2.2. Analytical methods
All water samples, due to their turbidity, were filtered through 0.7-µm-pore-size glass fiber filters (Whatman GFF, pre-combusted in 400°C, 24 h).. Samples were kept in cool storage (+4°C) and analysed approximately two months since collection time. Specific electric conductivity (SpC), pH and dissolved oxygen were measured in the field with a multiparameter YSI Pro Plus probe with a 4-sport Quattro Cable (11m).
Dissolved organic carbon (DOC) was determined through a total organic carbon (TOC) measurement in filtrate with a TOC-VCSH/CSN analyser (Shimadzu; the method of catalytic oxidation with oxygen at 680°C and nondispersive infrared spectroscopy detection). Calibration was performed with a potassium phthalate standard (Shimadzu), with an R2 = 0.998. CVs of five sample replicates were below 2%. Samples with DOC concentrations above the analytical range were diluted with MilliQ water (Merck Life Sciences). Only a few samples with a high suspended sediment content were analysed for POC (details inSuppl. Mat. 1 ).
Major ions (F-, Cl-, NO2-, Br-, NO3-, PO43-, SO42-, Na+, NH4+, K+, Ca2+ and Mg2+) were determined using Dionex ICS 3000 (Dionex, USA), calibrated with dilutions of a 1000 mg L-1 single-ion standard solutions. Analytical accuracy was checked with the TraceCERT® Multi Anion Standard 1 for IC (69734-100ML) and Multi Cation Standard 1 for IC (91286-100ML) (all solutions by Merck Life Science).
Elemental analysis was performed in acidified (with high-purity nitric acid, Suprapur, Merck Life Science) filtrate using inductively coupled plasma optical emission spectrometry (ICP-OES 9820, Shimadzu, Japan) for Be, P, S and Ti. Inductively coupled plasma mass spectrometry (ICP-MS 2030, Shimadzu, Japan) was applied for Ag, Al, As, Ba, Bi, Cd, Co, Cr, Cu, Fe, Hg, Li, Mn, Mo, Ni, Pb, Sb, Se, Si, Sr, V and Zn. For calibration of the ICP-MS, the ICP IV multielement standard (Merck Life Sciences) and single-element standards of As, Be, Sb, Se, Mo, and V (Sigma-Aldrich, USA) were used. Single-element 1000 mg L−1 standards (Sigma-Aldrich, USA)were applied for the ICP-OES. Internal standards controlling for the MS signal were solutions of Sc, Rh, Tb, and Ge in suprapure 1% HNO3 (Merck). Accuracy of the analysis was checked with Sigma-Aldrich CRMs (certified reference materials) Trace Metals ICP-Sample 1 and Trace Metals ICP-Sample 2, with 98-104% recoveries. High-concentration samples were diluted with Milli-Q water (Direct 8 Purification System Water, Merck Life Sciences, France). All metals, non-metals and metalloids concentrations measured have been corrected for a procedural blank value if it exceeded the method limit of quantification (LOQ); in all other determinations, the blank correction was not required. The details of all measurement conditions and QA/QC parameters, including method LOQ, are given in the Supplementary material (Suppl. Mat. 1, Table S3 ).