Pesticides negatively affect biodiversity and ecosystem functions of surface waters. The current pesticide risk assessment is afflicted with uncertainties. This PhD thesis (i) introduces an enhanced version of the Chemcatcher® passive sampler for the quantification of pesticides in running waters, (ii) reveals the contribution of wastewater treatment plants (WWTP) to insecticide loads in running waters and the associated consequences for stream macroinvertebrates, and (iii) confirms the recolonisation process of in-stream drift hypothesised for downstream community recovery from pulsed pesticide contamination. After an introduction into the fields of pesticides, running water ecology, and pesticide monitoring in running waters, the thesis continues with three studies on pesticide exposure and effects in lotic ecosystems.

In the first study, the pesticide stress of stream macroinvertebrates was determined using the bioindicator SPEARpesticides and modified Chemcatcher® passive samplers. To inhibit the manifestation of a biofilm on the sorbent medium of the sampling device, a diffusion-limiting membrane was installed atop the receiving phase. The results show that Chemcatchers with a shielding membrane reliably quantify pesticide pollution. It is concluded that monitoring programmes using this passive sampler version can benefit from the properties of the membrane.

In the second study, the ecological impact of WWTP-induced pesticide pollution was assessed. WWTPs discharge pesticides from agricultural and urban uses into receiving waters. The results show that both macroinvertebrate community structure and organic matter breakdown are adversely affected by pesticides from WWTP effluents. It is concluded that further wastewater treatment steps are needed to eliminate pesticides from treated wastewater prior to its release into aquatic ecosystems.

In the third study, the impact mitigation potential of stream reaches not contaminated with pesticides was examined. In-stream drift is the main source of recolonising taxa and has been hypothesised to be the main driving force behind downstream community recovery from short-term pesticide stress. The results confirm that forested stretches accelerate the recovery of downstream macroinvertebrate communities by replenishing taxa vulnerable to pesticides. It is concluded that riparian forest needs to be protected (and newly grown where applicable), accounting for its ecological importance for lotic biodiversity in agricultural landscapes. Overall, the combined use of Chemcatcher® passive samplers and SPEARpesticides is recommended for the assessment of pesticide exposure and effects in running waters.