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Ecotoxicology of chlorpyrifos and atrazine to some aquatic invertebrates under the influence of water characteristics

Boonthai, Chuleemas
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In New Zealand, potential problems associated with the occurrence of pesticide residues in freshwater systems have become a concern in recent years. The application of aquatic ecotoxicology plays a major role in evaluating such hazards or safety of a pesticide to aquatic life. The lack of aquatic toxicity data for native species continues to be a major limitation to conducting reliable hazard and risk assessments in New Zealand. Accurate predictions of pesticide fate and toxicity in aquatic environments are hindered by a lack of information on how site-specific water characteristics affect the biological activity of these compounds. Therefore, the ecotoxicology of two pesticides, chlorpyrifos and atrazine, on New Zealand aquatic invertebrates under varying water characteristics was studied. The aim was to identify the effect of chlorpyrifos and atrazine in commercial grade formulations on aquatic invertebrates in terms of acute and chronic toxicity values under the influence of water characteristics by investigating the toxicological response and the physiological response of several aquatic invertebrates to the two pesticides. The aquatic invertebrates used in this study were: cladoceran, Daphnia carinata, midge larva, Chironomus zealandicus, mayfly nymph, Deleatidium sp., caddisfly larva, Olinga feredayi, common red damselfly nymph, Xanthocnemis zealandica, common backswimmer, Anisops assimilis, common waterboatman, Sigara arguta, freshwater snail, Physella acuta, and an ostracod, Cypricercus sanguineus. The acute toxicity of chlorpyrifos to some New Zealand aquatic invertebrates was much higher than found for atrazine. Mayfly nymphs were the most sensitive species and a snail was the most tolerant species to chlorpyrifos and atrazine. The degree of sensitivity of aquatic invertebrates to chlorpyrifos decreased in the order: mayfly nymphs> caddisfly larvae> cladoceran > midge larvae> common backswimmer > common waterboatman > common red damselfly nymphs> ostracod> freshwater snail. The degree of sensitivity of aquatic invertebrates to atrazine was almost the same order with the exception of the common red damselfly nymph, which was more sensitive than common waterboatman. The toxicity of chlorpyrifos and atrazine increased with test duration. The acute toxicity of chlorpyrifos and atrazine to the selected aquatic invertebrates varied with biological parameters such as stage of life cycle and non-biological parameters such as differing time exposure, sampling location and season. Aquatic invertebrates that are herbivores were more sensitive to both pesticides than the carnivores and detritivores. Compared with aquatic invertebrate species found in other countries, some New Zealand species such as the cladoceran, mayfly nymphs, midge larvae and common backswimmer, were more sensitive to chlorpyrifos and atrazine while the others had similar sensitivities. The chronic bioassays measuring the reproduction of cladoceran, D. carinata and growth of midge, C. zealandicus over 14 and 10 day periods were successfully used to determine the NOEC, LOEC and MATC for chlorpyrifos and atrazine. The MATC values of chlorpyrifos and atrazine for the cladoceran were 0.005 µg/L and <0.005 mg/L, respectively, and the MATC values of chlorpyrifos and atrazine for the midge larvae were and 0.05 µg/L and 0.05 mg/L, respectively. The effect of varying water characteristics on the acute toxicity of chlorpyrifos and atrazine to mayfly nymphs, a cladoceran, midge larvae, and common backswimmer, was investigated. The common backswimmer was more tolerant to chlorpyrifos and atrazine than the other three species and more tolerant to atrazine than chlorpyrifos. A similar pattern of tolerance existed for the mayfly, cladoceran and midge but at lower pesticide concentrations. Temperature, pH, dissolved oxygen, and dissolved organic matter affected the toxicity of both pesticides. The trend showed that the toxicity increased with increasing water temperature, lower pH, lower dissolved oxygen and higher dissolved organic matter. However, there were differences between the species. Therefore, the effect of water quality factors on pesticide toxicity needs to be considered in pesticide hazard assessment to ensure adequate protection of aquatic organisms. Acetylcholinesterase (AChE) and glutathione-S-transferase (GST) levels were investigated in selected species of New Zealand aquatic invertebrates using a modified microplate assay based on the Ellman technique and Habig method as a biomarker of chlorpyrifos and atrazine exposure. After 2 days exposure to different concentrations of chlorpyrifos and atrazine, mayfly nymphs, midge larvae, a cladoceran, common waterboatman, and common red damselfly nymphs, showed a general trend of depression in AChE level and a trend of induction in GST level but there were differences between the species. Overall, there was no significant difference in GST level between treatments (p> 0.05). The results indicate that acetylcholinesterase may be useful as a biomarker but the choice of species is critical. The results also showed that atrazine may reduce the level of AChE. These results suggest that the restriction of AChE as a biomarker for organophosphate and carbamate pesticides may not be valid and the use of this enzyme as a biomarker may be able to be extended.
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