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. 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. 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. 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

This research was supported by the Spanish Ministry of Economy and Competitiveness and FEDER foundings through the SCARCE Consolider-Ingenio CSD2009-00065 and ABSTRACT CGL2012-35848 projects, and the European Communities 7th Framework Programme Funding under Grant agreement no. 603629-ENV-2013-6.2.1-Globaqua. We especially thank the people who assisted in the field and in the laboratory. A thorough review of the manuscript by Prof. Roger I. Jones and two unknown reviewers is deeply appreciated too. We also want to acknowledge financial support in terms of predoctoral grants from the University of the Basque Country (I. Aristi), the Basque Government (M. Arroita), as well as a postdoctoral grant ‘Juan de la Cierva’ (jci-2010-06397) (D. von Schiller) and a Marie Curie European Reintegration Grant (PERG07-GA-2010-259219) (V. Acuña). This work was partly supported by the Basque Government (Consolidated Research Group: Stream Ecology 7-CA-18/10)