Greenhouse gas assemblages (CO₂, CH₄ and N₂O) in the continental shelf of the Gulf of Cadiz (SW Iberian Peninsula)

This study examines the simultaneous water-atmosphere exchange of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) on the continental shelf of the Gulf of Cadiz, as well as the effect it has in terms of the radiative balance in the atmosphere, between 2014 and 2016. The experimental database consists of new measurements of the spatial and seasonal distribution of CO₂ partial pressure (pCO₂) and N₂O concentration in 2016. pCO₂ shows a wide range of variation influenced mainly by seasonal thermal variations (8.0 μatm ⁰C-1), as well as with the relative intensity of biological activity. There is experimental evidence of a progressive increase of pCO₂ over the last 2 decades, with an estimated gradient of 4.2 ± 0.7 μatm y^-1. During 2016, the Gulf of Cadiz acted as a slight source of CO₂ to the atmosphere, with a mean flux of 0.4 ± 2.2 mmol m^-2 d^-1. The analysis of concentration variations in the water column shows that nitrification is the main N₂O production process in the study area, although in the more coastal zone there are signs of inputs related to continental and sediment contributions, most probably induced by denitrification processes. In 2016, the Gulf of Cadiz acted as a weak sink of atmospheric N₂O, with a mean flux of -0.1 ± 0.9 μmol m^-2 d^-1. From previous studies, performed with a similar methodology, an interannual database (2014-2016) of water-atmosphere fluxes of CO₂, CH₄ and N₂O, normalized to the mean wind speed in the area, has been generated. Considering their respective Global Warming Potential (GWP) a joint greenhouse gasses (GHG) flux, expressed in CO₂ equivalents of 0.6 ± 2.0 mmol m^-2 d^-1, has been estimated, which extended to the area of study indicates an approximate emission of 67.9 Gg CO₂ y^-1. However, although there is a high uncertainty associated with the spatial, temporal and interannual variations of CO₂, CH₄ and N₂O fluxes in the Gulf of Cadiz, the exchange of greenhouse gasses could be influencing a radiative forcing increase in the atmosphere. When considering the available information on local and global estimates, the uncertainty about the effect of the joint exchange of GHGs to the atmosphere from the coastal seas increases significantly.