Under pressure: assessment of chemical stress on restored river sections using effect‐based methods

Aquatic ecosystems are affected by multiple stressors, including hydrological and morphological degradation, high nutrient loading, and chemical pollution. To improve freshwater habitats, hydromorphological restorations have been increasingly implemented. However, follow‐up assessments often show little to no improvement in ecological status, even years after restoration measures have been implemented. The success of restoration projects can be compromised by other stressors, such as insufficient water and sediment quality, which often receive less attention compared to nonchemical stressors. In this study, the impact of chemical stress on the outcome of five river restorations was evaluated ecologically, chemically, and ecotoxicologically. Overall, the habitat structure was considerably improved through the restoration measures, whereas the species communities did not show a consistent trend toward an improved ecological status. Effect‐based methods were used for an integrative assessment of the exposure to chemical mixtures in water and sediment samples of restored stream sections. Differences in toxicity between restored and non‐restored sections were found but did not show a consistent trend among the applied assays. In contrast, the chemical analysis showed that the sections of the same stream were similar in their chemical composition, and differences within a stream were primarily due to sediment contamination. The results of this study suggest that chemical pollution is a relevant factor preventing the success of restoration measures and, ultimately, the improvement of the ecological status of rivers. They also demonstrate the applicability of EBMs in water quality monitoring to detect mixture toxicity in streams and link chemical and ecological assessment.