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

Highlights: • 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. 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.