Combined effects of urban pollution and hydrological stress on ecosystem functions of Mediterranean streams.

Urban pollution and hydrological stress are common stressors of stream ecosystems, but their combined effects on ecosystem functioning are still unclear. We measured a set of functional processes and accompanying environmental variables in locations upstream and downstream of urban sewage inputs in 13 streams covering a wide range of water pollution levels and hydrological variability. Sewage inputs seriously impaired stream chemical characteristics and led to complex effects on ecosystem functioning. Biofilm biomass accrual, whole-reach nutrient uptake and metabolism (ecosystem respiration) were generally subsidized, whereas organic matter decomposition and biofilm phosphorus uptake capacity decreased with increasing pollutant concentrations. Hydrological stress affected stream ecosystem functioning but its effect was minor compared to the effects of urban pollution, due to the large inter-site variability of the streams. Changes appeared mainly linked to the concentration of pharmaceutically active compounds, followed by other chemical characteristics and by hydrology. The results point to the need to further improve sewage treatment, especially as climate change will stress riverine organisms and reduce the dilution capacity of the receiving streams.

Impact of wastewater effluent pollution on stream functioning: A whole-ecosystem manipulation experiment.

The ecological effects of wastewater treatment plant (WWTP) effluents on stream ecosystems cause growing concern. However, it is difficult to assess these effects as most streams receiving WWTP effluents are also affected by other stressors. We performed a whole-ecosystem manipulation experiment following a BACI design (Before-After/Control-Impact) in order to exclude the influence of other potentially confounding factors. We diverted part of the effluent of a large tertiary urban WWTP into a small, unpolluted stream, and studied its effects on ecosystem structure and functioning over two years (i.e., one year before and one year after the effluent diversion). Although highly diluted (final concentration in the receiving stream averaged 3%), the effluent promoted biofilm chlorophyll-a and biomass (2.3 and 2.1 times, respectively), exo-enzymatic activities (phosphatase 2.2 and glucosidase 4.2 times) and invertebrate-mediated organic matter decomposition (1.4 times), but reduced phosphorus uptake capacity of the epilithic biofilm down to 0.5 of the initial values. Biofilm metabolism, reach-scale nutrient uptake and microbially-mediated organic matter decomposition were not affected. Our results indicate that even well treated and highly diluted WWTP effluents can also affect the structure of the biofilm community and stream ecosystem functioning.

Immediate and legacy effects of urban pollution on river ecosystem functioning: A mesocosm experiment.

Effluents from urban wastewater treatment plants (WWTP) consist of complex mixtures of substances that can affect processes in the receiving ecosystems. Some of these substances (toxic contaminants) stress biological activity at all concentrations, while others (e.g., nutrients) subsidize it at low concentrations and stress it above a threshold, causing subsidy-stress responses. Thus, the overall effects of WWTP effluents depend mostly on their composition and the dilution capacity of the receiving water bodies. We assessed the immediate and legacy effects of WWTP effluents in artificial streams, where we measured the uptake of soluble reactive phosphorus (SRP) by the biofilm, biomass accrual, benthic metabolism and organic matter decomposition (OMD). In a first phase (32 d), the channels were subjected to a gradient of effluent contribution, from pure stream water to pure effluent. WWTP effluent affected the ecosystem processes we measured, although we found no clear subsidy-stress patterns except for biofilm biomass accrual. Instead, most of the processes were subsidized, although they showed complex and process-specific patterns. Benthic metabolism and OMD were subsidized without saturation, as they peaked at medium and high levels of pollution, respectively, but they never fell below control levels. SRP uptake was the only process that decreased with increasing effluent concentration. In a second phase of the experiment (23 d), all channels were kept on pure stream water to analyse the legacy effects of the effluent. For most of the processes, there were clear legacy effects, which followed either subsidy, stress, or subsidy-stress patterns. SRP uptake capacity was stressed with increasing pollution legacy, whereas algal accrual and benthic metabolism continued being subsidized. Conversely, biofilm biomass accrual and OMD showed no legacy effects. Overall, the WWTP effluent caused complex and process-specific responses in our experiment, mainly driven by the mixed contribution of subsidizers and stressors. These results help improving our understanding of the effects of urban pollution on stream ecosystem functioning.

River ecosystem processes: A synthesis of approaches, criteria of use and sensitivity to environmental stressors.

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.