## 3 Results and Discussion
### Suspended Particulate Matter: Distribution, Grain Size and Sources  
The maximum values of SPM were found in autumn, when the river discharge was highest (7.64 \(\pm\) 3.05 mg L\({}^{-1}\)) whereas lower values were observed during the sampling campaigns performed under conditions of low discharge both in spring (3.48 \(\pm\) 0.95 mg L\({}^{-1}\)) and summer (2.81 \(\pm\) 0.68 mg L\({}^{-1}\)) (Table S1). At the seven investigated sites along the Nalon River estuary, notable differences in the SPM values were not observed with increasing depth, suggesting that the water column was vertically well-mixed most likely due to the shallow depth of the water. This was consistent with the salinity distribution and confirmed by the surface-bottom SPM ratios which were generally low (0.52-0.88, 0.60-1.34 and 0.36-0.92 in spring, summer and autumn, respectively). The SPM grain size spectra did not notably change among the three different sampling campaigns, whereas the SPM was found to have a different grain size distribution along the Nalon River estuary (Figure S1). At the estuary mouth (site 0) as well as in the lower and central sectors (sites 1, 2, 3 and 4) surface SPM samples were skewed and sorted whereas a poorly sorted distribution was usually observed at the bottom testifying to the marine origin of the suspended particles. The only exception is represented by site 3 where skewed and sorted spectra were observed throughout the water column most likely due to low hydrodynamics at this site where a clear dominance of riverine or marine particles did not appear to occur. Conversely, the SPM samples collected in the upper sector of the estuary (sites 5 and 6) appeared to be poorly sorted both in the surface layer and at the bottom. There, the grain size distribution and selection appeared to be inhibited by the shallow water depth and the absence of a relevant freshwater flow. Marked differences in PHg, PFe and PMn concentrations along the water column were not generally observed at the different investigated sites suggesting that mixing processes between different water masses as well as resuspension of bottom sediments were the main factors governing trace element distribution along the Nalon River estuary. Particulate Hg was found to be quite constant by comparing the results relating to spring (1.59 \(\pm\) 0.53 \(\upmu\)g g\({}^{-1}\)), summer (1.80 \(\pm\) 0.47 \(\upmu\)g g\({}^{-1}\)) and autumn (1.20 \(\pm\) 0.66 \(\upmu\)g g\({}^{-1}\)) conditions (Table S2). Particulate Hg appeared to be mainly associated with fine particles (2-8 \(\upmu\)m) especially in autumn when a significant correlation was found (N = 17, r = 0.787, \(p\) < 0.01). This is consistent with previous research which highlighted that Hg distribution along the Nalon River estuary was strictly correlated to decreasing grain size leading to Hg accumulation in fine sediments restricted to the port areas of San Esteban de Pravia and San Juan de la Arena [33]. Moreover, the amount of PHg was found to be comparable to the total Hg concentration in the surface sediments ranging between 0.10 and 1.33 \(\upmu\)g g\({}^{-1}\)[33]. From the comparison of several aquatic systems affected by anthropogenic activities it can be seen that the PHg concentrations obtained from this research were one order of magnitude lower than those observed at the Isonzo River mouth (0.09-37.3 \(\upmu\)g g\({}^{-1}\), Gulf of Trieste, Italy [56]; 12.1 \(\upmu\)g g\({}^{-1}\), [57]) and at the estuarine systems along the Portuguese coast (0.80-15.3 \(\upmu\)g g\({}^{-1}\), [58]). Conversely, comparable PHg contents were measured at the Tagus estuary in Portugal (0.36-8.63 \(\upmu\)g g\({}^{-1}\), [14]) as well as at the Tinto (0.06-1.12 \(\upmu\)g g\({}^{-1}\)) and Odiel (0.14-6.52 \(\upmu\)g g\({}^{-1}\)) estuaries along the SW Spanish coast [1]. Particulate MeHg was found to be notably lower with respect to the total amount of Hg (Table S2).