Assessment of toxic effects of imidacloprid on freshwater zooplankton: An experimental test for 27 species

The neonicotinoid pesticide imidacloprid has been used worldwide since 1992. As one of the most important chemicals used in pest control, there have been concerns that its run-off into rivers and lakes could adversely affect aquatic ecosystems, where zooplankton play a central role in the energy flow from primary to higher trophic levels. However, studies assessing the effects of pesticides at the species level have relied on a Daphniacentric approach, and no studies have been conducted using species-level assessments on a broad range of zooplankton taxa. In the present study, we therefore investigated the acute toxicity of imidacloprid on 27 freshwater crustacean zooplankton (18 cladocerans, 3 calanoid copepods and 6 cyclopoid copepods). The experiment showed that a majority of calanoid copepods and cladocerans were not affected at all by imidacloprid, with the exception of one species each of Ceriodaphnia and Diaphasoma, while all six cyclopoid copepods showed high mortality rates, even at concentrations of imidacloprid typically found in nature. In addition, we found a remarkable intra-taxonomic variation in susceptibility to this chemical. As many cyclopoid copepods are omnivorous, they act as predators as well as competitors with other zooplankton. Accordingly, their susceptibility to imidacloprid is likely to cause different responses at the community level through changes in predation pressure as well as changes in competitive interactions. The present results demonstrate the need for species-level assessments of various zooplankton taxa to understand the complex responses of aquatic communities to pesticide disturbance.