## 4 Discussion
### CH\({}_{4}\) Emission Drivers at Massaciuccoli Lake System  
The chemical features of the waters collected from Massaciuccoli Lake system resulted from multiple factors, including: (i) chemical variability of the feeding waters, (ii) direct mixing with seawater, (iii) anthropogenic inputs from urban, agricultural and industrial areas and (iv) biogeochemical processes exacerbated by eutrophication, e.g., [47; 48; 87]. The water isotope signature evidenced that most waters were significantly affected by isotope fractionation due to evaporation processes, except those less exposed to solar radiation, i.e., from the Case Rosse spring (SPW) and most tributary channels (TCW) (Figure 6). Figure 4: \(\Phi\)CH\({}_{4}\) (in g m\({}^{-2}\) day\({}^{-1}\)) values from different aquatic systems (rivers and streams, lakes and reservoirs) as reported in [PERSON] et al. [17] compared with those measured at Porta and Massaciuccoli Lakes (present study). Figure 5: \(\Phi\)CO\({}_{2}\) vs. \(\Phi\)CH\({}_{4}\) (in g m\({}^{-2}\) day\({}^{-1}\)) binary diagrams for samples collected from (**a**) Porta. Lake, with symbols as in Figure 1a) and (**b**) Massaciuccoli Lake system. Symbols as in Figure 1b. The water springs in the area included both Ca\({}^{2+}\)-HCO\({}_{3}\)\({}^{-}\) waters, e.g., the SPW sample collected from the Case Rosse spring, and Ca\({}^{2+}\)-SO\({}_{4}\)\({}^{-}\) waters, such as the one (named Villa Spinola), which feeds the Quiesa channel (TCW4). However, the chemical composition of the waters collected from Massaciuccoli Lake system was largely dominated by Na\({}^{+}\) and Cl\({}^{+}\). The Na\({}^{+}\)/Cl\({}^{-}\) mass ratios ranged from 0.50 to 0.69, suggesting the direct influence of seawater (Na\({}^{+}\)/Cl\({}^{-}\) mass ratio = 0.56) in agreement with [PERSON] [47]). Accordingly, the TDS values progressively increased approaching the coastline, the lowest values being measured in the TCW samples and the highest ones in those from ECW, including the Burlamacca canal. Coherently, the average Cl\({}^{-}\)/Br\({}^{-}\) mass ratio value in the collected samples (308) was close to that of seawater (292). Nevertheless, higher Cl\({}^{-}\)/Br\({}^{-}\) mass ratio values (\(\geq\)384) were occasionally observed, especially in samples from Cava Sisa (LSW1, LSW2, LSW3, LSW4 and OSW2) and TCW1, likely related to anthropogenic inputs and/or Br assimilation processes by green plants and phytoplankton [47] and references therein. In fact, biogeochemical processes shaped the chemical features of waters and dissolved gases within the lake, as observed along the OSW1 vertical profile where decomposition processes at the lake bottom produced a chemokine at around 10 m depth, separating the anoxic and NH\({}_{4}\)-rich waters from the shallow levels. The variations of the \(\delta^{13}\)C-TDIC values along the water column supported the occurrence of decomposition processes at the lake bottom, with a \({}^{12}\)C-enriched TDIC, whereas enrichments in the heavier \({}^{13}\)C isotope were observed in shallow waters due to photosynthetic activity and exchanges with the atmosphere. Coherently, the \(\delta^{13}\)C-CO\({}_{2}\) values in shallow waters along the OSW1 vertical profile were characterized by a \({}^{13}\)C-enrichment with respect to bottom waters. Methanogenesis occurring in bottom waters and lake sediments produced an increase in dissolved CH\({}_{4}\) concentrations enriched in the lighter carbon isotope downwards the water column. Different from Porta Lake, no clear relationship was recognized between dissolved O\({}_{2}\) contents and water column depth (Figure 7a), likely due to the wide variability of physicochemical features encountered in Massaciuccoli Lake system, suggesting that depth was not sufficient to predict the mixing degree of the water column. Similarly, both  
Figure 6: \(\delta\)D-H\({}_{2}\)O vs. \(\delta^{18}\)O-H\({}_{2}\)O (in% vs. V-SMOW) binary diagram for water samples collected from Massaciuccoli Lake. Symbols as in Figure 1b.