Effects of cadmium on life-cycle parameters in a multi-generation study with Chironomus riparius following a pre-exposure of populations to two different tributyltin concentrations for several generations

So far only a few studies have been performed to assess the effects of dynamic pollutant exposure on life-history parameters of invertebrates. In a previous multi-generation approach with the midge Chironomus riparius we tested if a chronic tributyltin pre-exposure alters the ability of a population to cope with subsequent cadmium stress. In the experiment two separate chironomid populations were exposed via sediments to different TBT-concentrations (4.46 and 8.93 mu g Sn/kg dw) for several generations, followed by subsequent cadmium exposure (1.2 mg Cd/kg dw) for three generations. While the TBT-exposure to 4.46 mu g Sn/kg dw had only small effects on the development and reproduction of C. riparius the higher TBT-concentration of 8.93 mu g Sn/kg dw led to negative effects on life-history traits. Therefore, a higher adverse effect of the higher TBT-concentration and thus a higher susceptibility to other stressors could be assumed. Within, this paper only the results of the second stressor experiment were presented; clear effects of Cd on development and reproduction of C. riparius were determined independent of the pre-exposure scenario. While no differences in Cd-sensitivity were found between the population without pre-exposure to TBT and the population pre-exposed to the low TBT-concentration (4.46 mu g Sn/kg dw), the pre-exposure of midges to the higher TBT-concentration (8.93 mu g Sn/kg dw) resulted in a significantly higher susceptibility to subsequent Cd-stress. These results document that the exposure history may influence the reaction to altered chemical stress. Our findings are relevant to understand and predict the evolutionary fate of populations in rapidly changing, human-impacted environments. However, the fact that chemical-induced reduced genetic diversity, which is not necessarily linked to genetic adaptation, leads to a reduced fitness under altered stress conditions, is to our knowledge a novel finding.