Thallium accumulation in different organisms from karst and lowland rivers of Croatia under wastewater impact

Environmental context Despite thallium’s high toxicity, water solubility and bioaccumulation tendency, its ecotoxicological relevance is largely unknown. We investigated thallium concentrations in fish, amphipods and parasitic worms from a karst freshwater ecosystem, and estimated total and cytosolic thallium concentrations in intestine of fish from the karst and lowland rivers impacted by wastewater inflow. The study provides the first data on dietary thallium accumulation in the intestine of freshwater fishes. Abstract The aim of the present research was to investigate the bioaccumulation of thallium (Tl), a technology-critical element, in fish intestine and muscle, gammarids and the fish intestinal parasites, acanthocephalans, and to evaluate their potential as indicators of metal exposure in aquatic environments. Moreover, total and cytosolic (metabolically available and potentially toxic fraction) Tl concentrations were measured and compared between the intestines of brown trout (Salmo trutta Linnaeus, 1758) from the karst Krka River and Prussian carp (Carassius gibelio Bloch, 1782) from the lowland Ilova River. Since there is a scarcity of information on subcellular metal partitioning in the fish intestine, the results of Tl concentrations in acid digested intestinal tissue, homogenate and cytosol represent preliminary data on Tl diet-borne uptake in salmonid and cyprinid fish. In both rivers, samplings were performed upstream (reference site) and downstream (contaminated site) of the wastewater impact in autumn and spring. Total Tl concentrations were much higher in brown trout than Prussian carp, and higher proportions of cytosolic Tl concentrations were observed in the intestinal tissue of brown trout (45–71 %) compared with that of Prussian carp (32–47 %). Therefore, both indicators showed species- and site-specific differences. Considering different bioindicator organisms, the most effective Tl accumulation was evident in acanthocephalans compared with the fish tissues and gammarids, which confirmed the potential of fish parasites as bioindicators of metal exposure. Trends of spatial and temporal Tl variability were mostly comparable in all indicator organisms and for total and cytosolic Tl concentrations in the intestine of salmonid and cyprinid fish species, which confirmed their application as useful biological tools in metal exposure assessment.