Distribution of N2O in the eastern shelf of the Gulf of Cadiz (SW Iberian Peninsula)

Abstract

Distribution of N2O has been determined in eight cruises along three transects (Guadalquivir, Sancti Petri and Trafalgar) in the Gulf of Cadiz, during 2014 and 2015. The mean N2O value for this area was 10.0±0.9nM, with large spatial and temporal variations. Stratification in the water column has been observed; the concentration of this gas increases with the depth, because of the presence of the Eastern North Atlantic Central Water (ENACW) and the Mediterranean Outflow Waters (MOW). The N2O production measured in this study is mainly due to nitrification. N2O yields from nitrification were estimated from the linear correlation of the excess of N2O (ΔN2O) with Apparent Oxygen Utilization (AOU) and nitrate (NO3−), with values of their slopes ranged between 0.010 and 0.021% and 0.017–0.025% respectively. There is an onshore – offshore gradient of N2O; the highest values were found at the shallower stations, indicating coastal input and benthic remineralization. The seawater-air flux of N2O is affected by several variables (temperature, AOU and NO3−), and the average flux calculated is 2.7±2.0μmolm−2d−1. The fluxes show a decrease with increasing distance from the coast, and with proximity to the Strait of Gibraltar. The study area behaves as a source of N2O to the atmosphere, with a global emission of 0.18Ggyear−1.