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 (NO¯3) 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 NO¯3 and nitrite (NO¯2) 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 NO¯2 concentrations were high, ranging from 6 to 10 μM, and NO¯3 was in places strongly depleted to near 0μM. We observed for the first time a positive linear relationship between NO¯2δ15N and 18O at our coastal stations, analogous to that of NO¯3 N and O isotopes during NO¯3 uptake and dissimilatory reduction. This relationship is likely the result of rapid NO¯2 turnover due to higher organic matter flux in these coastal upwelling waters. No such relationship was observed at offshore stations where slower turnover of NO¯2 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 ( μ) for N-loss as calculated from the 15N of NO¯3, dissolved inorganic N, or biogenic N2, values for μ 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.