Ana-Marija Cindrić

We investigated trace elements, dissolved organic matter (DOM) and Cu organic speciation in estuarine surface microlayer (SML) over one day, during dry and wet season. We identified particulate material as important contributor to trace metal enrichment and diel variability in the SML (in following order: Fe, Al, Pb, Mn, Zn, Co, Cu, Cr), with indications of particle size dependence. In the dissolved phase, DOM composition influenced enrichment of Cu, Fe, Mn, Pb, Co and Ni. Pronounced diel variability was observed in the SML but not in the underlaying water. Short-term changes in SML seemed jointly regulated by solar irradiation, microbial activity and surface stability. Seasonally, increased riverine input enhanced the contribution of terrestrial DOM and the enrichment of particle-associated trace elements. Bubble scavenging strongly modified the transfer of organic matter and trace elements to the SML, but it did not act as uniformly enriching process. The effect of bubbling was both positive and negative depending on the composition and stability of the SML. Cu-binding ligands were enriched together with DOM, with humic ligands playing an important role in Cu complexation and maintaining free Cu below toxic levels despite pronounced Cu enrichment. Overall, trace-element dynamics in the estuarine SML were controlled by the combined effects of particle reactivity, DOM abundance and composition, riverine input, solar irradiation, microbial activity and bubble scavenging. This strong diel variability demonstrates that sampling time is a critical factor when assessing trace-element and DOM enrichment in the SML.

Rare earth elements and yttrium (REY) are emerging contaminants of concern due to their widespread use in modern technologies, persistence, and unknown ecological effects. This study presents the first assessment of REY in seawater and sediments of Pula Bay, Croatia, a semi-enclosed, industrialized coastal system. Surface seawater and sediment samples were analyzed using ICP-MS, following optimized preconcentration and digestion protocols. PAAS-normalized REY patterns and λ Polynomial Modelling identified natural and anthropogenic fractionation signatures. Dissolved ΣREE in seawater ranged from 17.6 to 45.9 ng L⁻¹, with naturally elevated concentrations from continental runoff and evidence of anthropogenic Gd (up to 33%) linked to sewage outputs. Sediment ΣREE concentrations varied from 134.8 to 218.2 mg kg⁻¹, with spatial variation reflecting terrigenous and anthropogenic inputs. Local MREE_PAAS and HREE_PAAS enrichment associated with industrial and municipal inputs distinguished anthropogenic contributions from the lithogenic background. While seawater remains largely unaffected, pollution and risk assessments indicate moderate to high sediment contamination by MREEs and HREEs, showing potential concern for benthic organisms near industrial and urban hotspots. These findings highlight the combined influence of natural and anthropogenic processes on REY distribution in coastal systems and underscore the need for further studies of potential REY effects in impacted coastal environments.

In this study, we examined for the first time the spatio-temporal distribution of trace metal (TM) contaminants (Zn, Cd, Pb, Cu, Ni and Co) in the seawater column of Pula Bay. The bay has been known for decades as one of the most industrialized regions on the Croatian side of the Adriatic. Water samples were collected at 20 sites (at two depths) in four different seasons. The main physico-chemical parameters and DOC were measured along the TMs. The spatial distribution clearly showed that areas with industrial and nautical activities are sources of Zn, Pb and Cu, while no increase was observed for Cd, Ni and Co. Compared to the reference area outside the bay, the increase in dissolved concentrations ranged from a factor of 1.1 for Ni and Co to 8.5 for Pb. A clear difference in TM concentrations was observed between seasons, with concentrations being higher in warmer periods than in colder periods. The potential bioavailability/toxicity of TMs was examined using a passive sampling technique: diffusive gradients in thin films (DGT). In addition, a single-factor pollution index based on the available EQSs was used for both the dissolved TMs and DGT to assess the potential risk to the environment.

To evaluate the level of contamination and predict the potential toxicity risk, selected metal concentrations (Cd, Pb, Cr, Mn, Co, Ni, Cu, Zn, and As) were determined in 40 surface sediment samples from the stratified karstic Krka River estuary (Croatia). In addition, diffusive gradients in thin films (DGT) probes were deployed in situ to understand the mobilization mechanisms and bioavailability of metals in the sediment. The results show significant spatial differences between the upper and lower estuary, with the latter being more affected by anthropogenic pollution. The pollution assessment using the enrichment factor (EF), the geoaccumulation index (Igeo), and the pollution load index (PLI) showed a strong enrichment of metals in the lower part of the estuary, especially of Mn, Cu, Zn, Pb, and As. The statistical analysis (PCA) revealed the former ferromanganese factory and the port as major sources of pollution in the area. Nickel, Co, and Cr, although slightly elevated, may be attributed to the natural origin. The metal mobility in the estuarine sediment was primarily governed by early diagenetic processes (aerobic organic matter mineralization, Fe and Mn oxyhydroxide reduction), which caused the release of metals from the sediment into the pore water and subsequently into the overlying water column.

