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SUMMARY1. The effluents of wastewater treatment plants (WWTPs) include a complex mixture of nutrients and pollutants. Nutrients can subsidise autotrophic and heterotrophic organisms, while toxic pollutants can act as stressors, depending, for instance, on their concentration and interactions in the environment. Hence, it is difficult to predict the overall effect of WWTP effluents on river ecosystem functioning. 2. We assessed the effects of WWTP effluents on river biofilms and ecosystem metabolism in one river segment upstream from a WWTP and three segments downstream from the WWTP and following a pollution gradient. 3. The photosynthetic capacity and enzymatic activity of biofilms showed no change, with the exception of leucine aminopeptidase, which followed the pollution gradient most likely driven by changes in organic matter availability. The effluent produced mixed effects on ecosystem-scale metabolism. It promoted respiration (subsidy effect), probably as a consequence of enhanced availability of organic matter. On the other hand, and despite enhanced nutrient concentrations, photosynthesis-irradiance relationships showed that the effluent partly decoupled primary production from light availability, thus suggesting a stress effect. 4. Overall, WWTP effluents can alter the balance between autotrophic and heterotrophic processes and produce spatial discontinuities in ecosystem functioning along rivers as a consequence of the mixed contribution of stressors and subsidisers.

Amultitude of pharmaceuticals enter surface waters via discharges of wastewater treatment plants (WWTPs), and many raise environmental and health concerns. Chemical fate models predict their concentrations using estimates of mass loading, dilution and in-stream attenuation. However, current comprehension of the attenuation rates remains a limiting factor for predictive models.We assessed in-stream attenuation of 75 pharmaceuticals in 4 river segments, aiming to characterize in-stream attenuation variability among different pharmaceutical compounds, as well as among river segments differing in environmental conditions. Our study revealed that in-streamattenuation was highly variable among pharmaceuticals and river segments and that none of the considered pharmaceutical physicochemical and molecular properties proved to be relevant in determining the mean attenuation rates. Instead, the octanol–water partition coefficient (Kow) influenced the variability of rates among river segments, likely due to its effect on sorption to sediments and suspended particles, and therefore influencing the balance between the different attenuation mechanisms (biotransformation, photolysis, sorption, and volatilization). The magnitude of themeasured attenuation rates urges scientists to consider themas important as dilutionwhen aiming to predict concentrations in freshwater ecosystemsThis research was supported by aMarie Curie European Reintegration Grant (PERG07-GA-2010-259219) and by a Marie Curie Career Integration Grant (PCIG9-GA-2011-293535) within the 7th European Community Framework Programme, aswell as by the SpanishMinistry of Economy and Competitiveness through the projects SCARCE (Consolider-Ingenio 2010 CSD2009-00065) and ENDERUS (CTM-2009-13018), and the postdoctoral grants “Juan de la Cierva” (jci-2009-05604 and jci-2010- 06397), and by the European Union through the European Regional Development Fund (FEDER). This work was partly supported by the Generalitat de Catalunya (Consolidated Research Group: Water and Soil Quality Unit 2009-SGR-965

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