Bioavailability of cadmium and zinc to midge larvae under natural and experimental conditions: Effects of some environmental factors

In this paper the effects of environmental factors on cadmium and zinc uptake by larvae of chironomids are discussed. The results of several laboratory experiments and field studies were pooled and analysed using uptake and accumulation models. In the field studies, the relationship between metal concentrations in larvae and sediment was studied on samples from several watercourses. The effect of different sediment characteristics on these relationships was investigated In the laboratory experiments, larvae of Chironomus riparius were exposed to metals via the water, and the effects evaluated of three changing environmental factors, i.e. salinity temperature, and pH. Non-linear regression models were constructed to determine the relative importance of the different environmental factors contributing to the variation in metal uptake or accumulation. For the field data, the amount of variation that could be explained by these models was limited. Only for zinc was a significant amount of variation (up to 66 \%) explained relating accumulated zinc to easily extractable zinc and considering total organic carbon (TOC) in the model. For the laboratory data, relating uptake levels to the metal ion activities explained no more than 6\% and 24\% of the total variation in respectively cadmium and zinc uptake. The integration of the different effects of the environmental factors in the models explained 67\% of the total variation in cadmium uptake and 56\% of the total variation in zinc uptake. Factors contributing most significantly to the explained variation were temperature, pH, and salinity of exposure, calcium ion activity and salinity of acclimation. The high, unexplained variation under field conditions is probably due to the large variation in exposure conditions in natural environments and a lack of knowledge concerning the relative importance of the different exposure routes under these circumstances.