## 3 Results
### The Chemical Features of Porta Lake
The chemical composition was dominated by Na+ and Cl-, with higher concentrations of NH4+ and NO3- with respect to those recorded in the OMW and OSW surface water samples, i.e., up to 0.48 (LSW8) and 1.3 (LSW5) mg/L, respectively (Table 3). The isotopic composition of water was comparable with that of samples from open waters, the \(\delta^{18}\)O-H\({}_{2}\)O and \(\delta\)D-H\({}_{2}\)O values being from \(-\)1.80 (LSW8) to \(-\)0.99 (LSW4)% vs. V-SMOW and from \(-\)14.4 (LSW8) to \(-\)10.1 (LSW4)% vs. V-SMOW, respectively (Table 3). The \(\delta^{13}\)C-TDIC values ranged from \(-\)14.2 (LSW9) to \(-\)11.7 (LSW1)% vs. V-PDB (Table 3). The chemical composition of dissolved gases was dominated by N2 in all of the collected surface waters, with concentrations ranging from 0.54 (LSW4 and TCW3) to 0.66 (LSW8) mmol/L (Table 4). The measured N2/Ar ratios were varying around that of ASW, i.e., from 38 to 43. The second most abundant gas was O2, with concentrations ranging from 0.056 (LSW7) to 0.31 (TCW4) mmol/L, with the sole exception of the LSW7 sample that was characterized by CH4 as the second most abundant gas after N2. Overall, the dissolved CH4 concentrations varied from 0.003 (OMW2) to 0.23 (LSW7) mmol/L, with values averagely higher in the LSW, TCW and ECW samples with respect to those measured in the SPW, OMW and OSW samples (Table 4). The dissolved CO2 concentrations ranged from 0.006 (SPW) to 0.15 (LSW7) mmol/L (Table 4). The carbon isotopic composition of CO2 and CH4 showed a general increase in CO2 and CH4 concentrations, with \(\delta^{13}\)C-CO2 and \(\delta^{13}\)C-CH4 values ranging from \(-\)22.0 (LSW4) to \(-\)9.20 (LSW3)% vs. V-PDB and from \(-\)60.4 (LSW7) to \(-\)33.6 (SPW)% vs. V-PDB, respectively (Table 4). Similar to surface waters, the dissolved gas chemistry along the depth profiles of the OMW1, OMW2 and OSW1 sites was dominated by N\({}_{2}\). However, both CH\({}_{4}\) and CO\({}_{2}\) concentrations showed an increasing trend along the water column at Massaciuccoli Lakeand Cava Sisa. The dissolved CO2 concentration at Massaciuccoli Lake increased from 0.02 mmol/L at the water-atmosphere interface to 0.08 (OMW1) and 0.10 (OMW2) mmol/L in the bottom waters, whereas those of CH4 increased ca. 2.5 times, i.e., from 0.0035 and 0.0031 mmol/L to 0.0088 and 0.0078 mmol/L in OMW1 and OMW2, respectively. At Cava Sisa, the increase in CO2 and CH4 concentrations with depth were up to one and two orders of magnitude from the surface (0.07 and 0.0087 mmol/L, respectively) to the bottom (0.23 and 0.56 mmol/L), respectively, with CH4 becoming the second most abundant gas at \(\geq\)8 m depth. The O2 concentrations showed an opposite trend with respect to CO2 and CH4, progressively decreasing with depth; at OSW, hypoxic conditions were found below 2 m depth, whereas no detectable dissolved O2 was measured at \(\geq\)10 m depth (Table 4). The \(\delta^{13}\)C-CO2 and \(\delta^{13}\)C-CH4 values were relatively stable along the water column at Massaciuccoli Lake, with an overall decrease of \(\leq\)0.2 and \(\leq\)4.6% vs. V-PDB, respectively (Table 4). Conversely, a larger enrichment in lighter carbon isotopes was observed at OSW1, with \(\delta^{13}\)C-CO2 and \(\delta^{13}\)C-CH4 values varying from \(-\)14.6 and \(-\)43.7% vs. V-PDB, respectively, in surface waters to \(-\)19.6 and \(-\)63.4% vs. V-PDB, respectively, in bottom waters (Table 4).