Nitrate dual isotopes and nitrification rates in central Chile

Abstract. Nitrification is an essential process for nutrient availability in the ocean. Due to its low oxygen requirements, the importance of this microaerophilic process increases in upwelling regions, associated with oxygen minimum zones (OMZ). Yet, the environmental conditions influencing the magnitude of this process seem to vary largely across similar ecosystems, calling for more system-specific measurements. In the present study, nitrification was studied during a coastal upwelling event on the shelf off Concepción, central Chile (~ 36.5° S). A transect of four stations, from the coast to the shelf edge, was sampled in austral summer 2023, with upwelling of Equatorial Subsurface Water impacting all stations to varying degrees. First measurements of nitrate (NO₃⁻) dual isotopes revealed that upwelled waters bear high concentrations of NO₃⁻ with a δ¹⁵N-NO₃⁻ of 13.1 ± 0.3 ‰, a δ¹⁸O-NO₃⁻ value of 12.1 ± 0.5 ‰. Increase in δ¹⁵N- and δ¹⁸O-NO₃⁻ towards the surface and at oxygen depleted depths suggest that assimilation in surface waters and denitrification in the OMZ are the primary controls on NO₃⁻ isotopic composition, while NO₂⁻ accumulation and δ¹⁵N- reveal intermediate transformations such as partial NO₂⁻ oxidation. Despite low overall nitrification rates, a localized peak at the most offshore station sampled (78.7 nmol L⁻¹ h⁻¹) indicates active ammonium oxidation under nanomolar oxygen levels, only detectable thanks to the use of a high-precision oxygen sensor. These findings highlight the dynamic nature of nitrogen cycling in the Chilean upwelling system and the importance of high-resolution sampling for such regions. This study offers critical parameters for improving productivity models and interpreting nitrogen isotope records in marine sediments in the region.