Greenhouse gas dynamics in a coastal lagoon during the recovery of the macrophyte meadow (Mar Menor, SE Spain)

The Mar Menor is a hypersaline coastal lagoon with salinity values ranging from 41.9 to 45.5. The system is subjected to a high anthropic pressure that causes an intense eutrophication process, followed by a recovery of the macrophyte meadows. This study focuses on the distribution of the main greenhouse gases (CO2, CH4 and N2O) and was carried out in the extreme seasonal conditions of winter and summer during the year 2018. Sediment-water-atmosphere exchanges and biochemical processes in the water column appeared to be the main factors to explain the variability of these gases. Dissolved Inorganic Carbon (DIC), CH4 and N2O benthic fluxes values obtained in this study, were of 91 ± 29 mmol m−2 d−1, 3.9 ± 1.9 μmol m−2 d−1 and −0.65 μmol m−2 d−1, respectively, along with an important seasonal variation observed, with an increase of DIC and CH4 benthic fluxes during the summer season. Mean values of partial pressure of CO2 (pCO2) in surface water were of 579 μatm in winter and 464 μatm in summer, therefore we can establish that the Mar Menor acts as a source of this gas emitting 3.3 ± 3.0 mmol CO2 m−2 d−1 to the atmosphere. In spite of this, the Mar Menor has a strong autotrophic behaviour partly due to the recovery of the macrophyte meadows, presenting an estimated NEP of 101 mmol m−2 d−1. Regarding to CH4, the mean fluxes to the atmosphere were of 8.0 ± 5.8 μmol m−2 d−1 and there was evidence of CH4 production in the water column that increased in summer. Last of all, in the case of N2O the system acts as a sink with values of −0.65 ± 0.5 μmol m−2 d−1, presenting an intake of N2O that is usually detected in pristine systems. [Display omitted] •This coastal lagoon acts as a source of CO2 and CH4, but as a sink for N2O.•The benthic fluxes of CO2 and CH4 are an important contribution to the water column.•There is a significant intake of N2O in the sediment.•The Mar Menor is strongly autotrophic during the recovery of macrophyte meadows.•Unbalanced sediment-water-air CH4 fluxes suggest water column production.