## 4 Discussion
### CH\({}_{4}\) Emission Drivers at Massaciuccoli Lake System
Global meteoric water line (GMWL: \(\delta D=8\times\delta^{18}O+10\)[88]), local meteoric water line (LMWL: \(\delta D=7.6\times\delta^{18}O+7.5\)[47]) and local evaporation line (LEL: \(\delta D=5.4\times\delta^{18}O-5.9\)[47]). \(\delta^{13}\)C-CH\({}_{4}\) values and CH\({}_{4}\) diffusive fluxes at the water-air interface varied independently from the water column depth (Figure 7b,c), confirming that this physical environmental feature alone was not informative of the extent of CH\({}_{4}\) release to the atmosphere. As observed at Porta Lake, the CH\({}_{4}\) emissions were expected to be controlled by both (i) water mixing/stratification, regulating the oxidation of CH\({}_{4}\) along its ascent towards the water-atmosphere interface, and (ii) rate of microbial activity, determining the extent of CH\({}_{4}\) production mostly occurring within the bottom sediments. In the case of Massaciuccoli Lake system, the former was likely controlled by morphological settings rather than water depth, whereas the latter was likely modulated by several factors, including quality and quantity of available organic matter. The measured \(\delta^{13}\)C-CH\({}_{4}\) values pointed to ca. \(-65\%\) vs. V-PDB as the CH\({}_{4}\) contents increased (Figure 8), suggesting the involvement of largely and readily available organic substrates for acetoclastic methanogenesis. Notably, the highest \(\Phi\)CH\({}_{4}\) value, up to two orders of magnitude higher than those measured in the other sites, was recorded at LSW7, i.e., a sampling site located within an area largely covered by need beds [50] (Figure 1b), confirming the relevant dual role of reed beds in (i) providing labile organic matter enhancing CH\({}_{4}\) production while (ii) sheltering winds and hindering water oxygenation. Notably, the \(\Phi\)CH\({}_{4}\) values at Massaciuccoli Lake were within the range reported in literature for lakes and reservoirs (Figure 4), with the sole exception of LSW7. On the other hand, whilst the increase in the \(\delta^{13}\)C-CH\({}_{4}\) values at decreasing dissolved CH\({}_{4}\) contents (Figure 8) was in agreement with a Rayleigh-type fractionation process (Equation (7)), with a \(\epsilon_{CH_{4}-CO_{2}}\) value of about 5, similar to that observed by [PERSON] [74] based on field observations, the fractionation factor (\(\epsilon_{C}\)) between CO\({}_{2}\) and CH\({}_{4}\) tended to ca. 45% vs. V-PDB (Table 4) at decreasing \(\delta^{13}\)C-CH\({}_{4}\) values, in agreement with the \(\epsilon_{C}\) values expected for freshwater environments dominated by acetate fermentation [74]. The \(\Phi\)CO\({}_{2}\) values at Massaciuccoli Lake system largely varied from negative values up to 43.9 g m\({}^{-2}\) day\({}^{-1}\), the highest values being observed within the reed beds (LSW7). Overall, high CH\({}_{4}\) fluxes were associated with high \(\Phi\)CO\({}_{2}\) values (Figure 5b), confirming a strict relationship among the diffusive emissions of these two GHGs from waterbodies and their common origin related to the degradation of organic matter, as supported by the measured \(\delta^{13}\)C-CO\({}_{2}\) values (Table 4). Although Massaciuccoli Lake system displayed lower \(\Phi\)CO\({}_{2\text{eq}}\) values with respect to those of Porta Lake, with local areas where photosynthetic activity produced negative fluxes (i.e., CO\({}_{2}\) uptake from the atmosphere), relevant hot spots of intense GHGs emissions were recognized (LSW7). Figure 7: Water column depth (in cm) vs. (**a**) O\({}_{2}\) (in mmol/L), (**b**) \(\delta^{13}\)C-CH\({}_{4}\) (in% vs. V-PDB) and (**c**) \(\Phi\)CH\({}_{4}\) (in g m\({}^{-2}\) day\({}^{-1}\)) binary diagrams for samples collected from Massaciuccoli Lake. Symbols as in Figure 1b.