Experimental induction of morphological deformities in Chironomus riparius larvae by chronic exposure to copper and lead

Five consecutive generations of Chironomus riparius Meigen larvae were chronically exposed from egg to fourth instar to four sublethal concentrations of copper (0, 1, 10, 100 mu g L-1) and lead (0, 5, 50, 500 mu g L-1) in artificially spiked water (static with renewal), with diatomaceous earth as substrate and tetraphyl(R) as food, in order to test the induction of morphological deformities by these metals. The use of diatomaceous earth was suboptimal because it caused high mortalities (>60%), independent of metal stress. The higher copper concentrations had a positive effect on the survivals relative to the control. Split medial mentum teeth were recorded in more than 10% of the larvae, but could not be related to metal stress. Deformities of the mentum and the mandibles were recorded in second, third, and fourth instars exposed to both metals. Concentration and generation effects were noted for unusual number of mentum teeth (0-5.3%, lead), unusual number of mandible inner teeth (0-10.4%, copper and lead), and small open mentum gap (0-6.5%, copper). These experiments demonstrated the potential of both an essential and a non-essential metal to induce weak deformities in a small proportion of a C. riparius population as well as the induction of deformities which are independent of metal stress or fluctuating over the generations. The study shows the potential of midge deformities as a biomonitoring tool, but at the same time warns for a careful interpretation of deformity scores because of the influence of population dynamics on the final outcome of deformity frequencies and of the existence of deformities not related to pollution.