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

Abstract. O₂ deficient zones (ODZs) of the world's oceans are important locations for microbial dissimilatory nitrate (NO₃⁻) reduction and subsequent loss of combined nitrogen (N) to biogenic N₂ gas. ODZs are generally coupled to regions of high productivity leading to high rates of N-loss as found in the coastal upwelling region off Peru. Stable N and O isotope ratios can be used as natural tracers of ODZ N-cycling because of distinct kinetic isotope effects associated with microbially mediated N-cycle transformations. Here we present NO₃⁻ and nitrite (NO₂⁻) stable isotope data from the nearshore upwelling region off Callao, Peru. Subsurface oxygen was generally depleted below about 30 m depth with concentrations less than 10 µM, while NO₂⁻ concentrations were high, ranging from 6 to 10 µM, and NO₃⁻ was in places strongly depleted to near 0 µM. We observed for the first time a positive linear relationship between NO₂⁻δ¹⁵N and δ¹⁸O at our coastal stations, analogous to that of NO₃⁻ N and O isotopes during NO₃⁻ uptake and dissimilatory reduction. This relationship is likely the result of rapid NO₂⁻ turnover due to higher organic matter flux in these coastal upwelling waters. No such relationship was observed at offshore stations where slower turnover of NO₂⁻ facilitates dominance of isotope exchange with water. We also evaluate the overall isotope fractionation effect for N-loss in this system using several approaches that vary in their underlying assumptions. While there are differences in apparent fractionation factor (ε) for N-loss as calculated from the δ¹⁵N 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 of sedimentary N-loss at our inshore stations which incurs highly suppressed isotope fractionation.