Deltamethrin toxicity to the midge Chironomus riparius Meigen-effects of exposure scenario and sediment quality.

We investigated how exposure scenario and sediment characteristics affected deltamethrin bioavailability and toxicity to Chironomus riparius. We designed whole-sediment experiments, including separate spiked-water and spiked-sediment experiments and using both a peat-based artificial and a natural sediment. Deltamethrin was highly toxic to larvae in artificial sediments, with LC(50)-values (28 d) of 11 microg/kg for sediment-exposures and 16 pg/L for water-exposures. In contrast, deltamethrin-induced mortality was absent in experiments with natural sediment, both in spiked water and in spiked sediment. This was attributed to the higher organic matter content of the natural sediment, 12.5+/-0.05%, compared with that in the artificial sediments, 4.1-4.8%, resulting in an efficient and fast sorption and lower bioavailability. Deltamethrin degradation was absent in artificial sediment, while in natural sediment, 50% degraded during 10 days of test-vessel acclimation. Despite a faster degradation, the highest measured concentration in spiked-natural sediment was more than seven times higher than the LC(50)-value obtained in tests with spiked-artificial sediment, indicating that low bioavailability, and not compound degradation, was the main reason for the observed lack of deltamethrin toxicity.