Use of cholinesterase activity to monitor exposure of Chironomus riparius (Diptera: Chironomidae) to a pesticide mixture in hypoxic wetland mesocosms

Cholinesterase (ChE) inhibition has been demonstrated to be useful as a biomarker for exposure to organophosphorus (OP) and carbamate insecticides in many environments; however, there are questions about whether ChE activity may be affected by common environmental conditions such as low oxygen concentrations or by concurrent exposure to multiple chemicals. This article describes experiments in which larvae of Chironomus riparius were exposed to hypoxia (dissolved oxygen concentrations of 0.1 mg L-1; <1% saturation) in wetland mesocosms and in the laboratory (1.90 mg l-1; 19% saturation), and in which C. riparius larvae were exposed to mixtures of the OP, chlorpyrifos and the herbicides, atrazine and metolachlor, in hypoxic wetland mesocosms and in normoxic conditions in the laboratory. Larvae exposed to hypoxia for up to 35 h had no change in ChE activity, nor did atrazine or metolachlor cause ChE inhibition. There was no significant difference in ChE inhibition between larvae exposed to a mixture of chlorpyrifos (0.5 μg l-1), atrazine and metolachlor and larvae exposed to chlorpyrifos (0.5 μg l-1) alone. Larval chironomids are good candidates for monitoring of ChE inhibitors in shallow aquatic systems. Chironomid larvae are easy to handle and can be caged in areas where pesticide inflow is a concern. Chironomids can be placed in hypoxic aquatic systems as they are able to tolerate hypoxia for several hours up to a few days. Finally, in this study, larval ChE activities were significantly depressed following a range of exposures to chlorpyrifos, indicating that chironomid ChE activities are a sensitive indicator of exposure to OPs.