## 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  
River discharge can seriously affect the occurrence and distribution of trace elements as well as their partitioning between solid and dissolved phases in estuarine environments. Moreover, trace element distribution in estuaries can be notably affected by sediment resuspension events [19; 60; 61] which control both the movement and redistribution of particles in shallow aquatic systems playing a crucial role in regulating trace element mobility and bioavailability [22]. In this research, a 3W-PCA (59.4% of explained variance) was used as an additional chemometric tool to highlight the effects of different seasonal conditions of river flow (spring, summer and autumn) on the behaviour of dissolved and particulate variables (Hg, MeHg, Fe, Mn, DOC and POC) as well as the physico-chemical parameters along the Nalon River estuary (Figure 3). The seven investigated sites along the Nalon River estuary are clearly represented by the multivariate model and their position in the 3W-PCA output appears to be regulated by salinity which varies along Axis 2 as seen in Figure 3.  
Notable differences in the dissolved and particulate concentrations were not observed among the seven investigated sites along the Nalon River estuary (Figures 4 and 5, Table S2). This is also evident in the 3W-PCA (Figure 3) output and most likely due to the fact that the water column was generally vertically well-mixed at each site as the result of the relatively shallow water depth along the river axis. Moreover, the 3W-PCA output also clearly denotes the difference among the three levels of river flow during the year and especially between summer and autumn (Figure 3). Generally, concentrations of DHg (4.02 \(\pm\) 1.33 ng L\({}^{-1}\)), DMeHg (18.1 \(\pm\) 6.8 pg L\({}^{-1}\)), DFe (7.56 \(\pm\) 2.39 \(\mu\)g L\({}^{-1}\)), DMn (2.86 \(\pm\) 0.62 \(\mu\)g L\({}^{-1}\)) and DOC (0.64 \(\pm\) 0.18 mg L\({}^{-1}\)) prevailed in summer under conditions of low discharge. Conversely, they reached minimum values during high river discharge in autumn (DHg = 2.13 \(\pm\) 0.70 ng L\({}^{-1}\), DMeHg = 5.69 \(\pm\) 2.10 pg L\({}^{-1}\), DFe = 2.50 \(\pm\) 0.63 \(\mu\)g L\({}^{-1}\), DMn = 0.81 \(\pm\) 0.27 \(\mu\)g L\({}^{-1}\), DOC = 0.37 \(\pm\) 0.08 mg L\({}^{-1}\)) most likely due to dilution effects related to higher river discharge (Figure S2). Dissolved Hg was found to be one order of magnitude lower compared to other aquatic systems in the Iberian Peninsula such as the Tagus estuary (3.61-65.4 ng L\({}^{-1}\), [14]), the Portuguese coast (1.00-28.1 ng L\({}^{-1}\), [58]), the Tinto estuary (1.66-19.8 ng L\({}^{-1}\), [1]) and the Odiel estuary (3.29-40.0 ng L\({}^{-1}\), [1]). In contrast, slightly lower concentrations of DHg were observed in the Gulf of Cadiz (0.42-0.76 ng L\({}^{-1}\), [1]). The most toxic and bioavailable DMeHg reached concentrations in the range between 2.48 and 34.5 pg L\({}^{-1}\) which were higher than those measured in the Tagus estuary (0.06-6.90 pg L\({}^{-1}\), [14]) but notably lower if compared to other estuarine systems affected by the legacy of mining activities, such as the Isonzo River mouth (Gulf of Trieste, Italy, <LOD-1323 pg L\({}^{-1}\), [56]), or chlor-alkali plants as in the case of the Wuli estuary and Jinzhou Bay in NE China (640 and 45-280 pg L\({}^{-1}\), respectively, [62]). Figure 3: Plot summarising the results of 3W-PCA performed on dissolved and particulate Hg, MeHg, Fe and Mn as well as DOC, POC and physico-chemical parameters measured in the water column along the Nalon River estuary under different seasonal conditions (spring, summer and autumn). Figure 4: Distribution of DHg, DMeHg, DFe, DMn and DOC 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).