Seasonal changes in the tissue-metal (Cd, Zn and Pb) concentrations in two ecophysiologically dissimilar earthworm species: pollution-monitoring implications

Abstract

Metal (Cd, Pb, Zn) concentrations and contents were measured each month for thirteen consecutive months in two ecophysiologically distinct earthworm species, sampled from the vicinity of a disused Pb/Zn mine. Aporrectodea caliginosa, an endogeic species, enters diapause during unfavourable conditions; Lumbricus rubellus, an epigeic species, does not enter diapause. Cd concentrations were higher in A. caliginosa than in L. rubellus throughout the year: Pb concentrations were also usually higher in the endogenic species. Zn concentrations were fairly similar in both species, except during Winter and early Spring, when L. rubellus has significantly higher concentrations. Analysis of covariance indicated that the concentrations of Cd, Pb and Zn varied significantly over the thirteen-month period in L. rubellus, but only the non-essential metals (Pb, Cd) varied significantly in A. caliginosa. During diapause (observed only in the July sample), tissue concentrations of Cd and Zn in A. caliginosa were significantly lower than in 'active' worms; Pb concentrations were significantly higher during diapause. Since diapause was accompanied by a significant decrease in dry body mass, it was concluded that Pb is retained by earthworm tissues during the resting phase; but Cd and Zn are actively eliminated from the tissues before or during diapause. The implications of these findings on the use of earthworms for monitoring the relative degrees of metal contamination at different sites are discussed.