The effects of metal contamination on earthworm populations around a smelting works: quantifying species effects

Abstract

The effects of metal contaminants on the population density and species composition of earthworms were studied at 22 sites around a primary smelting works situated at Avonmouth, southwest England. All worms were absent from six sites within 1 km of the factory and numbers were also reduced significantly at an additional four sites 2 km from the plant. Total earthworm density was found to be inversely related to concentrations of metals in soils. some species of earthworms were found to be more ‘sensitive’ to metals and were absent from sites where more ‘tolerant’ species persisted. For example, Lumbricus rubellus, Lumbricus castaneus and Lumbricus terrestris were all present at sites close to the smelter where Aporrectodea rosea, Aporrectodea caliginosa and Allolobophora chlorotica were absent. The first three ‘tolerant’ species have more active calcium secretion glands in their gut than the three ‘sensitive’ worms. Calcium is known to be involved in the sequestration and elimination of various metals through the chlorogogenous tissue. Toxicity tests were carried out with a ‘sensitive’ and a ‘non-sensitive’ earthworm species ( Lumbricus rubellus and Aporrectodea rosea respectively) to examine if earthworm species distribution could be related to their susceptibility to the metals. In addition, the OECD recommended species Eisenia fetida was tested, to compare the sensitivity of this worm to the two commonly occurring soil species. Zinc was the metal used for these tests, since earlier comparisons of metal toxicity between the laboratory and field have shown that of the four metals released from the smelting works (Cd, Cu, Pb, Zn), it is zinc that is limiting the distribution of worms. Test results indicated that Aporrectodea rosea is more sensitive to the effects of zinc than Lumbricus rubellus. Thus, it appears that differences in the distribution of species of earthworms around the smelter can be related to variation in sensitivity to zinc. Both species were affected by zinc at lower concentrations than Eisenia fetida.