Biodiversity and the health of freshwater ecosystems is strongly impaired by human activities, compromising the stability of these ecosystems and the ecosystem services they provide. Global and European efforts to halt the biodiversity decline and protect ecosystem health were not very successful, especially for rivers, so that for less than ten percent of the German rivers good ecological status was reached in 2021. Present-day agriculture has been identified as the main driver for this deterioration, as evident from a multitude of studies. However, the agricultural effects differ between the organism groups and depending on environmental conditions like soil and climatic conditions. Moreover, and most importantly, agriculture is not uniform. The specific agricultural types and practices differ between regions, which in turn leads to differences in the intensity of agrochemical usage as suggested by many small-scale studies. Consequently, the magnitude of agricultural effects on biodiversity and health of river ecosystems most probably depends on agricultural types and practices and differs between regions. For the effective mitigation of these negative effects, several knowledge gaps need to be closed, which were addressed in six chapters, shortly described in the following.

First, the current knowledge on the effect of agriculture on river biota was summarized and analysed in a meta-analysis (Schürings et al., 2022). According to this meta-analysis described in the first chapter, agriculture has an overall medium to high negative effect on river biota, and results indicate that the effects of agriculture differ between agricultural types, practices, the organism groups, and biological metrics considered. Second, a pan-European dataset was used to establish an agricultural typology, based on agricultural production and agriculture-related freshwater pressure by nutrients, pesticides, water abstraction and hydromorphological alterations (Schürings et al., 2023). This chapter identified how agricultural types differ in their pressures exerted on freshwaters and shows that accounting for agricultural pressure intensity nearly doubles the correlation with the ecological status. Third, the effects of different agricultural types on the ecological status according to the EU Water Framework Directive (WFD) were investigated, using high resolution German-wide land use data, distinguishing between different crop types (Schürings et al., 2024a). The effects on the ecological status clearly differed between crop types, which typically are associated with different agrochemical application rates. Macroinvertebrates and macrophytes were most strongly affected by pesticide application intensive crops and diatoms were most affected by nutrient intensive crops. Fourth, the results presented in Markert et al. (2023) provided evidence that urban areas and different agricultural crop types with typical agrochemical application rates are indeed related to the micropollutant concentrations monitored in rivers, which often exceeded Environmental Quality Standards. Fifth, crop type-specific differences in agrochemical application rates reported in literature were used to generate an agricultural intensity index (Schürings et al., 2024b). This index improved the correlative strength between present-day agriculture and the ecological status with most pronounced relations for macroinvertebrates in small mountain streams. Sixth, experiences from implementing environmental legislations like the WFD were used to advice for a successful implementation of the EU Nature Restoration Law (Hering et al., 2023). This final chapter highlights that joining restoration efforts with a shift to more sustainable agriculture, whose importance is reasoned in the previous chapters, would offer unprecedented opportunities for successful protection of ecosystem health.

In conclusion, this thesis provides overwhelming evidence for the negative effects of present-day agriculture on river biota, portraying influencing factors and highlighting strong relationships between agricultural effects on river biota and agrochemical application, particularly of pesticides. Therefore, to mitigate these effects, a transition of present-day agriculture to more sustainable practices, such as organic farming or agroecology is of vital importance. Such a transition would be beneficial both for the future viability of agriculture itself but also for the protection and restoration of healthy ecosystems, including the successful implementation of the European environmental legislation such as the Nature Restoration Law.