Spatial and seasonal variability of CH 4 in the eastern Gulf of Cadiz (SW Iberian Peninsula).

Methane (CH4 ) concentrations were measured along three sections of the eastern Gulf of Cadiz (designated "Guadalquivir", "Sancti Petri" and "Trafalgar") during eight cruises in 2014 and 2015. The concentration of CH4 varied from 3.6 to 19.7nmolkg-1 (CH4 saturation percent of 122 and 916%), showing seasonal variation. The highest values were found in December 2014 and November 2015. In most of the sampling area the highest concentration of CH4 was found in subsurface waters at depths close to the thermocline, and in the bottom waters near the coast. The seawater-air flux of CH4 ranged between 0.8 and 59.7μmolm-2 d-1 , showing seasonal variation in function of the temperature of the surface water. In the "Guadalquivir" and "Sancti Petri" sections, the CH4 fluxes increased with proximity to the coast; this may be a result of continental inputs and CH4 emissions from sediments. The whole study area behaves as a source of CH4 to the atmosphere with mean values of 0.5 and 0.6GgCH4 yr-1 in 2014 and 2015, respectively.