Microplastic contamination worsens the ecotoxicity of chlorpyrifos to cladoceran Ceriodaphnia cornuta (Sars, 1885) and Echinisca triserialis (Brady, 1886).

The presence of microplastics (MPs) along with agrochemicals is being widely reported from lakes, ponds, and rivers. Microplastic (MP) in association with agrochemicals could have synergistic or antagonistic effects on non-target aquatic organisms and pose ecological risks. The present study explored the combined effects of MP, the insecticide chlorpyrifos (CPF) and MP preconditioned with CPF (MP^^CPF) on two cladocerans, Ceriodaphnia cornuta, and Echinisca triserialis in short-term and long-term laboratory bioassay tests. The experimental protocol included an acute bioassay at five different concentrations of MP (25, 40, 100, 200, and 500 particles mL^-1), CPF (0.01, 0.05, 0.1, 0.5, and 1 µgL^-1) and MP^^CPF. Whereas, CPF (0.01 µg L^-1), MP (25 particles mL^-1), and MP^^CPF in 0.01 µg L^-1 of CPF were selected for the chronic bioassay. Upon exposure to CPF, C. cornuta and E. triserialis displayed a dose-dependent mortality pattern with a 48 h LC₅₀ value of 0.06 µgL^-1_. No mortality was observed in the MP only treatment, but when MPs were preconditioned with CPF, the 48 h LC₅₀ value was 0.02 µgL^-1. Chronic exposure to MP, CPF, or MP^^CPF resulted in reduced survival and reproductive output in both the cladocerans. However, C. cornuta was more vulnerable than E. triserialis. A significant delay in age at first reproduction and shorter generation time were observed in the presence of MP^^CPF, suggesting MP-mediated enhanced toxicity of CPF, wherein CPF could have accumulated onto the MP surface, thus, intensifying its toxicity. The enhanced toxicity of organic pollutants by MPs in aquatic environments especially in pelagic organisms is a matter of concern.