Structural changes in a macrozoobenthos assemblage after imidacloprid pulses in aquatic field-based microcosms

Imidacloprid pulses were introduced to an aquatic microcosm to evaluate the response of the lentic benthic invertebrate assemblage. The microcosms were exposed to three weekly pulses of imidacloprid at concentrations of 0.6, 1.4, 3.2, 7.5, 17.3, and 40 µg/L. The authors monitored for total abundance (larvae and adults), number of species, and emergence as endpoints. Imidacloprid applications occurred on sunny days in ideal conditions for photolysis, and they observed a rapid initial decrease in imidacloprid within 6 hours (mean DT50 of 28h). The clear water and lentic community made this experiment more similar to spray drift conditions than stormwater runoff. Since the imidacloprid dissipated quickly, the authors reported time-weighted average concentrations, which ranged from 0.2 to 12 µg/L. The highest imidacloprid applications (17.3 and 40 µg/L applied; 5.2 and 12 µg/L time-weighted averages) caused a significant decrease in the total number and abundance of Chironomidae species. Orthocladiinae species diversity and abundance also declined at the 12 µg/L average concentration (40 µg/L applied).  Ablabesmyia spp. declined significantly at the highest concentration and Radix spp. gastropods increased (12 µg/L avg., 40 µg/L applied). Adult Ephemeroptera were sensitive to imidacloprid, with Caenis spp. adults absent from microcosms with more than 0.4 µg/L time-weighted average (more than 1.4 µg/L applied).  Since detections in surface waters have been higher than these concentrations, the study represents a best-case scenario for many areas. Ephemeroptera began to show effects at 3.2 µg/L applied (1 µg/L average) and the rest of the community saw effects at 7.5 µg/L applied (2.3 µg/L average). The authors suggested that the effects were likely cumulative, but that the microcosms could still be colonized until the third pulse, so it is possible that some invertebrates were only exposed to one or two pulses.