Polycyclic aromatic hydrocarbons, toxic and potentially toxic elements in Pelophylax esculentus complex embryos from the UNESCO-Protected Drava River Floodplain (Croatia)

River floodplains are dynamic ecosystems that trap and store pollutants transported through riverine processes, atmospheric deposition, and adjacent land use. Their role as sediment and nutrient retention zones also promotes the accumulation of persistent organic pollutants and (potentially) toxic elements. Amphibians, whose eggs and larvae develop directly in aquatic environments, are highly sensitive to such pollution and are therefore widely used as bioindicators of freshwater ecosystem health. This study provides a preliminary assessment of 16 priority polycyclic aromatic hydrocarbons (PAHs) and toxic and potentially toxic elements (arsenic [As], selenium [Se], cadmium [Cd], lead [Pb], mercury [Hg], and chromium [Cr]) in embryos of the European green frogs (Pelophylax esculentus complex) collected from the UNESCO-protected floodplain area of Drava River in continental Croatia. Several PAHs were detected in all embryo samples, including naphthalene, acenaphthene, acenaphthylene, fluorene, phenanthrene, fluoranthene, and pyrene, with concentrations ranging 0.012-23.22 µg/kg. When grouped by molecular weight, low-molecular-weight PAHs dominated the overall pollution profile, whereas high-molecular-weight compounds were detected sporadically. Diagnostic PAH ratios suggested mixed pollution sources, with patterns consistent with both petrogenic and pyrogenic inputs. In contrast, most analysed elements (As, Cd, Pb, and Hg) remained below limit of detection (LOD) in all samples. Only Se and Cr were quantified, with concentrations ranging 12.52-17.41 µg/kg and 10.35-12.00 µg/kg, respectively. Overall, the results indicate that amphibian embryos in the floodplain area of the Drava River are exposed to detectable levels of PAHs, while element concentrations appear low. These findings provide initial insight into pollution exposure in early amphibian life stages and highlight the need for further monitoring and expanded ecological risk assessments in floodplain ecosystems.