Efficient shedding of accumulated metals during metamorphosis in metal-adapted populations of the midge Chironomus riparius

Metal accumulation and loss during metamorphosis were investigated in Chiro nomus riparius populations in a metal contaminated lowland river. Cadmium a nd zinc levels were measured in imagoes and larvae at reference and metal-e xposed sites. It was hypothesized that the relationship between metal conce ntrations in biota and environmental compartments would be influenced by th e presence of metal-adapted chironomids. In contrast to the large interpopu lation differences in larval body burdens of cadmium, body burdens in imago es vanished to background levels for all midge populations. This indicated that any cadmium accumulated in larval stages was lost during metamorphosis . This nearly 100% efficiency in shedding of cadmium is most likely caused by an increased metal handling capacity present in exposed midges. In agree ment with the cadmium measurements, larval body burdens of zinc showed also highly significant interpopulation differences. In contrast with the cadmi um values, however, body burdens of zinc in imagoes showed highly significa nt interpopulation differences and differences were even recorded between t he two exposed sites, indicating interpopulation differences in shedding ca pacity for zinc. It is concluded that the highly efficient shedding of accu mulated metals reflected the metal adaptation recorded in earlier studies o f metal-exposed C. riparius populations from the River Dommel. Based on the differences in metal accumulation and the differences found in shedding of metals between the two exposed midge populations, it was concluded that po pulation differentiation due to metal stress is a gradual process rather th an an all-or-nothing situation.