Exploring Methane Emission Drivers in Wetlands: The Cases of Massaciuccoli and Porta Lakes (Northern Tuscany, Italy)

Wetlands are hotspots of CH4 emissions to the atmosphere, mainly sustained by microbialdecomposition of organic matter in anoxic sediments. Several knowledge gaps exist on how environmental drivers shape CH4 emissions from these ecosystems, posing challenges in upscaling effortsto estimate global emissions from waterbodies. In this work, CH4 and CO2 diffusive fluxes, alongwith chemical and isotopic composition of dissolved ionic and gaseous species, were determinedfrom two wetlands of Tuscany (Italy): (i) Porta Lake, a small wetland largely invaded by Phragmitesaustralis reeds experiencing reed die-back syndrome, and (ii) Massaciuccoli Lake, a wide marsharea including open-water basins and channels affected by seawater intrusion and eutrophication.Both wetlands were recognized as net sources of CH4to the atmosphere. Our data show that themagnitude of CH4 diffusive emission was controlled by CH4 production and consumption rates,being mostly governed by (i) water temperature and availability of labile carbon substrates and (ii)water column depth, wind exposure and dissolved O2 contents, respectively. This evidence suggeststhat the highest CH4 diffusive fluxes were sustained by reed beds, providing a large availability oforganic matter supporting acetoclastic methanogenesis, with relevant implications for global carbonbudget and future climate models.