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Vertical modeling of the nitrogen cycle in the eastern tropical South Pacific oxygen deficient zone using high‐resolution concentration and isotope measurements
Author(s) -
Peters Brian D.,
Babbin Andrew R.,
Lettmann Karsten A.,
Mordy Calvin W.,
Ulloa Osvaldo,
Ward Bess B.,
Casciotti Karen L.
Publication year - 2016
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1002/2016gb005415
Subject(s) - anammox , denitrification , nitrate , heterotroph , nitrite , environmental chemistry , oxygen minimum zone , environmental science , isotopes of nitrogen , nitrogen , stable isotope ratio , ammonium , chemistry , oceanography , oxygen , geology , denitrifying bacteria , quantum mechanics , paleontology , physics , organic chemistry , bacteria
Marine oxygen deficient zones (ODZs) have long been identified as sites of fixed nitrogen (N) loss. However, the mechanisms and rates of N loss have been debated, and traditional methods for measuring these rates are labor‐intensive and may miss hot spots in spatially and temporally variable environments. Here we estimate rates of heterotrophic nitrate reduction, heterotrophic nitrite reduction (denitrification), nitrite oxidation, and anaerobic ammonium oxidation (anammox) at a coastal site in the eastern tropical South Pacific (ETSP) ODZ based on high‐resolution concentration and natural abundance stable isotope measurements of nitrate (NO 3 − ) and nitrite (NO 2 − ). These measurements were used to estimate process rates using a two‐step inverse modeling approach. The modeled rates were sensitive to assumed isotope effects for NO 3 − reduction and NO 2 − oxidation. Nevertheless, we addressed two questions surrounding the fates of NO 2 − in the ODZ: (1) Is NO 2 − being primarily reduced to N 2 or oxidized to NO 3 − in the ODZ? and (2) what are the contributions of anammox and denitrification to NO 2 − removal? Depth‐integrated rates from the model suggest that 72–88% of the NO 2 − produced in the ODZ was oxidized back to NO 3 − , while 12–28% of NO 2 − was reduced to N 2 . Furthermore, our model suggested that 36–74% of NO 2 − loss was due to anammox, with the remainder due to denitrification. These model results generally agreed with previously measured rates, though with a large range of uncertainty, and they provide a long‐term integrated view that compliments incubation experiments to obtain a broader picture of N cycling in ODZs.