he amount of wastewater processed in treatment plants is increasing following more strict environmental regulations. Treatment facilities are implementing upgrades to abate the concentrations of nutrients and contaminants and, thus, reduce their effects on receiving systems. Although many studies characterized the chemical composition and ecotoxicological effects of treated wastewater, its environmental effects are still poorly known, as receiving water bodies are often subjected to other stressors. We performed a field manipulative experiment to measure the response of invertebrate assemblages to one year of tertiary-treated wastewater discharges. We poured treated wastewater from an urban wastewater treatment plant into the lower-most 100-m of a previously unpolluted stream (3.6 % daily flow on average) while using another upstream reach as control. The positive correlation between effect sizes of abundance changes and IBMWP scores suggested assemblage modifications were following taxa tolerance to ecological impairment. The treatment increased the temporal variability of SPEARorganic, EPT relative abundance, and invertebrate functional redundancy. Our results show that even in this best-case scenario of tertiary-treated and highly diluted wastewater, the abundance of the most sensitive taxa in the aquatic assemblages is reduced. Further improvements in wastewater treatments seem necessary to ensure these effluents do not modify receiving water ecosystems.We greatly appreciate the kind and continuous support provided by all the Apraitz WWTP operators before and during fieldwork. We also thank many volunteers from the University of the Basque Country (UPV/EHU) and the University Rey Juan Carlos for their assistance with fieldwork and laboratory analyses. This research was part of the 603629-ENV-2013-6.2.1 (GLOBAQUA) project funded by the European Community's Seventh Framework Programme. We also acknowledge the financial support from the Basque Government (Consolidated Research Group: Stream Ecology 7-CA-18/10). Ioar de Guzmán was supported by a pre-doctoral fellowship from the Basque Government
