Pesticides are an important pillar of food production today. The use of pesticides has increased in recent years as the world's population has grown and food production has increased. The large‐scale application of pesticides in nature leads to pesticides also reaching non‐target areas. Here, they can cause lasting damage to existing ecosystems and natural communities. One important pathway is the flushing of pesticide residues from agricultural fields into adjacent streams. It has been observed that the surface runoff leads to high concentrations of pesticides in surface waters and alters the community there. This raises the question on how exactly the input takes place and how risk mitigation measures such as vegetated buffer strips (VBS) can optimally retain pesticides in the field. These questions, among others, were the objectives of the ”Kleingewässermonitoring” (KgM), in which small streams in agricultural environments were sampled on a large‐scale basis in 2018 and 2019 using rainfall‐related and grab samples.

We evaluated the pesticide exposure at the KgM‐stream sections based on the results of six publications. The difference between rainfall‐related samples and dry weather samples was examined at over 100 stream sections over Germany (publication 1, 2 & 3). We assessed the period of highest pesticide exposure to aquatic invertebrates and algae/aquatic plants (hereafter algae), by analysing real spray series and connected these to the seasonal and short‐term exposure patterns of the KgM‐samples (publication 2). We connected the observed exposure to the invertebrate community and evaluated the current Environmental Risk Assessment (ERA) which is used during the approval process of pesticides (publication 3). Possible additional effects from toxic pesticide mixtures were assessed by cumulating the risk in the KgM‐samples to invertebrates and algae (publication 4). We identified flaws in the monitoring program of the Water Framework Directive (WFD) to assess pesticide risks (publication 5). Finally, we investigated the effects of VBS along the KgM‐stream sections in retaining pesticides (publication 6).

We found that the strongest factor influencing in‐stream pesticide exposure is their application. Seasonal and short‐term exposure peaks depend on the time of pesticide application (publication 2). In rainfall‐related samples, not only concentrations are generally higher by a factor of 10 (90% percentile) (publication 1 & 3), but they are particularly higher during the seasonal peak in May/June (publication 1 & 2). The peak exposures significantly influenced the composition of the invertebrate community (publication 3). As a result, the current ERA is not sufficient to protect aquatic communities, as there are widespread exceedances of thresholds which are derived insufficiently (publication 3). The simultaneous occurrence of pesticide exposure and the resulting exceedance of the regulatory thresholds increases the risk by a factor of 3.2 (publication 4). The monitoring program of the WFD currently lacks event‐related sampling, relevant analyte spectrum and missing availability of regulatory thresholds (publication 5). The presence of VBS could drastically minimize the risk if they were extended to an average width of 18 meter and if inputs from dry ditches were also reduced (publication 6).

The results of this thesis show that the general targets of the ERA are not achieved, because during the regulation process many sources of errors are present and partly unprotective thresholds are given. Not only that the exposure of pesticides is too high, but the recommendations of the risk mitigation measures are also flawed. The data of this thesis shows for the first time, that real‐world VBS in their current form cannot adequately contain the exposure. This thesis gives profound guidance on how to monitor pesticides in streams, improve regulations of pesticides and design risk mitigation measures to better contain the input of pesticides through surface runoff. This could significantly reduce the input of pesticides and improve the conditions for aquatic organisms. In this way, the good ecological status stipulated by the WFD could be achieved in the long term.