## 4 Discussion
### CH4 Emission Drivers at Porta Lake
The best fit of the data was obtained with a \(\epsilon_{CH_{4}-CO_{2}}\) value of about 8, in line with the values expected for carbon fractionation factors related to CH\({}_{4}\) oxidation (<10 [74]). Although this evidence confirmed the relevance of water column depth in controlling the CH\({}_{4}\) emissions within the wetland, which showed a variation range of one order of magnitude, different trends were observed in \(\Phi\)CH\({}_{4}\) values (Figure 2c), as detailed below. The data registered in summer displayed a higher increase in CH\({}_{4}\) diffusive fluxes as the water column depth decreased (trend A) with respect to winter data. Such seasonal variation in the measured CH\({}_{4}\) diffusive fluxes was to be ascribed to the higher temperatures recorded in summer, which are expected to accelerate methane production rate from sediments, e.g., [83] and references therein. On the other hand, winter data showed two distinct behaviors: trends B and C in Figure 2c, with those samples collected from sites located close to the lake shores and in the reeds being characterized by higher \(\Phi\)CH\({}_{4}\) values with respect to other sites of similar depth. Such behavior might be related to the diverse hydrological regimes encountered within the lake, with higher \(\Phi\)CH\({}_{4}\) values where dense riparian reeds sheltering the lake waters from winds hinder mixing and oxygenation processes and produce more  
Figure 2: 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 Porta Lake. Symbols as in Figure 1a. In (**c**), the dashed contour lines highlight the trends (A, B and C) described in detail in the text. stagnant waters (trend C), as testified by the lower O\({}_{2}\) contents (Figure 2a) with respect to open waters (trend B; Figure 2c). Moreover, the sites displaying anomalous \(\Phi\)CH\({}_{4}\) values (trend C) were located in areas characterized by visible retreat of need beds (Figure 1a). Accordingly, [PERSON] et al. [76] demonstrated that methanogenesis rates in die-back sites were higher than those in the surrounding healthy reeds, suggesting that RDBS-affected wetlands might be intense sources of CH\({}_{4}\). This hypothesis was also supported by the data obtained at Porta Lake in summer, which displayed the highest \(\Phi\)CH\({}_{4}\) values in sites of dying back reeds (LP6 and LP7). Even though their role in regulating lake CH\({}_{4}\) emissions has been long debated, e.g., [84; 85] and references therein, macrophytes were recognized as important sources of autochthonous labile carbon in aquatic environments, e.g., [77] and the macrophyte detritus was demonstrated to improve CH\({}_{4}\) production in anoxic sediments, e.g., [78]. Similarly, [PERSON] et al. [86] found that macrophyte litter determined a \(\geq\)400 times higher CH\({}_{4}\) production than terrestrial sources. As shown in Figure 4, where the CH\({}_{4}\) diffusive fluxes measured at Porta Lake were compared with those recently compiled from literature data by [PERSON] et al. [17], the \(\Phi\)CH\({}_{4}\) values from the present study largely exceeded the average value reported for diffusive fluxes from diverse aquatic ecosystems and were anomalously high with respect to the range reported in literature for lakes and reservoirs. On average, the \(\Phi\)CH\({}_{4}\) values at Porta Lake even exceeded those exceptionally high values recently reported for a tropical lake in Kenya (ca. 0.97 g CH\({}_{4}\) m\({}^{-2}\) day\({}^{-1}\)[24]). Figure 3: \(\delta^{13}\)C-CH\({}_{4}\) (in % vs. V-PDB) vs. CH\({}_{4}\) (in mmol/L) binary diagram for samples collected in summer and winter from Porta Lake. Symbols as in Figure 1a. The Rayleigh fractionation evolution curve for CH\({}_{4}\) oxidation to CO\({}_{2}\) related to methanotrophic activity, as described in the text, is also reported (dashed line).