Low-Level Gradients of Metal Contamination in Temperate Lakes of Southern South America: Evidence of Bioaccumulation in the Bivalve Diplodon chilensis (Hyriidae)

Glacial-origin lakes in southern South America are increasingly exposed to anthropogenic pressures, but early signs of contamination often remain undetected in apparently pristine systems. In this study, we assessed the bioaccumulation of trace metals (Cu, Mn, Zn, Cr) in the tissues of the native freshwater mussel Diplodon chilensis and their relationship with sediment metal concentrations across 15 sites in six temperate lakes. Sediment quality was largely classified as unpolluted according to the geoaccumulation index (87% of values ≤ 0), yet high metal loads were found in mussel tissues, with Mn and Zn reaching 3325 mg·kg−1 and 350 mg·kg−1 respectively. Bioaccumulation factors were especially high for Mn (42.2) and Zn (32.1), reflecting efficient uptake from the environment. Multivariate analyses revealed spatial patterns driven by sediment composition and gradients of human influence, while regression models highlighted a significant role of fine sediment fractions in Mn bioaccumulation. These results demonstrate that D. chilensis can detect bioavailable metal fractions even in low-impact systems, underscoring its potential as a sentinel species. The integrative approach combining sediment chemistry, tissue analyses, and quantitative indices provides a replicable, cost-effective framework for the early detection of metal contamination in temperate lakes worldwide.