Seasonal ventilation controls nitrous oxide emission in the NW Iberian upwelling

Abstract

Despite their small spatial extent, coastal upwelling systems are an important source of oceanic nitrous oxide (N₂O) to the atmosphere. To date, hot-spot N₂O emissions have been reported for low oxygen waters of the eastern boundary upwelling systems at their tropical latitudes, but there is a limited number of studies in their “oxygenated” temperate latitudes. This is the first study of the N₂O cycle in the NW Iberian Upwelling system, where we investigated the seasonality of the N₂O concentrations and their emissions to the atmosphere, along with the spatial differences in this coastal region in response to the upwelling. Monthly observations were collected from February 2017 to July 2018, in two hydrographic sections within the Ría of Vigo and Ría of A Coruña, two coastal embayments with contrasting response to the upwelling of the Eastern North Atlantic Central Water (ENACW) in the region. N₂O concentrations ranged between 8.56 to 12.53 nmol kg⁻¹ (94–121 % of saturation) in the shelf, and from 8.62 to 17.60 nmol kg⁻¹ (94–203 % of saturation) inside the rías, with the highest N₂O concentration at the bottom, which increase as the upwelling progress from April to October. The air-sea fluxes of N₂O varied between −1.6 to 3.26 µmol m⁻² d⁻¹ in the shelf and −1.53 to 7.00 µmol m⁻² d⁻¹ inside the rías. Local differences on the ventilation and remineralization pattern drives the seasonality of N₂O and differences between Ria of Vigo and Ria of A Coruña, being the higher values of N₂O concentrations and air-sea fluxes registered in the inner Ria of Vigo. Our study reports the N₂O emissions of an upwelling system in a temperate latitude, where the upwelling waters are central waters relatively well ventilated in terms of oxygen content, behaving as a moderate low net source of N₂O to the atmosphere compared to tropical upwelling latitudes, characterised by a lower oxygen content.