Ecological effects of imidacloprid on a tropical freshwater ecosystem and subsequent recovery dynamics

This study aimed to investigate the effect of imidacloprid on structural (invertebrates and primary producers) and functional (organic matter decomposition and physicochemical parameters) characteristics of tropical freshwaters using acute single species and mesocosm studies performed in Ethiopia. The recovery of affected endpoints was also studied by using a mesocosm study period of 21 weeks. Our acute toxicity test showed that Cloeon dipterum (96-h EC50 = 1.5 μg/L) and Caenis horaria (96-h EC50 = 1.9 μg/L) are relatively sensitive arthropods to imidacloprid. The mesocosm experiment evaluated the effects of four applications of imidacloprid with a weekly interval and the results showed that the macroinvertebrate and zooplankton community structure changed significantly due to imidacloprid contamination in mesocosms repeatedly dosed with ≥0.1 and ≥ 0.01 μg/L, respectively (time weighted average concentrations of 112 days (TWA_112d) of ≥0.124 and ≥ ≈0.02 μg/L, respectively). The largest responses were found for C. dipterum, C. horaria, Brachionus sp. and Filinia sp. Chlorophyll-a concentrations of periphyton and phytoplankton significantly increased in the ≥0.1 μg/L treatments levels which are indirect effects as a result of the release of grazing pressure. A significant, but quantitatively small, decrease of organic matter decomposition rate was observed in mesocosms treated with repeated doses of 1 μg/L (TWA_112d of 2.09 μg/L). No recovery was observed for the macroinvertebrates community during the study period of 21 weeks, but zooplankton recovered after 9 weeks. We observed spatio-temporal related toxicity differences between tropical and temperate aquatic taxa, with tropical taxa generally being more sensitive. This suggests that use of temperate toxicity data for the risk assessment of imidacloprid in tropical region is not recommended.