Biodilution of heavy metals in a stream macroinvertebrate food web: Evidence from stable isotope analysis

Analysis of carbon (δ13C) and nitrogen (δ15N) stable isotopes provides an increasingly important means of understanding the complex trophic structure of macroinvertebrate communities in streams.We coupled a stable isotope approach with a contaminant analysis of six metals (Pb, Ag, Zn, Hg, Cu, As) to trace the accumulation and dilution of metals from an abandoned mine across trophic levels of the benthic community in Ginzan Creek, Japan. The δ15N signature increased with trophic level, with mean increases of 4.70‰from producers to primary consumers and 3.06‰ from primary to secondary consumers. Tissue Pb and Ag concentrations were negatively correlated with δ15N, indicating biodilution of both metals through the food web. Although macroinvertebrate taxon body mass was negatively correlated with tissue metal concentration at several sites, it did not increase with trophic level (as δ15N) in any of the sites, suggesting that changes in body mass were not the cause of biodilution. Our findings suggest invertebrates at higher trophic levels may exhibit increasingly efficient excretion of metals. Autotrophic epilithon (mean δ13C=−21.3‰) had a much higher concentration of mined metals than did riparian vegetation (mean δ13C= −29.3‰); nonetheless, a carbon-mixing model indicated that taxa feeding on autochthonous carbon sources did not accumulate more metal than allochthonous feeders. It is likely that the notably high metal concentration of allochthonous FPOM plays an important role in the trophic transfer of metals. Our data suggest the strong potential for stable isotope analysis to enhance our understanding of metal transfer through stream macroinvertebrate food webs.