Freshwater ecosystems are strongly affected by anthropogenic activities, which can significantly alter environmental conditions with adverse effects on riverine biota. The status of both the freshwater communities and the environmental stressors are subject to frequent monitoring programs. However, despite considerable efforts in monitoring and restoration improving both water quality and habitat conditions, less than 10 % of German rivers have so far achieved a good ecological status as defined by the European Water Framework Directive (WFD). Thus, further reduction of adverse anthropogenic impacts and improvement of the ecological status of river ecosystems are necessary, which require the identification of predominant stressors and the selection of effective management measures. The aim of this thesis was to analyze the sources and effects of multiple stressors on riverine biota using comprehensive datasets of WFD-related monitoring programs in Germany. Firstly, the relative importance of each stressor group for the biological quality elements macroinvertebrates, benthic diatoms and fishes was assessed to provide additional evidence on relevant stressor groups for river basin management. Secondly, the influence of anthropogenic land uses, including urban areas, wastewater treatment plant (WWTP) effluents and intensive agriculture in the catchments, on stressor levels was analyzed to inform the identification of targeted management measures. A particular focus of these analyses has been on micropollutants, such as pesticides, pharmaceuticals and industrial chemicals, and their mixtures, as knowledge gaps remain for the sources and ecological effects of this stressor group in a multi-stressor context. Additionally, the multi-stressor datasets included physico-chemical, hydrological and morphological stressors.

Water quality showed dominating effects on the status of the three organism groups in each multi-stressor analysis. Especially, physico-chemical variables, including concentrations of nutrients, salt ions and oxygen, were often associated with the strongest biological responses. Furthermore, the analyses showed a high relevance of hydrological alterations, particularly of changes in the flow variability as well as the frequency and duration of high and low flow events. Differences were observed for the stressor responses of the three biological quality elements. This included stronger responses of macroinvertebrate and fish communities to the physical water quality (oxygen concentration), whereas benthic diatoms particularly responded to nutrient concentrations. Relative effects of the stressor groups also distinctly differed between the ecological metrics used to describe community characteristics, such as sensitivity, functional traits, community composition and biodiversity, and between individual fish species. Especially, sensitivity metrics (e.g. the Pollution Sensitivity Index on the basis of diatoms or the SPEARpest Index on the basis of macroinvertebrates, both responding to micropollutants) showed strong responses, which are well suited for stressor-specific diagnoses of biological deterioration. The percentage of urban areas and WWTP effluents were strongly associated with micropollutant concentrations and calculated ecotoxicological risks, especially for pharmaceuticals. Additionally, further water quality-related variables (oxygen and nutrient concentrations) and hydrological variables (flow variability and high flow frequency) were linked to WWTP effluents. In contrast, the influence of urban areas and WWTP effluents on pesticide levels were less pronounced but still evident for individual substances, for example biocides with applications in urban areas (e.g. facade paints). Stronger associations were found between pesticide concentrations and the percentage of agricultural land, especially when differentiating between individual crop types. Effects of the percentage of forests and grasslands in the catchments were negligible.

The multi-stressor analyses compiled additional evidence of the adverse effects of anthropogenic stressors, water quality deterioration and hydrological alteration in particular, on riverine ecosystems. The results highlight the need for additional management measures addressing both stressor groups to achieve a good ecological status. Furthermore, differences in stressor responses between organism groups and individual fish species indicate the influence of the choice of biological quality elements and metrics on the identification of relevant stressors. Ideally, diagnoses of biological deterioration and selection of targeted management measures should consider all three organism groups and different ecological metrics. For selected stressors (e.g., micropollutants) and organism groups (e.g., fishes), the development of additional stressor-specific metrics is recommended. To reduce individual stressor levels different sources need to be addressed in management plans, with advanced wastewater treatments potentially mitigating water quality deterioration associated with WWTP effluents, whereas additional measures, such as restoring riparian vegetation, are required to reduce diffuse pollution in agricultural areas. Moreover, the analyses revealed uncertainties in the assessment of micropollutants, such as limitations in the number and selection of substances and the frequency of grab samples used for the chemical monitoring, which may lead to an underestimation of ecological effects of this stressor group. Enhanced monitoring programs, particularly considering micropollutants and hydrological variables, may be implemented as part of the investigative monitoring to specifically analyze the stressor’s effects at selected sites to facilitate targeted diagnoses of the cause of biological deterioration and evidence-based developments of management measures to achieve and maintain a good ecological status by 2027 and beyond.