Are pesticides the dominant stressors in German lowland streams?

The Helmholtz Centre for Environmental Research (UFZ) conducted a monitoring study of small lowland streams (ger. Kleingewässermonitoring, KGM) in Germany during the 2018 and 2019 growing seasons that included collecting water samples for pesticides and urban contaminants. This study was commissioned and funded by the German Environmental Agency (Umweltbundesamt, UBA). A publication by Liess and co-authors in 2021 concluded that 83% of agricultural streams included in the monitoring study were in an unacceptable ecological condition. This conclusion was based on a comparison of benthic macroinvertebrate (BMI) communities between reference and agricultural sites. The authors observed a decline in vulnerable BMI species, which they attributed to agricultural nonpoint source pollution by pesticides. We conducted an extensive re-analysis of the raw data and found that many of the study results were not reproducible because of a lack of detailed reporting of data processing methods. Other data processing steps and decisions by the authors lacked supporting rationales. Further, we found that the indicators of pesticide pressure for some stream sections were derived from samples taken after BMI communities had been sampled and are therefore not indicative of a causal relationship between pesticide pressure and community status. We recommend that the original dataset be filtered to include only stream sections with complete and temporally consistent data for both pesticide and macroinvertebrate sampling. This approach would enhance the transparency of the analysis and enable the scientific community to better assess the evidence supporting the authors' conclusions regarding the dominant stressors affecting BMI community structure in German lowland streams. Although the KGM dataset provides valuable insights into the status of small streams in agricultural landscapes, a more rigorous data selection and statistical analysis process is needed to derive robust and scientifically defensible findings for environmental management.