CHRONIC TOXICITY OF CADMIUM TO CHIRONOMUS-RIPARIUS (DIPTERA, CHIRONOMIDAE) AT DIFFERENT FOOD LEVELS

The interacting effects of cadmium toxicity and food limitation on the midge, Chironomus riparius, were studied during chronic exposure in l aboratory experiments. If the food was supplied ad libitum, both larva l developmental time and mortality of the larvae were negatively affec ted by cadmium concentrations of 2.0-16.2 mug/L. The number of eggs de posited per female and the mean life span of the imagines were not aff ected by cadmium. Integration of these separate effects into a populat ion growth rate showed a clear reduction with increasing cadmium conce ntrations. Food limitation of unexposed larvae at high population dens ity reduced fitness, judged on all parameters studied and consequently reduced the population growth rate (up to 85%). The effects on larvae , which were exposed to both cadmium and food limitation, differed con siderably from the response to the individual stress factors. Exposure to cadmium increased mortality among food-limited first and second in star larvae. Consequently, the amount of food available for each survi ving larva increased. At the two lowest concentrations studied (2.0 an d 5.6 mug Cd/L), these indirect positive effects of cadmium overruled the direct negative effects and caused an increase of the fitness of t he food-limited exposed larvae compared to the food-limited, unexposed controls. At a concentration of 16.2 mug Cd/L, the negative effects o f cadmium on food-limited midges balanced the positive effects of redu ced food limitation. At this concentration, the population growth rate did not differ significantly from the food-limited control any more. It is concluded that the indirect positive effects of cadmium on food limitation could eliminate negative, direct effects of low cadmium con centrations on food-limited chironomid populations.