Following the Mixtures of Organic Micropollutants with Bioassays in a Large Lowland River from Source to Sea.

Human-impacted rivers often contain a complex mixture of organic micropollutants, including pesticides, pharmaceuticals and industrial compounds, along with their transformation products. Combining chemical target analysis for exposure with bioassays for effect assessment offers a holistic view of water quality. This study targeted the River Elbe in Central Europe, known for its anthropogenic pollution exposure, to obtain an inventory of micropollutant contamination during base flow and to identify hotspots of contamination. We identified tributaries as sources of chemicals activating the aryl hydrocarbon receptor quantified with the AhR-CALUX assay, including historically contaminated tributaries and a newly identified Czech tributary. Increased neurotoxicity, detected by differentiated SH-SY5Y neurons' cytotoxicity and shortened neurite length, was noted in some Czech tributaries. A hotspot for chemicals activating the oxidative stress response in the AREc32 assay was found in the middle Elbe in Germany. An increase in oxidative stress inducing chemicals was observed in the lower Elbe. While effect-based trigger values (EBT) for oxidative stress response, xenobiotic metabolism and neurotoxicity were not exceeded, estrogenicity levels surpassed the EBT in 14% of surface water samples, posing a potential threat to fish reproduction. Target analysis of 713 chemicals resulted in the quantification of 487 micropollutants, of which 133 were active in at least one bioassay. Despite this large number of bioactive quantified chemicals, the mixture effects predicted by the concentrations of the quantified bioactive chemicals and their relative effect potency explained only 0.002-1.2% of the effects observed in the surface water extracts, highlighting a significant unknown fraction in the chemical mixtures. This case study established a baseline for understanding pollution dynamics and spatial variations in the Elbe River, offering a comprehensive view of potential chemical effects in the water and guiding further water quality monitoring in European rivers.