RESUMEN
Small streams in agricultural landscape can experience short and repeated pulses of fluctuating pesticide concentrations. A single pesticide pulse may not have adverse effects on macrozoobenthos species but repeated pulses may have, especially if the organisms have not yet fully recovered when the second pesticide pulse occurs. Against this background, a comprehensive indoor stream mesocosm study was carried out in order to evaluate the cumulative effects of repeated insecticide pulses on a macrozoobenthos community. Weekly 12h pulses of 12 µg/L of the insecticide imidacloprid were set 3 times in 4 stream mesocosms in 2 series, one in spring and one in summer. Another 4 mesocosms served as controls. Prior to each pulse series, the mesocosms were stocked with macroinvertebrates from an uncontaminated reference stream using straw bags as attraction devices. The straw bag method proved suitable for establishing a functional macroinvertebrate community in the stream mesocosms. The caddisfly species Neureclipsis sp. reacted immediately and most sensitively after a single imidacloprid pulse whilst insect larvae such as ephemerids and dipteran larvae were negatively affected only after repeated imidacloprid pulses. Effects on insect larvae were more pronounced in the summer series most likely due to increased temperature. Abundance was a less sensitive endpoint than sublethal endpoints such as emergence. The results of the study underline that pulse effects are driven by a number of variables like pulse height, pulse duration, number of pulses, time in between pulses and by the species and live stage specific ability of temperature dependent detoxification which all should be taken into account in the risk assessment of pesticides.