The State of Tocantins is becoming one of the main agricultural centers in Brazil, and the availability of water resources have strengthened agricultural production in the state. Monoculture areas extend mainly along the Araguaia and Tocantins watersheds, which are the two main drainage systems. To achieve high productivity, agricultural crops are highly dependent of pesticide use. As such, agricultural pesticides may reach the aquatic ecosystems through leaching and drift after application, also through the adsorption to biological materials and soil clay particles that are carried to aquatic system by erosion. Consequently, the aquatic ecosystem may be affected due to the toxic effect of these pesticides to aquatic species of different trophic levels. As a result of a field survey of several areas of cultivation in the State of Tocantins, it was found that the insecticide thiamethoxam (TMX), the fungicide cyproconazole (CPZ) and the herbicide paraquat (PQ) are the main pesticides used in many crops, in different areas of cultivation. The aim of this study was to evaluate the toxicity of TMX, CPZ and PQ for freshwater invertebrates, in order to contribute to ecological risk assessment of agricultural pesticides in the ecosystem of the Araguaia-Tocantins watershed. Since information on the lethal and sub-lethal toxicity of the different compounds, particularly for ecologically relevant aquatic species, is crucial for risk assessment of pesticide in aquatic ecosystems, ecotoxicological tests were carried using laboratory tests with two freshwater invertebrates, the dipteran Chironomus riparius and the planarian Dugesia tigrina. Preliminary studies revealed that environmentally relevant concentrations of TMX is did not induced toxicity to D. tigrina and thus, effects of this insecticide were evaluated only using C. riparius (survival, growth, emergence and evaluation of biochemical responses). In the evaluation of CPZ toxicity, the endpoints evaluated in C. riparius included survival, growth and emergence, while in D. tigrina effects were evaluated in terms of survival, regeneration and behavioral responses (locomotion and feeding). For the assessment of PQ toxicity, chronic tests were conducted with C. riparius (growth, emergence and adult weight) and D. tigrina (regeneration, locomotion, and feeding). Our results showed that TMX is highly toxic to C. riparius, since environmentally relevant concentrations affects not only the development rates, but also survival. The CPZ fungicide is moderately toxic to C. riparius (delayed emergence time) and D. tigrina (decreased locomotor activity, feeding activity and delayed regeneration). The herbicide PQ also caused a delay in emergence time and decrease in body weight of C. riparius imagos, as well as reduction in locomotor activity, feeding activity and regeneration of D. tigrina. This study adds ecotoxicological data on the effects of TMX, CPZ and PQ to freshwater invertebrates and suggests that in the long term, these pesticides can potentially disrupt food chains and community structure in freshwater ecosystems. The application of these pesticides namely in the Araguaia-Tocantins watershed should thus be closely monitored in agricultural areas in order to avoid impacts on aquatic ecosystems.
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