Trace element bioaccumulation in the freshwater mussel Unio elongatulus after long-term transplantation: implications for biomonitoring and nature-based remediation

Abstract

Freshwater mussels are widely used as bioindicators of aquatic contamination due to their high filtration capacity and ability to accumulate trace elements. However, their potential role in contaminant sequestration and environmental remediation remains poorly explored under natural exposure conditions. In this study, we investigated trace element bioaccumulation dynamics in the freshwater mussel Unio elongatulus (Pfeiffer, 1825) through a long-term active biomonitoring experiment. Approximately 2,000 individuals collected from an uncontaminated reference site in Lake Maggiore (Italy) were translocated to Lake Orta, a historically metal-contaminated system, and retrieved after two years of natural exposure. Element concentrations were quantified in sediments, soft tissues and shells of donor, transplanted and resident mussel populations. Sediments showed the strongest spatial differentiation between lakes, reflecting differences in catchment characteristics and long-term geochemical inputs. After prolonged exposure, element concentrations in soft tissues of translocated mussels converged toward those of the resident population, indicating that tissue accumulation primarily reflects current environmental availability rather than population origin. Shell composition displayed clearer spatial differentiation and recorded longer-term exposure patterns. Bioconcentration factors revealed strong accumulation for several elements, including Mn, Sr and Ba, whereas metals such as Fe, Co and Mo showed limited biological uptake. These results demonstrate that Unio elongatulus integrates trace element exposure across multiple temporal scales and confirm the value of combining sediment, tissue and shell analyses for environmental assessment. This multi-compartment approach improves the interpretation of element dynamics in freshwater ecosystems and supports the use of freshwater mussels as effective tools for biomonitoring and nature-based remediation of contaminated waters.

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