The growth and emergence responses of the midge Chironomus riparius Meigen were used to evaluate the toxicity of natural and formulated reference sediments which were either obtained from polluted sites or spiked with copper in the laboratory. A significant reduction in larval growth was only observed in one of the two polluted field sediments tested. The reasons for this are discussed. However, copper spiking resulted in significant growth reductions of larvae exposed to the two formulated reference sediments at copper concentrations of 3 and 4 mg Cu2+ 1−1. Growth of larvae exposed in natural reference sediment was not significantly affected at equivalent concentrations.
The emergence of adults was adversely affected in polluted field sediment compared to formulated reference sediment. Male, female and total adult median emergence times (EmT50) were significantly greater in the former treatment indicating a delay in emergence, attributable to sediment contamination. In addition, survivorship of the larvae in the polluted sediment was reduced, resulting in a lower percentage of adults emerging compared to the formulated reference sediment. The results of this study illustrate the usefulness of chronic response criteria, incorporating the more sensitive life-stages of C. riparius, in the assessment of sediment toxicity.