THE UPTAKE OF CADMIUM BY THE MIDGE LARVAE CHIRONOMUS-RIPARIUS AS A FUNCTION OF SALINITY

The effect of salinity on the uptake of cadmium by the freshwater midge larvae Chironomus riparius was studied. Fourth instar larvae were exposed to a salinity range up to 10 ppt, by diluting chemically defined seawater with chemically defined river water. The influence of prior acclimation at different salinities on cadmium uptake rate was examined for three salinities of acclimation. To discriminate between the effect of the free cadmium ion concentration, the free cadmium ion activity and the calcium ion concentration, we tested solutions of different composition. Within each acclimation group, cadmium uptake decreased with increasing salinity in all exposure solutions. At a constant salinity of exposure, cadmium uptake increased with increasing salinity of acclimation. Changes in the free ion activity explained most of the observed variation (52%). The cadmium uptake was also influenced by the salinity of acclimation. Changes in calcium concentration had only a minor effect on cadmium uptake. The integration of the different effects explained almost 80% of the total variation in cadmium uptake by midge larvae. The remaining variation could not be explained and was attributed to natural variation in cadmium uptake among midge larvae.