Feeding activity of midge larvae (Chironomus riparius Meigen) in metal-poll- uted river sediments

A method was developed for monitoring the feeding activity of larvae of the midge Chironomus riparius. The egestion rate (mg dry wt feces/mg dry wt larva/h) of the deposit-feeding larvae was measured and used as an indication of the feeding activity. Both the egestion rate and survival of several metal-adapted and reference larvae were measured in five test sediments with various Cd and Zn concentrations. The reference larvae suffered increased mortality in two contaminated sediments by comparison with field-exposed larvae. The feeding of all individuals decreased in the most polluted test sediment but also in another supposedly clean reference sediment. No differences were found in egestion rates between the reference and field-exposed larvae. This observation, however, does not reliably answer the hypothesis whether the feeding activity of the adapted and nonadapted midge larvae differs in metal-contaminated sediments because the data are based on total egestion during the short (96-h) experiment and the progress of feeding was not followed.