## 3 Results and Discussion
### Variation in Size Distribution between Stratified Layers  
For most metals (Cd, Pb, Ni, Mn, Fe, Al), an increase of the truly dissolved fraction from FWL (M1 and M2) to SWL (M5 and M6) was observed (Figure 2). For Cd, an increase of the truly dissolved species with salinity is usually ascribed to the competitive effect of chloride ions causing its release from particles [8; 11]. For Pb, Mn, Fe and Al, an increase of the \(>\)5 \(\upmu\)m size fraction was observed in the FSI (M2 and M3), indicating a close association of these metals. A transfer of metals from the low size fractions into particles in the FSI could occur via colloidal aggregation [5]. Fe and Mn may be the main carriers of Pb, and the behavior of Pb in the salinity gradient may be controlled to a significant extent by their aggregation in the mid-salinity region [6; 24], indicated here by a decrease of their dissolved and truly dissolved concentrations and an increase of abundance of their high size fractions (Figure 2, Supplementary Materials Figures S5 and S6). This is consistent with a previous study that showed that the Pb distribution in the Krka estuary might be controlled by Mn and Fe particulate forms [8; 26]. Fe and Mn oxides are known scavengers of various risk elements, such as Pb here. However, while cleaning the water column, they settle in the sediment and can increase the enrichment factors of these elements, which could finally mask or distort the anthropogenic contamination signals in sediment composition. In the SWL, the particulate fractions of Pb, Mn, Fe and Al again decreased, suggesting that low size metal species are being released in response to particle desorption and the defoloculation of colloids at high salinity; a similar behavior was observed for Mn, Fe and Al in the San Francisco Bay [5]. In contrast to other metals, Zn showed an increase of the truly dissolved fraction in the FSI, whereas in the SWL, a decrease of the truly dissolved fraction along with an increase of the highest size fraction (\(>\)5 \(\upmu\)m) was prominent. A significant decrease of the dissolved Zn fraction at high salinity was observed also by [PERSON] et al., 2013 [14] in a small Mediterranean river mouth. [PERSON] et al., 2013 [14] also observed a release of Co from particles in the mixing zone, and the same was found in the Hudson River estuary, San Francisco Bay [43] and two Japanese estuaries [44]. In the Krka estuary, Co showed a uniform distribution of its truly dissolved fraction and a decrease of the particulate fraction with salinity (21-13%), coupled with an increase of the colloidal fraction (13-19%). A much higher increase of colloidal Cd with salinity was observed in the Gulf of Trieste [37]. Finally, Cu was the only metal that showed an increase of the contribution of colloids to its dissolved fraction in the FSI. This could be due to the binding \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Zn** & **Cd** & **Pb** & **Cu** & **Ni** & **Co** & **Mn** & **Fe** & **Al** \\ \hline \multirow{3}{*}{15\%} & \multirow{3}{*}{9\%} & (A) Average \% of particulate fraction in total fraction & & & & \\  & & & 11\% & 7\% & 17\% & 42\% & 84\% & 75\% \\ \cline{4-7}  & & & (B) Average \% of colloidal fraction in total fraction & & & \\
10\% & 8\% & 17\% & 29\% & 8\% & 15\% & 2\% & 6\% & 7\% \\ \cline{4-7}  & & & (C) Average \% of colloidal fraction in dissolved fraction & & & \\
12\% & 9\% & 37\% & 32\% & 9\% & 18\% & 4\% & 37\% & 31\% \\ \hline \end{tabular}
\end{table} 
Table 1: Average percentages of (A) particulate (\(>\)0.1 \(\upmu\)m) and (B) colloidal (3 kDa–0.1 \(\upmu\)m) trace metals in the total metal pool (unfiltered sample) and (C) percentages of colloidal trace metals in the dissolved metal pool (\(<\)0.1 \(\upmu\)m). with biologically-produced organic ligands, such as colloidally-associated thiols or other cluster forming sulfide complexes [10; 45], since, according to previous studies, the highest biological activity in the estuary is associated with the FSI [32; 46; 47].