Distribution, reactivity and vertical fluxes of methane in the Guadalquivir Estuary (SW Spain)

The influence of temperature, salinity, sediment-water-atmosphere exchanges and oxidation rate on the variability of methane (CH₄) in the Guadalquivir Estuary has been studied. The database corresponds to 3 intensive samplings carried out in summer (2021 and 2022) and winter (2022). An increase in CH₄ concentration towards the interior of the estuary has been observed, more intense during summer (19-371 nmol L^-1). The influence of temperature and salinity on the variability of CH₄ concentration is negligible, with contributions below 1 nmol L^-1. Water-atmosphere fluxes increase inland in summer (28-574 μmol m^-2 d^-1), being generally higher than in winter (18-80 μmol m^-2 d^-1). Similarly, benthic fluxes remain relatively constant in winter (10 ± 6 μmol m^-2 d^-1) and increase inland in summer (7-212 μmol m^-2 d^-1). In the innermost station of the estuary, with salinities lower than 1, there is a significant increase in benthic fluxes, with values above 9000 μmol m^-2 d^-1. CH₄ oxidation rates increase towards low salinities, being especially high in summer (489 nmol L^-1 d^-1). Based on the information obtained, CH₄ variability in the Guadalquivir Estuary is mainly controlled by water-atmosphere fluxes, benthic fluxes and oxidation in the water column. The uncertainty associated with the quantification of these processes does not allow an adequate assessment of the influence of lateral inputs, although there is experimental evidence of their importance in the Guadalquivir.