The combined stress effects of salinity and copper on the earthworm Eisenia fetida

Most studies on the effects of salinity or copper on soil organisms have hitherto focused on testing of individual substance toxicity which cannot adequately predict the toxicity of mixtures of contaminants due to their possible interaction in soil or at the point of entry into organisms. In order to assess the joint toxic effect of salinity and Cu to earthworms, specimens of Eisenia fetida were exposed in OECD artificial soil spiked with a range of sub-lethal concentrations of NaCl and Cu singly and as mixtures. Mortality, weight change, and internal copper concentrations were assessed in worms while Cu concentrations in the soil (total, DTPA and CaCl2 extractable) were also determined. No worm mortality occurred during this study in both individual and joint toxicity tests. Weight change and cocoon production were significantly affected by increased NaCl and Cu, as individual substances. In combination, the contaminants had mostly additive effects on these worm parameters. Among the three extraction methods used, only CaCl2 extraction showed that salinity had a significant increase on the estimated bioavailability of Cu in the soil. Despite increased labile Cu with increased salinity, the Cu contents of worms were in most instances not related to the bioavailable fraction as estimated by the CaCl2 method. It is concluded that the effects of salinity, resulting from increased NaCl, on the toxicity of copper to these earthworms were mainly additive but also depended on the concentrations of both substances.