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

Abstract 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—NO 3 - , nitrite—NO 2 - , and phosphate—PO 4 3- ), gas ratio (N 2 /Ar) measurements, and stable N and O isotopes in NO 3 - , NO 2 - , and nitrogen gas (N 2 ). The dissolved inorganic nitrogen deficit calculated from [PO 4 3- ] ([DIN] def,P ) exceeded the concentration of N 2 gas biologically produced in the ODZ (local [N 2 ] bio ) throughout the ODZ at most stations, likely due to release of PO 4 3- from sediments driving up [DIN] def,P . Calculating DIN deficit using water mass analysis and local oxygen (O 2 ) consumption ([DIN] def,OMP ) yielded better agreement with local [N 2 ] bio than [DIN] def,P , except at the maximum [N 2 ] bio , where [DIN] def,OMP misses contributions of anaerobic ammonia oxidation (anammox) to N 2 production. We used the mismatch between [DIN] def,OMP and [N 2 ] bio to estimate a 29% contribution of anammox to [N 2 ] bio . Stable isotopic measurements of NO 2 - , NO 3 - , and N 2 were used alongside [N 2 ] bio and new estimates of [DIN] def to calculate N and O isotope effects for NO 3 - reduction ( 15 e NAR and 18 e NAR , respectively), and N isotope effects for DIN removal ( 15 e DIN-R ). While the various methods for estimating [DIN] def had little effect on the isotope effects for DIN removal, differences between 15 e NAR and 15 e DIN-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 NO 2 - oxidation and NO 2 - accumulation on expression of isotopic fractionation in the ODZ.