A size partitioning of several trace metals (Zn, Cd, Pb, Cu, Ni, Co, Mn, Fe and Al) between five size fractions (5 µm) was studied in the vertical salinity gradient of the highly stratified Krka River estuary. The results indicated a dominant river source for Zn, Co, Mn, Fe and Al and a diluting effect on Cd, Pb and Ni. The truly dissolved fraction (5 µm particles. Pb, Mn, Fe and Al were closely related, showing a precipitation and colloidal aggregation in the surface layers and dissolution in the seawater layer. The highest percentage (30–37%) of colloids (3 kDa–0.1 µm) in the dissolved pool was found for Pb, Cu, Fe and Al. Differences in size distribution between low and high river flow periods revealed that Zn, Pb, Co, Mn, Fe and Al are introduced by the river mostly in the 3 kDa–5 µm size range. Therefore, a low percentage of colloidally bound metals compared to other coastal areas can be explained by a limited riverine input of terrigenous material, characteristic for this estuary. Correlation with PARAFAC components revealed associations of Cu with protein-like substances and Co with humic-like substances. The accumulation of Cu at the freshwater-seawater interface coupled with an increase of its colloidal fraction was observed, apparently governed by biologically produced organic ligands.

The abundance and distribution of dissolved Re (DRe) were determined in the freshwater part of the Krka River (Croatia), which drains a karst landscape, and in the salinity gradient of its highly stratified estuary. Due to the low DRe concentration, a batch procedure consisting of a pre-concentration step using an anion exchange resin (Dowex) and analysis of DRe in 8 M HNO₃ eluate using high-resolution inductively coupled plasma mass spectrometry (HR ICP-MS) was applied. Due to potentially inconsistent recoveries, which ranged from 60 to 87%, quantification was performed using the isotope dilution technique (ID). DRe concentrations in the Krka River increased downstream, from 6.2 pM at the spring site to 11.9 pM upstream of the estuary region. Weathering of the surrounding carbonate lithology is assumed to be the source of the natural Re. Two specific anomalies were registered: a strong increase in DRe concentration due to anthropogenic input near the town of Knin (27.5 pM) and a decrease at a downstream site caused by subsurface input of freshwater from the Zrmanja River, resulting in a relatively low DRe concentration (8.5 pM). In the estuarine region, a near-conservative behavior of DRe was found in the salinity gradient of the upper surface layer, with DRe concentrations ranging from 18 to 38 pM. Anthropogenic input was suspected within the estuarine segment near the urban area, causing a small positive deviation from the conservative line. In the bottom seawater layer, a minor decrease in DRe concentration in the most upstream estuarine regions was apparent, implying weak scavenging of Re.

Commercially important bivalve Noah’s Ark shell (Arca noae Linnaeus, 1758) represents a high-quality seafood product, but the data on levels of metal contaminants that could pose a human health risk and also on some essential elements that are important for health protection are lacking. This study examined the concentrations of Cd, Pb, Cr, Ni, Cu, Co, and Zn in the soft tissue of A. noae from harvesting area in the central Adriatic Sea, to survey whether heavy metals are within the acceptable limits for public health and whether tourism could have an impact on them. The concentrations of analysed metals varied for Cd: 0.15–0.74, Pb: 0.06–0.26, Cr: 0.11–0.34, Ni: 0.09–0.22, Cu: 0.65–1.95, Co: 0.04–0.09, and Zn: 18.3–74.7 mg/kg wet weight. These levels were lower than the permissible limits for safe consummation of seafood, and only for Cd, some precautions should be taken into account if older shellfish were consumed. Increase of Cd, Cr, and Cu in shell tissue was observed during the tourist season at the site closest to the marine traffic routes, indicating that metal levels in shellfish tissue should be monitored especially carefully during the peak tourist season to prevent eventual toxic effects due to increased intake of metals, specifically of Cd.