Effects of the insecticide chlorpyrifos on Amazonian freshwater invertebrates: A mesocosm experiment

Chlorpyrifos is one of the most widely used insecticides globally, yet its toxicity for tropical aquatic ecosystems in Latin America has not been thoroughly evaluated. In this study, we conducted a 70-day mesocosm experiment to assess the effects of chlorpyrifos on zooplankton and macroinvertebrate communities representative of the Ecuadorian Amazon. Mesocosms were exposed to a single application of chlorpyrifos at four nominal concentrations (0.01, 0.1, 1, and 10 μg/L). Results showed rapid chlorpyrifos dissipation, with a half-life of 1.2 days. For some Rotifera species, including Polyarthra luminosa and Anuraeopsis sp., a consistent No Observed Effect Concentration (NOEC) of 0.01 μg/L was identified. Cyclopoida (e.g., Macrocyclops sp.) also displayed treatment-related declines, with a NOEC of 0.1 μg/L. Among macroinvertebrates, the most sensitive taxon was the Ephemeropteran Caenis sp. (NOEC: 0.1 μg/L), while other insect taxa (Odonata, Diptera) also showed significant treatment-related declines. Based on the response of some Rotifera species and the zooplankton community, we determined a mesocosm NOEC of 0.01 μg/L. The response of Rotifera taxa was explained by indirect effects resulting from a decline in copepod populations within the experimental systems, which led to an increase in Cladocera (e.g., Moina sp.), effective grazers on small rotifers. Our findings suggest a mesocosm NOEC slightly lower than those reported in other tropical and temperate studies (0.1 μg/L), potentially due to differences in species composition and interactions. Finally, our study highlights that measured chlorpyrifos concentrations in Latin American surface waters exceed levels at which both direct and indirect effects have been observed. Additional monitoring and regulatory measures are needed to control chlorpyrifos use and environmental release in tropical regions with intensive agriculture and high rainfall.