ammonium isotope datasets. The methods of comparing nitrogen cycling studies will become significantly clearer with such a standard making interpretation uniform by removing significant uncertainties.