Differences in Cd accumulation among species of the lake-dwelling biomonitor Chaoborus

We measured substantial differences in Cd accumulation among four species o f the phantom midge Chaoborus that were exposed in the laboratory to the sa me Cd concentration in naturally contaminated prey. The two large-bodied sp ecies accumulated more Cd than did the two small-bodied species, in spite o f the fact that all species ingested prey at the same rate. To determine wh y this was the case, we fitted our experimental data to a bioaccumulation m odel that allowed us to compare the species with respect to their rate cons tants for growth and Cd efflux, their Cd assimilation efficiency, and their Cd concentrations at steady state. Differences among species were explaine d mainly by the fact that the small-bodied species assimilated a much lower proportion of the Cd that they ingested with prey (approximately 6%) than did the large-bodied species (45 and 58%). A comparison between Cd concentr ations measured in Chaoborus species in the field and predictions from the model suggests that differences in Cd concentrations among coexisting Chaob orus species in nature are explained by differences both in the rate at whi ch they assimilate Cd and in their feeding habits.