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.

River ecosystems are subject to multiple stressors that affect their structure and functioning. Ecosystem structure refers to characteristics such as channel form, water quality or the composition of biological communities, whereas ecosystem functioning refers to processes such as metabolism, organic matter decomposition or secondary production. Structure and functioning respond in contrasting and complementary ways to environmental stressors. Moreover, assessing the response of ecosystem functioning to stressors is critical to understand the effects on the ecosystem services that produce direct benefits to humans. Yet, there is more information on structural than on functional parameters, and despite the many approaches available to measure river ecosystem processes, structural approaches are more widely used, especially in management. One reason for this discrepancy is the lack of synthetic studies analyzing river ecosystem functioning in a way that is useful for both scientists and managers. Here, we present a synthesis of key river ecosystem processes, which provides a description of the main characteristics of each process, including criteria guiding their measurement as well as their respective sensitivity to stressors. We also discuss the current limitations, potential improvements and future steps that the use of functional measures in rivers needs to face.

Large dams regulate river hydrology and influence water chemistry, sediment dynamics, channel form and biotic communities. These effects may translate into important changes in river ecosystem processes, especially in rivers naturally subject to strong seasonality, such as those under Mediterranean climate. The effects of flow regulation on ecosystem metabolism [i.e. gross primary production (GPP) and ecosystem respiration (ER)] were analysed by means of open-stream measurements. Organic matter accrual and metabolism were measured in reaches upstream and downstream from large reservoirs in three tributaries of the Ebro River (NE Iberian Peninsula) during three sampling campaigns. Dams reduced downstream hydrological variability, dampened floods and increased the duration of interflood periods. Benthic organic matter increased twofold and chlorophyll-a eightfold. GPP increased by 59% on average, whereas ER increased by 75%. In general, flow regulation intensified the capacity of downstream river reaches to store and process materials and energy, therefore increasing the amount of organic carbon processed and altering the whole flux of materials and energy along the river continuumThis research was supported by the Spanish Ministry of Economy and Competitiveness through the project SCARCE Consolider-Ingenio CSD2009-00065. The compilation of environmental data was possible thanks to the Water Agencies of Ebro and the Water Agency of Catalonia. Laurie Boithias, Anna Giorgi, Lorea Flores and Lampros Nakis provided support in the field or in the laboratory. We also want to acknowledge financial support in terms of pre-doctoral 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-2009-05604 and jci-2010-06397) (D von Schiller) and a Marie Curie European Reintegration Grant (PERG07-GA-2010-259219) (V Acu~na). This work was partly supported by the Generalitat de Catalunya (Consolidated Research Group 2009 SGR-00759) and the Basque Government (Consolidated Research Group: Stream Ecology 7-CA-18/10

No abstract

Dams regulate downstream hydrology and modify water quality, which in turn can impinge on the biota, especially in rivers naturally subject to large hydrological variability, such as those under Mediterranean climate. The effect of dams on biofilms was analysed in three tributaries (Cinca, Siurana and Montsant) of the Ebro River (NE Spain). We hypothesized that flow regulation would lead to lower spatial variability of biofilms on the streambed and to a decrease in their metabolic rate per unit biomass, especially during low flow periods. Biofilm characteristics were studied in five transects evenly spaced along river reaches upstream (control) and downstream (impact) of dams in each river, along with riverbed granulometry, hydraulics and water chemistry. Chlorophyll-a, respiratory activity, photosynthetic capacity and efficiency, and extracellular enzymatic activities (β-d-glucosidase, alkaline phosphatase and leucine-amino-peptidase) of epilithic biofilms were measured in different seasons. Spatial variability of chemical and biological variables was reduced downstream of the dams. Chlorophyll-a concentration, photosynthetic efficiency and respiration capacity were higher in impact than in control reaches, but generally, low inorganic phosphorus concentrations resulted in comparable phosphatase activities downstream and upstream of dams. On the other hand, β-d-glucosidase and leucine-amino-peptidase activities were higher at impact reaches. Biofilms were thicker and metabolically more active at the impact reaches, with higher ability to transform dissolved organic matter. Overall, results from this study provide evidence that dams can largely affect the structure and activity of river biofilms, with foreseeable important consequences for river ecosystem functioningThis research was supported by Spanish Ministry of Economy and Competitiveness through the projects SCARCE (Consolider-Ingenio 2010 CSD2009-00065) and CARBONET (CGL2011-30474-C02-01). 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’ of the Spanish Ministry of Economy and Competitiveness (jci-2009-05604 and jci-2010-06397) (D von Schiller) and a Marie Curie European Reintegration Grant (PERG07-GA-2010-259219) (V Acuña