## 3 Results and Discussion
### Dissolved and Particulate Hg and MeHg: The Effect of the River Discharge
#### 3.3.1 Occurrence and Distribution of Dissolved and Particulate Hg and MeHg
Figure 5: Distribution of PHg, PMeHg, PFe, PMn and POC in the water column from the upper sector of the estuary of the Nalon River to its mouth (from site 6 to site 0) under different seasonal conditions (spring, summer and autumn). The amount of DMeHg was also found to be significantly low if compared to the total concentration of Hg in the dissolved fraction. Indeed, the percentage of DMeHg was always \(<\)1% (0.07-0.43%, 0.08-0.81%, and 0.15-0.58% in spring, summer and autumn, respectively). This would testify to the minimal risk of MeHg bioaccumulation since the amount of MeHg in the water column can be seen as a strong predictor of its concentration in the biota [63]. Moreover, the bioavailability of MeHg depends on several factors and its dissolved concentration is usually the result of the balance between methylation and demethylation processes [64]. In aquatic systems, most of the MeHg originates from in situ production in the sediment compartment [2; 11; 12; 65] near the SWI [66] and methylation is not simply influenced by the total amount of Hg [67; 68] since several parameters (e.g., temperature, pH, Eh, inorganic and/or organic complexing agents) contribute to MeHg production, mobility and fate [69; 70; 71]. Once released in the water column, the amount of DMeHg also depends on demethylation processes due to sunlight [72] which could be enhanced at the Nalon River estuary where the water depth is generally shallow. Regarding the SPM, the particulate forms of the investigated variables reached the maximum values on average in autumn under higher river discharge (PHg = 8.37 \(\pm\) 4.20 ng L\({}^{-1}\), PFe = 848 \(\pm\) 413 \(\upmu\)g L\({}^{-1}\), PMn = 445 \(\pm\) 175 \(\upmu\)g L\({}^{-1}\), POC = 1.48 \(\pm\) 0.10 mg L\({}^{-1}\)) and were found to be notably lower and quite constant in summer (PHg = 5.05 \(\pm\) 1.84 ng L\({}^{-1}\), PFe = 195 \(\pm\) 83 \(\upmu\)g L\({}^{-1}\), PMn = 131 \(\pm\) 54 \(\upmu\)g L\({}^{-1}\), POC = 0.40 \(\pm\) 0.10 mg L\({}^{-1}\)) (Figure S2 and Table S2). The only exception is represented by PMeHg which appeared to be mostly present in the saltwater in the lower sector of the estuary as well as at the estuary mouth, especially in summer under conditions of low river discharge (25.8 \(\pm\) 19.1 pg L\({}^{-1}\)) rather than in autumn (18.2 \(\pm\) 11.1 pg L\({}^{-1}\)) when the river flow was highest (Figure 3, Figure 5 and Figure S2). In this context, the surface sediments can be considered a secondary source of MeHg to the overlying water column, most likely due to sediment resuspension induced by natural and/or anthropogenic events [22; 37; 64; 68; 73]. Indeed, it has been demonstrated that the mobilisation of sediment particles during a tidal resuspension event can notably affect the PMeHg concentrations in the water column [74]. In the case of the Nalon River, the tidal currents prevailing under conditions of low discharge could be responsible for perturbed conditions at the bottom of the estuary mouth. This could enhance the resuspension of fine particles which may be a valid explanation for the highest values of PMeHg in the lower sector of the estuary. Although slightly lower values were observed during resuspension experiments, it has been demonstrated that PMeHg concentrations sharply decreased after 24 h [37]. In addition, [PERSON] et al. [22] stated that the dynamics between the dissolved and the particulate phases of Hg and MeHg during resuspension experiments also depended on the type of particles involved which can accumulate and may or may not release Hg and MeHg. Moreover, as previously mentioned for the dissolved fraction, the percentages of PMeHg with respect to the total amount of PHg were generally low (0.12-0.65%, 0.09-1.56% and 0.61-3.47% in spring, summer and autumn, respectively) and did not seem to represent a relevant environmental issue in terms of potential MeHg bioaccumulation.