## 4 Conclusions  
This study was aimed at quantitatively assessing the size distribution of various trace metals (Zn, Cd, Pb, Cu, Ni, Co, Mn, Fe and Al) in the vertical salinity gradient of the highly stratified Krka River estuary, which is characterized by a low riverine input of trace elements, suspended particulate matter and organic carbon. The particularity of this study was the distribution of size fractions, which spanned ~5 orders of magnitude ranging from >5 um to <3 kDa. Overall, the dominant river source was indicated for Zn, Co, Mn, Fe and Al, while the river showed a diluting effect on the concentration of Cd, Pb and Ni in the estuary. The truly dissolved fraction (<3 kDa) dominated the Zn (75%), Cd (83%), Cu (60%), Ni (85%), Co (68%) and Mn (56%) fractionation, while Pb, Fe and Al were mainly present in a particulate form larger than 0.1 um (56, 85 and 75%, respectively). Although the truly dissolved fraction dominated the Mn fractionation, the fraction between 3 kDa and 5 um accounted for less than 10% on average, indicating the preferential formation of larger Mn particles under estuarine conditions. Colloids, as a subgroup of the dissolved fraction, represented a small absolute fraction for all elements. However, their contribution to the dissolved pool was relatively high for Pb (37%), Cu (32%), Fe (37%) and Al (31%), indicating a significant contribution of high molecular weight forms. High molecular weight forms can be formed by biologically-produced organic ligands (such as reduced sulfur species), which was evident for Cu that correlated well with protein-like organic compounds. Interestingly, Co was the only metal that showed a strong association with humic-like substances in the dissolved phase. The comparison with other studies showed inconsistent results, suggesting that various environmental factors, such as chemical composition and the flux of organic matter and suspended particles, play a crucial role in defining the fractionation of trace metals in estuarine systems. In order to qualitatively resolve the observed quantitative distributions of size fractionation/species and fate of trace metals in the Krka River estuary, a more focused and detailed study encompassing the entire horizontal estuarine transect is required. **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/app12125816/s1](https://www.mdpi.com/article/10.3390/app12125816/s1)]([https://www.mdpi.com/article/10.3390/app12125816/s1](https://www.mdpi.com/article/10.3390/app12125816/s1)), Figure S1: Vertical salinity profiles during the time of the samplings, Figure S2: Concentrations of studied trace metals in the total fraction (unfiltered sample) in relation to salinity, Figure S3: Concentrations of studied trace metals in <5 uM fraction in relation to salinity, Figure S4: Concentrations of studied trace metals in <1.2 uM fraction in relation to salinity, Figure S5: Concentrations of studied trace metals in <0.1 uM fraction in relation to salinity, Figure S6. Concentrations of studied trace metals in truly dissolved fraction (<3 kDa) in relation to salinity, Figure S7: Average percentages of studied trace metals (samplings with FWL characterised by salinity <25) in each size fraction at each depth (M1-M6); Figure S8. Average percentages of studied trace metals (samplings with FWL characterised by salinity >25) in each size fraction at each depth (M1-M6). Table S1: Percentage of colloidal metal fraction in the dissolved phase - comparison between various coastal areas. **Author Contributions:** Conceptualization, S.M. and D.O. ; methodology, A.M.C. and J.P.; validation, A.M.C. and J.P.; formal analysis, S.M., A.M.C. and J.P.; investigation, S.M., A.M.C. and J.P.; resources, D.O. ; data curation, S.M. ; writing--original draft preparation, S.M. ; writing--review and editing, S.M., A.M.C., J.P. and D.O. ; visualization, S.M. and D.O. ; supervision, D.O. ; project administration, D.O. ; funding acquisition, D.O.