Do aquatic insects avoid cadmium-contaminated sediments?

The long-term colonization of profundal lake sediments having a range of spiked cadmium (Cd) concentrations (0.007 to 2.7 μol/g dry wt.) was measured in the field. Population densities of two of the most abundant colonizing insects (the chironomids Procladius [Holotanypus] sp., and Sergentia coracina) were unrelated to the Cd gradient, even though both taxa accumulated Cd in direct relation to its concentration in sediment. Cadmium concentrations in these taxa ranged from approximately 0.3 to 5 μg/g, and 1 to 65 μg/g, respectively, along the sediment Cd gradient. Cadmium concentrations in Chironomus (salinarius gp.) sp. larvae also responded positively to the sediment Cd gradient and ranged from 0.2 to 50 μg/g. In contrast with the two other taxa, the abundance of Chironomus (salinarius gp.) sp. larvae was negatively correlated with the sediment Cd gradient. To determine if the abundance pattern of Chironomus (salinarius gp.) sp. was the result of a behavioral or a toxic response, larvae of the three chironomid taxa were given a choice between field]control and Cd-spiked sediments in the laboratory. None of the taxa avoided the Cd-spiked sediments, suggesting that the lower abundance of Chironomus (salinarius gp.) sp. at high Cd concentrations in the field was due to Cd toxicity and not to avoidance of the Cd-rich sediments.