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O2 deficient zones (ODZs) of the world's oceans are important locationsfor microbial dissimilatory nitrate (NO3−) reduction and subsequentloss of combined nitrogen (N) to biogenic N2 gas. ODZs are generallycoupled to regions of high productivity leading to high rates of N-loss asfound in the coastal upwelling region off Peru. Stable N and O isotope ratioscan be used as natural tracers of ODZ N-cycling because of distinct kineticisotope effects associated with microbially mediated N-cycle transformations.Here we present NO3− and nitrite (NO2−) stable isotope datafrom the nearshore upwelling region off Callao, Peru. Subsurface oxygen wasgenerally depleted below about 30 m depth with concentrations lessthan 10 µM, while NO2− concentrations were high, ranging from6 to 10 µM, and NO3− was in places strongly depleted tonear 0 µM. We observed for the first time a positive linearrelationship between NO2−δ15N and δ18O at ourcoastal stations, analogous to that of NO3− N and O isotopes duringNO3− uptake and dissimilatory reduction. This relationship is likelythe result of rapid NO2− turnover due to higher organic matter fluxin these coastal upwelling waters. No such relationship was observed atoffshore stations where slower turnover of NO2− facilitates dominanceof isotope exchange with water. We also evaluate the overall isotopefractionation effect for N-loss in this system using several approaches thatvary in their underlying assumptions. While there are differences in apparentfractionation factor (ε) for N-loss as calculated from theδ15N of NO3−, dissolved inorganic N, or biogenic N2,values for ε are generally much lower than previously reported,reaching as low as 6.5 ‰. A possible explanation is the influence ofsedimentary N-loss at our inshore stations which incurs highly suppressedisotope fractionation

Nitrogen cycling in shallow low-oxygen coastal waters off Peru from nitrite and nitrate nitrogen and oxygen isotopes