Nitrous oxide and N-nutrient cycling in the oxygen minimum zone off northern Chile

Abstract

Measurements of dissolved gases (O 2, N 2O), nutrients (NO 3 −, NO 2 −, PO 4 3−), and oceanographic variables were performed off northern Chile (∼21°S) between March 2000 and July 2004, in order to characterize the existing oxygen minimum zone (OMZ) and identify processes involved in N 2O cycling. Both N 2O and NO 3 − displayed sharp, shallow peaks with concentrations of up to 124 nM (1370% saturation) and 26 μM, respectively, in association with a strong oxycline that impinges on the euphotic zone. NO 2 − accumulation below the oxycline's base reached up to 9 μM. The vertical distribution of physical and chemical parameters and the existing relationships between apparent oxygen utilization (AOU), apparent N 2O production (ΔN 2O), and NO 3 − revealed three main layers within the upper OMZ. The first layer, or the upper part of the oxycline, is located between the base of the mixed layer and the mid-point of the oxycline (around σ t =25.5 kg m −3). There the O 2 declines from ∼250 to ∼50 μM, and strong (but opposing) O 2 and NO 3 − gradients and their associated AOU–ΔN 2O and AOU–NO 3 − relationships indicate that nitrification produces N 2O and NO 3 − in the presence of light. The second layer, or lower part of the oxycline, represents the upper OMZ boundary and is located between the middle and the base of the oxycline (25.9< σ t <26.1 kg m −3). In this layer NO 3 − reduction begins at O 2 levels ranging from ∼50 to ∼11 μM and accumulation of 41–68% of the ΔN 2O pool occurs. The accumulation of N 2O (but not of NO 2 − or NH 4 +) and the observed AOU–ΔN 2O and AOU–NO 3 − relationships (which are opposite to those of the overlying first layer) suggest that a coupling between nitrification and NO 3 − reduction is involved in N 2O cycling in this second layer. The third layer is the OMZ core, where the O 2 concentration remains constant (O 2<11 μM). It coincides with σ t >26.2 kg m −3, which is typical of Equatorial Subsurface Water (ESSW). In this layer, N 2O and NO 3 − continue to decrease, but a large NO 2 − accumulation is observed. Considering all the data, a biogeochemical model for the upper OMZ off northern of Chile is proposed, in which nitrification and denitrification differentially mediate N 2O cycling in each layer.