## 4 Discussion  
### CH4 Emission Drivers at Porta Lake  
The chemical composition of Porta Lake waters was strictly controlled by the Ca2+-SO42 springs that feed the lake, although a further shallow water source was possibly affecting those waters showing a HCO3- composition (Table 1). Seasonal variations in the lake water level were found, associated with changes in TDS and Eh values. The Eh values were relatively low, especially in summer, testifying of the presence of reducing conditions related to organic matter accumulation at the lake bottom and associated decomposition and mineralization processes. Accordingly, the dissolved O2 concentrations were lower with respect to those expected for surface waters in equilibrium with air (Table 2), with the strongest O2 depletion observed where the lake depth was relatively low (Figure 2a). These sites were also characterized by high concentrations of NH4+ and CH4 (Tables 1 and 2) supplied by microbial activity in anoxic bottom sediments. It is well-known that CH4 production under anoxic conditions may occur through two distinct pathways, i.e., acetoclastic methanogenesis and hydrogenotrophic methanogenesis, depending on the degradability of organic matter, e.g., [70; 71; 72; 73]. The former, in which acetate serves as substrate, is considered to be more common in aquatic environments with large availability of labile organic matter; on the other hand, hydrogenotrophic methanogenesis, involving H2 as electron acceptor, relies on more refractory organic matter. The two pathways produce distinct stable carbon isotopic signatures of CH4, leading to \(\delta^{13}\)C-CH4 values varying from \(-\)110 and \(-\)60% vs. V-PDB in the case of hydrogenotrophy and from \(-\)60 to \(-\)50% vs. V-PDB in the case of acetoclasty, e.g., [72; 74; 75]. As the water column depth decreased, CH4 showed a progressive \({}^{12}\)C-enrichment, as expected for a microbial CH4 production (Figure 2b), pointing to an isotope signature of CH4 from methanogenesis around \(-\)60% vs. V-PDB, suggesting a prominent contribution from acetoclasty. Presumably, the presence of widespread reeds affected by RDBS at Porta Lake might play a significant role in enhancingCH\({}_{4}\) production and, hence, its release to the atmosphere, by providing a higher organic matter accumulation rate, making more labile carbon substrates available for methanogenesis, e.g., [29, 76, 77, 78]. These findings were in agreement with previous evidence that higher CH\({}_{4}\) ebullition rates in northern temperate lakes were predominantly supported by acetoclastic methanogenesis [79]. The large availability of autochthonous organic matter, providing abundant substrate for microbial activity, likely limited the competition between methanogenesis and sulfate reduction [80, 81, 82]. On the other hand, the overall decrease in \(\Phi\)CH\({}_{4}\) values at increasing water column depth (Figure 2c) was likely related to more efficient surface water mixing and/or stratification processes in open waters, testified by higher O\({}_{2}\) contents (Figure 2a), resulting in more efficiency of oxidation processes affecting CH\({}_{4}\) slowly diffusing from the lake bottom towards the water-atmosphere interface. Accordingly, the increase in \(\delta^{13}\)C-CH\({}_{4}\) values at decreasing dissolved CH\({}_{4}\) contents in surface waters (Figure 3) can adequately be described by a Rayleigh-type fractionation process (Figure 3), as follows:  
\[\delta^{13}C-CH_{4\_residual}=\left[\left(\delta^{13}C-CH_{4\_residual}+1000 \right)\times f^{\frac{-\epsilon_{CH_{4}-CO_{2}}}{1000}}\right]-1000 \tag{7}\]  
where \(\delta^{13}\)C-\(CH_{4\_initial}\) and \(\delta^{13}\)C-\(CH_{4\_residual}\) are the isotopic signatures of initial CH\({}_{4}\) (\(-60\%\) vs. V-PDB) and residual CH\({}_{4}\) after oxidation, \(f\) is the fraction of residual CH\({}_{4}\) and \(\epsilon_{CH_{4}-CO_{2}}\)is the fractionation factor for CH\({}_{4}\) oxidation to CO\({}_{2}\) during methanothrophy.