Estimating fixed nitrogen loss and associated isotope effects using concentration and isotope measurements of NO3–, NO2–, and N2 from the Eastern Tropical South Pacific oxygen deficient zone
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Estimating fixed nitrogen loss and associated isotope effects using concentration and isotope measurements of NO3–, NO2–, and N2 from the Eastern Tropical South Pacific oxygen deficient zone

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  • Journal Title:
    Deep Sea Research Part II: Topical Studies in Oceanography
  • Personal Author:
  • NOAA Program & Office:
  • Description:
    Quantifying the pathways of fixed nitrogen (N) loss in marine oxygen deficient zones (ODZs) and the isotopic fractionation caused by these processes are important for understanding the marine fixed N budget and its potential for change. In this study, a variety of approaches were used to quantify fixed N loss in the eastern tropical South Pacific Ocean (ETSP). The required measurements included nutrient concentration (nitrate—NO3-, nitrite—NO2-, and phosphate—PO43-), gas ratio (N2/Ar) measurements, and stable N and O isotopes in NO3-, NO2-, and nitrogen gas (N2). The dissolved inorganic nitrogen deficit calculated from (def,P) exceeded the concentration of N2 gas biologically produced in the ODZ (local bio) throughout the ODZ at most stations, likely due to release of PO43- from sediments driving up def,P. Calculating DIN deficit using water mass analysis and local oxygen (O2) consumption (def,OMP) yielded better agreement with local bio than def,P, except at the maximum bio, where def,OMP misses contributions of anaerobic ammonia oxidation (anammox) to N2 production. We used the mismatch between def,OMP and bio to estimate a 29% contribution of anammox to bio. Stable isotopic measurements of NO2-, NO3-, and N2 were used alongside bio and new estimates of def to calculate N and O isotope effects for NO3- reduction (15εNAR and 18εNAR, respectively), and N isotope effects for DIN removal (15εDIN-R). While the various methods for estimating def had little effect on the isotope effects for DIN removal, differences between 15εNAR and 15εDIN-R, and variations with depth in the ODZ were observed. Using a simple time-dependent ODZ model, we interpreted these patterns to reflect the influences of NO2- oxidation and NO2- accumulation on expression of isotopic fractionation in the ODZ.
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  • Source:
    Deep Sea Research Part II: Topical Studies in Oceanography, 156, 121-136
  • DOI:
  • ISSN:
    0967-0645
  • Format:
    PDF
  • Publisher:
    Elsevier BV
  • Document Type:
    Journal Article
  • Funding:
    Grant no. NA10OAR4320148
  • Rights Information:
    Accepted Manuscript
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    Library
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