Earthworms [Lumbricidae] as bioindicators: the relationship between in-soil and in-tissue heavy metal content

The heavy metal (Cd, Zn, Cu, Pb, Ni, Cr) accumulation in earthworms was measured and evaluated on 84 differently managed and polluted sites (field, grassland, deciduous forest, coniferous forest, alluvial sites, mine spoil) together with selected soil properties, like pH and Corg-contents. The uptake of heavy metals by the earthworms was correlated with in-soil metal amounts and it showed a considerable variation between land use types and the individual lumbricid species. An important positive correlation was determined between in-tissue contents of earthworms and in-soil contents for all studied heavy metals: Cd (R² = 0.72), Cu (R² = 0.65), Cr (R² = 0.54), Pb (R² = 0.51), Zn (R² = 0.47), Ni (R² = 0.45). Mostly Cd and Zn are accumulated by earthworms. The uptake of Cd and Zn by epigeic earthworms is stronger than by other endogeic and anecic species. The highest metal amounts of Cd were accumulated in the following species: Aporrectodea caliginosa (Savigny) (18 mg kg⁻¹ dw) Lumbricus castaneus (Savigny) (17 mg kg⁻¹ dw), Octolasion cyaneum (Savigny) (17 mg kg⁻¹ dw), Lumbricus rubellus (Hoffmeister) (14 mg kg⁻¹ dw). The highest uptake of Zn was found for the species: L. castaneus (623 mg kg⁻¹dw), Lumbricus terrestris (Linnaeus) (433 mg kg⁻¹ dw), A. caliginosa (416 mg kg⁻¹ dw), and L. rubellus (339 mg kg⁻¹ dw). The lowest contents of Cd (3 mg kg⁻¹dw) were calculated for Aporrectodea longa (Ude) and for Zn – Allolobophora chlorotica (Savigny) (168 mg kg⁻¹ dw). Transfer ratios (TR) (ratio of in-soil heavy metal content to the in-tissue metal content in earthworms) ranged for Cd – from 6 in alluvial forest sites to 64 in coniferous forest sites and for Zn – from 2 in alluvial grassland to 12 in coniferous forest. Cd and Zn are more strongly taken up by the epigeic species like Dendrobaena octaedra (Savigny) (TR: Cd = 47, Zn = 8), L. rubellus (TR: Cd = 21, Zn = 5), and L. castaneus (TR: Cd = 12, Zn = 5) than by the other endogeic and anecic species, with TR values accounted from 9 to 21 for Cd and 2 to 5 for Zn, respectively. For risk assessment of habitats cadmium is the more important heavy metal due to its high transfer rates into the tissues of earthworms.