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

We measured substantial differences in Cd accumulation among four species of the phantom midge Chaoborus that were exposed in the laboratory to the same Cd concentration in naturally contaminated prey. The two large-bodied species accumulated more Cd than did the two small-bodied species, in spite of the fact that all species ingested prey at the same rate. To determine why this was the case, we fitted our experimental data to a bioaccumulation model that allowed us to compare the species with respect to their rate constants for growth and Cd efflux, their Cd assimilation efficiency, and their Cd concentrations at steady state. Differences among species were explained 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 concentrations measured in Chaoborus species in the field and predictions from the model suggests that differences in Cd concentrations among coexisting Chaoborus species in nature are explained by differences both in the rate at which they assimilate Cd and in their feeding habits.