In many estuarine systems, riverine discharge is the main source of dissolved organic matter (DOM). In contrast, karstic Krka River, with self-purifying ability, is characterized by very low DOM concentrations (~30 µM) and a diluting effect on DOM in the estuary. Interestingly, within the estuary a marked in-situ production is observed, particularly in summer. Pristine nature of the estuary makes it a natural laboratory to study the DOM dynamics, originated from 3 sources: the river the seawater and in-situ production. With this work we aim at reporting the first information on DOM dynamics in the Krka estuary in two contrasting seasons, winter and summer 2019. The quality of DOM was assessed using absorbance and fluorescence measurements coupled with PARAFAC analysis. During winter, the estuary is characterized by extremely low DOC concentrations, for which optical properties indicate that is mainly of terrestrial origin. In summer, due to the low riverine discharge and high temperature, DOC concentrations are higher than in winter and a marked increase in absorption and protein-like fluorescence is observed in the estuary. The permanent stratification determines DOC accumulation in the surface layer with values up to 147 µM, suggesting a decoupling between production and removal processes. Our work opens intriguing questions about the main processes responsible for DOC accumulation in this system and highlights the need of new studies combing chemical and biological information.

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The determination of copper (Cu) speciation and its bioavailability in natural waters is an important issue due to its specific role as an essential micronutrient but also a toxic element at elevated concentrations. Here, we report an improved anodic stripping voltammetry (ASV) method for organic Cu speciation, intended to eliminate the important problem of surface-active substances (SAS) interference on the voltammetric signal, hindering measurements in samples with high organic matter concentration. The method relies on the addition of nonionic surfactant Triton-X-100 (T-X-100) at a concentration of 1 mg L−1. T-X-100 competitively inhibits the adsorption of SAS on the Hg electrode, consequently 1) diminishing SAS influence during the deposition step and 2) strongly improving the shape of the stripping Cu peak by eliminating the high background current due to the adsorbed SAS, making the extraction of Cu peak intensities much more convenient. Performed tests revealed that the addition of T-X-100, in the concentration used here, does not have any influence on the determination of Cu complexation parameters and thus is considered "interference-free." The method was tested using fulvic acid as a model of natural organic matter and applied for the determination of Cu speciation in samples collected in the Arno River estuary (Italy) (in spring and summer), characterized by a high dissolved organic carbon (DOC) concentration (up to 5.2 mgC L−1) and anthropogenic Cu input during the tourist season (up to 48 nM of total dissolved Cu). In all the samples, two classes of ligands (denoted as L1 and L2) were determined in concentrations ranging from 3.5 ± 2.9 to 63 ± 4 nM eq Cu for L1 and 17 ± 4 to 104 ± 7 nM eq Cu for L2, with stability constants logKCu,1 = 9.6 ± 0.2–10.8 ± 0.6 and logKCu,2 = 8.2 ± 0.3–9.0 ± 0.3. Different linear relationships between DOC and total ligand concentrations between the two seasons suggest a higher abundance of organic ligands in the DOM pool in spring, which is linked to a higher input of terrestrial humic substances into the estuary. This implies that terrestrial humic substances represent a significant pool of Cu-binding ligands in the Arno River estuary.

14Chromium is a toxic element naturally present in natural waters whose chemical speciation 15 regulates its cycling, mobility and bioavailability. We present here: 1-an improved analytical 16 method for chromium speciation (Cr(VI) vs Cr(III)) in estuarine samples by catalytic 17 adsorptive cathodic stripping voltammetric (cat-AdCSV) and 2-a study highlighting a 18 significant change of redox speciation during summer and winter. Initial measurements first 19 revealed that surface-active substances (SAS) present in estuarine samples strongly influenced 20 the analytical determination of Cr by partially masking the Cr peak through an increase of the 21 background current. We found that the application of a low negative accumulation potential (-22 1.65 V) resulted in much better voltammograms compared to those obtained using the usual 23 accumulation potential of -1.0 V . Using humic acid (HA) as a model SAS of natural origin, 24we show that this negative potential clearly prevents adsorption of SAS on the Hg-electrode 25 surface, which in turns benefits the adsorption of the in-situ formed Cr(III)-DTPA complex 26 and the resulting signal. The optimised method was applied to determine chromium redox 27 speciation and distribution along the 23 km long salinity gradient, well oxygenated, Krka 28 River estuary (Croatia). Cr(VI) was found to be the dominant redox species in both summer 29 and winter, with Cr(III) contribution being lower in summer (up to ~30%, average of ~5%) 30 than in winter (up to ~50%, average of ~30%). In summer, lower concentrations of Cr(VI) 31 were found in the freshwater end-member (2.5 nM) than in the seawater end-member (4-5 32 nM), while the opposite trend was found in winter. Hexavalent chromium exhibited a non-33 conservative behaviour along the salinity gradient for both seasons. Chromium predominantly 34 exists in dissolved phase, and contribution of particles reactive Cr(III) was minor. 35

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