## 3 Results and Discussion  
### Physico-Chemical Parameters of the Water Column  
The Nalon River discharge was found to be quite different among the three sampling campaigns: the highest river flow was observed in October (autumn, 69.0 m\({}^{3}\) s\({}^{-1}\)) followed by the sampling performed in April (spring, 34.8 m\({}^{3}\) s\({}^{-1}\)) and July (summer, 21.6 m\({}^{3}\) s\({}^{-1}\)). Salinity distribution varied slightly among different seasonal conditions of river discharge as well as along the whole estuary (Figure 2). Indeed, variations in the vertical salinity gradient were limited and the estuary appeared to be dominated by partially mixed estuarine circulation. In this context, conditions of turbulent mixing between different water masses as well as the effect of strong tidal currents at the bottom may inhibit vertical salinity stratification [55]. Moreover, the interaction among the wave motion, tidal currents and river inputs represented the main factors regulating the energy pattern in this estuary [33, 41] and the mixing zone between freshwater and saltwater reached a different extension along the river axis. The maximum extension of saltwater (at the bottom of site 3) was observed in summer, when the river discharge was lowest. When the river flow became higher, the extension of the mixing zone increased and brackish water dominated throughout the water column along the Nalon River estuary (from site 1 to site 4). In autumn, when the river discharge was highest, the extension of the mixing zone was comparable to that observed in spring with the only difference being that river freshwater dominated the surface water layer at site 3 and slightly lower values of salinity were observed at a higher depth at site 3 (7.14 \(\pm\) 0.93) and at site 4 (8.89 \(\pm\) 1.65). Commonly, saltwater prevailed at the estuary mouth (site 0) and in the lower estuary (sites 1 and 2) especially at the bottom. The central sector of the estuary (sites 3 and 4) was usually dominated by the occurrence of brackish water whereas the upper estuary was characterised by river freshwater (sites 5 and 6) (Figure 2). Saltwater always prevailed at the estuary mouth (site 0, always >31) and at site 1 under conditions of low discharge observed in summer (avg. 31.1 \(\pm\) 0.9) whereas brackish water was found in the surface layer in spring (25.6) and autumn (18.6) when the river flow was highest. The occurrence of brackish water along the water column was evident at site 2 (avg. 23.5 \(\pm\) 6.8, 25.5 \(\pm\) 10.6, and 21.7 \(\pm\) 8.7 in spring, summer and autumn, respectively) and site 3 in summer (avg. 20.5 \(\pm\) 8.8) and spring (avg. 18.6 \(\pm\) 1.5) whereas freshwater prevailed in the surface layer in autumn (0.49) where salinity increased with depth. Similar conditions were also evident at site 4 where brackish water was observed at the intermediate depth whereas freshwater always occurred in the surface layer (avg. 0.26 \(\pm\) 0.03). The upper sector of the estuary (sites 5 and 6) was always dominated by freshwater along the water column showing an average salinity of 0.21 \(\pm\) 0.02 during the year. Notable disparities in temperature were not observed although the bottom water displayed slightly lower values (13.7 \(\pm\) 0.5, 16.6 \(\pm\) 0.5, 12.7 \(\pm\) 0.4 \({}^{\circ}\)C in spring, summer and autumn, respectively) compared to those found in the surface layer (14.1 \(\pm\) 0.6, 18.4 \(\pm\) 0.6, and 13.8 \(\pm\) 0.3 \({}^{\circ}\)C in spring, summer and autumn, respectively) especially in July (Table S1). Slight variations in pH were measured among the different sampling campaigns  
Figure 2: Schematic areal distribution of salinity measured along the water column at the seven investigated sites along the Nalon River estuary under different seasonal conditions (spring, summer, autumn).