Tolerance Induction and Life Cycle Changes in Cadmium-Exposed Chironomus riparius (Diptera) during Consecutive Generations

Cultures of Chironomus riparius were exposed to cadmium during nine consecutive generations to determine whether cadmium tolerance could be induced. Selection for cadmium tolerance was assumed to influence the population dynamics of this species. Therefore, the responses and interactions of different population parameters (such as mortality, growth, and reproduction) were studied during the selection process. Exposure to cadmium during consecutive generations caused increasing effects on some life cycle parameters compared to a one-generation experiment. Tolerance to cadmium increased during exposure to 54.2 nM Cd and the tolerant population seemed to be stimulated by low cadmium concentrations (based on an acute growth experiment). Despite this tolerance development, mortality among cadmium-exposed tolerant chironomids remained high. These experiments illustrated that changes of the life cycle and tolerance can be expected as soon as single-generation NOEC values are exceeded, and in addition that "safe concentrations" based on a one-generation toxicity experiment could well underestimate the potential effects of a toxicant on midge populations.