Laboratory and field response of Chironomus riparius to a pyrethroid insecticide

Sediment toxicity test and bioassay protocols are currently being standardised in order to meet European legislative requirements for product assessments and environmental monitoring. Most existing test methods require that sediment-dwelling organisms are exposed to test substances bound to sediment in test systems with uncontaminated overlying water. These test systems have not been widely compared with responses of field populations under realistic exposure conditions. In this study, sediment toxicity data generated in the laboratory were compared with the response of a field population of the midge Chironomus riparius exposed to the pyrethroid insecticide permethrin. In the field this insecticide was applied to the overlying water column of artificial pond systems to give nominal concentrations of 1, 10, 50 and 100 μg/L. The field response, as measured by larval density and adult emergence, was not predicted by bulk sediment chemical concentration, sediment toxicity test or laboratory bioassay results. All three measurements underestimated acute toxic effect. Rather, toxicity to C. riparius in the field was predicted more accurately using a short-term water-only toxicity test. No chronic or sublethal effects due to sediment-bound permethrin were observed. This indicates that the primary route of exposure in the field was probably via the water column. Primary routes of exposure are difficult to predict a priori for different chemical types until the factors affecting bioavailability have been fully elucidated. Until this time, there is a need for the validation of predictive sediment toxicity tests and for the measurement of in situ biological response in conjunction with sediment chemistry and toxicity for environmental monitoring purposes, particularly when ongoing exposure via the water column is likely.