## 2 Materials and Methods
During the measurement, a nitrogen blanket was maintained over the sample. Samples were added to the electrochemical cell by the in-house constructed autosampler, and the addition of standards was conducted by an automated burette system (Cavro XE 1000 or XL 3000 syringe pumps, Tecan, San Jose, CA, USA). The purpose-developed VoltAA (Voltammetric AutoAnalyser) software was used for the preparation of the project file for the GPES software and the data handling after the automated analysis. Concentrations of Cu, Cd, Pb, Zn, Ni and Co in filtered and unfiltered samples were determined by means of the standard addition (\(n=4\)) method, using stripping voltammetry in differential pulse mode: anodic stripping voltammetry (DP-ASV) for Zn, Cd, Pb, and Cu, and adsorptive cathodic stripping voltammetry (DP-AdCSV) with Dimethylglyoxime as a complex ligand for Ni and Co. The analysis was validated by repeated measurements (\(n=7\)) of the UV-irradiated CASS-5. The determined metal concentrations (\(\pm\)standard deviation) were within the certified limits. A magnetic sector field high resolution inductively coupled plasma mass spectrometer (HR-ICP-MS; Element 2, Thermo Scientific, Bremen, Germany) was used for multielemental analysis. Samples were diluted 10 times with 2% \(v/v\) HNO\({}_{3}\)\(s.p.\) to minimize the matrix effect linked to the presence of salt. Indium at a concentration of 10 \(\mu\)g L\({}^{-1}\), was used as an internal standard. The concentrations of elements were determined by means of using matrix matching (10\(\times\) diluted UV-irradiated seawater) external calibrations. External calibration standards were prepared by dilution of the multi-element standard (NIST traceable) in 2% \(v/v\) HNO\({}_{3}\)\(s.p.\) at the following concentrations: 0, 0.1, 1 and 10 \(\mu\)g L\({}^{-1}\). CASS-5 was analyzed multiple times during the one sequence (\(n=8\)). The obtained trace element concentrations (\(\pm\)standard deviation) were within the certified limits. The measured trace metal concentrations in the unfiltered samples represent acid-leachable (quasi-total) metal concentrations, and within the text, they are referred to as total metal concentrations. Particulate and dissolved fractions are usually separated using 0.2 \(\mu\)m pore size filters, and for the separation of colloidal and truly dissolved fractions, membranes of nominal weight between 1-10 kDa are most frequently used [6; 21; 22; 24; 37]. In this work, the fraction >0.1 \(\upmu\)m was considered as particulate and <3 kDa as truly dissolved, making the 3 kDa-0.1 \(\upmu\)m size range a colloidal fraction. However, colloids are often components of complexed aggregates and can occur in sizes up to 1 \(\upmu\)m. Therefore, colloids in the range between 0.1 \(\upmu\)m and 1.2 \(\upmu\)m were also considered, as well as the two additional dimensions of particulate fractions, including particles in the 1.2 \(\upmu\)m-5 \(\upmu\)m range and particles larger than 5 \(\upmu\)m.  
In two sampling campaigns (February and July 2019), an additional fluorescence analysis of the dissolved organic matter was performed. The results were already published as a part of the study of organic matter dynamics in the estuary [30], and data from the studied site are here used for the correlation with metal concentrations in the dissolved phase. The procedure of fluorescence analysis is described in detail in [PERSON] et al., 2020 [30]. Briefly, fluorescence excitation-emission matrices were recorded using the Aqualog (Horiba-Jobin Yvon, Piscataway, NJ, USA) spectrofluorometer in a 1 \(\times\) 1 cm\({}^{2}\) quartz cuvette. The scan was performed in the excitation wavelength range of 250-450 nm, with 5 nm increments, and the emission wavelength range of 220-620 nm, with 3 nm increments. The fluorescence intensity was normalized to Raman units (R.U.) using the daily-measured Raman peak of MilliQ water (\(\lambda_{\text{ex}}\) = 350 nm, \(\lambda_{\text{em}}\) = 371-428 nm).