Nitrous oxide content and fluxes in Setúbal Bay, Portugal, during upwelling events

Abstract

To evaluate the influence of upwelling events on nitrous oxide (N₂O) levels and sea-air exchange in Setúbal Bay (SW Portugal), measurements of dissolved N₂O were carried out from the surface down to 200 m depth in May 2006 and May 2007. During the weak upwelling event (May 2006), higher N₂O concentrations (values up to 14 nmol L^–1) were observed in the upper and deeper layers. In the upper layers a positive correlation between ΔN₂O (N₂O excess) and apparent oxygen utilization was calculated, suggesting that nitrification contributed to N₂O production. During the stronger upwelling event (May 2007), N₂O values were lower and did not surpass 12 nmol L^–1. Production of N₂O could not be disentangled and apparently upwelling provided an effective pathway for ventilating N₂O from subsurface waters to the atmosphere. Surface waters were, in general, supersaturated with respect to the atmosphere (percent saturation values 95–160%), indicating that the study area was acting as a source of atmospheric N₂O. Higher upwelling-favorable winds (values up to 9 m s^–1) registered in May 2007 contributed to increase N₂O emissions, which attained a maximum value of 15.2 µmol m^–2 d^–1 in this period. Lower N₂O sea-air fluxes were estimated in May 2006 and values did not surpass 8.8 µmol m^–2 d^–1. Taking into account the N₂O flux in our study area, the contribution of Portuguese coastal upwelling to N₂O emission was estimated, varying from 0.040 to 0.102 Gg N yr^–1. These values are lower than those reported for most upwelling systems and represent a minor source to atmospheric N₂O.