Distribution of N 2 O in the eastern shelf of the Gulf of Cadiz (SW Iberian Peninsula).

Distribution of N2 O 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 N2 O 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 N2 O production measured in this study is mainly due to nitrification. N2 O yields from nitrification were estimated from the linear correlation of the excess of N2 O (ΔN2 O) 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 N2 O; the highest values were found at the shallower stations, indicating coastal input and benthic remineralization. The seawater-air flux of N2 O is affected by several variables (temperature, AOU and NO3 - ), and the average flux calculated is 2.7±2.0μmolm-2 d-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 N2 O to the atmosphere, with a global emission of 0.18Ggyear-1 .