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.

The interactive effects of climate change and land use on boreal stream fish communities

Ongoing and projected climate change is likely to greatly alter co-occurring stressor mechanisms, yet these potential interactions remain poorly understood in natural freshwater systems worldwide. As the global biodiversity crisis deepens, successful conservation efforts will hinge on developing mechanistic multiple stressor frameworks that have been ground-truthed in natural systems containing complex species dynamics and ecological processes. Our study examined the combined and interacting effects of potential climate and land use stressors on boreal stream fishes using data from over 300 catchments across a broad 250,000 km² region. To characterize boreal fish community health, we examined four indicators including species richness, total catch per unit effort, the proportion of lithophilic spawners (fish sensitive to sedimentation), and the assemblage tolerance index which provides a measurement of the overall community tolerance to disturbance. Land use stressors included total anthropogenic land use area and linear disturbance at multiple watershed scales as well as two site-specific habitat degradation indicators (dissolved oxygen and the proportion of fine substrate). Overall community richness and productivity were not negatively related to land use changes indicating potential compensatory dynamics (e.g. where intolerant species are replaced with more tolerant species as habitat quality degrades). In contrast, we observed declines for sensitive species, including highly valued salmonids, that varied depending on interactions between local climate, land use, and stream type. Sensitive species declines were concentrated in regions experiencing increased land use and warming, whereas increases were observed in cooler regions consistent with a subsidy-stress response. In addition, lithophilic spawners declined in watersheds experiencing warmer and wetter conditions owing to potential indirect effects on spawning habitat quality. Results from our study provide novel insight into complex climate and land use interactions occurring across a broad, real-world landscape, and highlight the potential for amplified species declines under future warming and land use scenarios.

The response patterns of stream biofilms to urban sewage change with exposure time and dilution

Urban wastewater inputs are a relevant pollution source to rivers, contributing a complex mixture of nutrients, organic matter and organic microcontaminants to these systems. Depending on their composition, WWTP effluents might perform either as enhancers (subsidizers) or inhibitors (stressors) of biological activities. In this study, we evaluated in which manner biofilms were affected by treated urban WWTP effluent, and how much they recovered after exposure was terminated. We used indoor artificial streams in a replicated regression design, which were operated for a total period of 56 days. During the first 33 days, artificial streams were fed with increasing concentration of treated effluents starting with non-contaminated water and ending with undiluted effluent. During the recovery phase, the artificial streams were fed with unpolluted water. Sewage effluents contained high concentrations of personal care products, pharmaceuticals, nutrients, and dissolved organic matter. Changes in community structure, biomass, and biofilm function were most pronounced in those biofilms exposed to 58% to 100% of WWTP effluent, moving from linear to quadratic or cubic response patterns. The return to initial conditions did not allow for complete biofilm recovery, but biofilms from the former medium diluted treatments were the most benefited (enhanced response), while those from the undiluted treatments showed higher stress (inhibited response). Our results indicated that the effects caused by WWTP effluent discharge on biofilm structure and function respond to the chemical pressure only in part, and that the biofilm dynamics (changes in community composition, increase in thickness) imprint particular response pathways over time.

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.

Testing wastewater treatment plant effluent effects on microbial and detritivore performance: A combined field and laboratory experiment

The amount of pollutants and nutrients entering rivers via point sources is increasing along with human population and activity. Although wastewater treatment plants (WWTPs) greatly reduce pollutant loads into the environment, excess nutrient loading is a problem in many streams. Using a Community and Ecosystem Function (CEF) approach, we quantified the effects of WWTP effluent on the performance of microbes and detritivores associated to organic matter decomposition, a key ecosystem process. We measured organic matter breakdown rates, respiration rates and exo-enzymatic activities of aquatic microbes. We also measured food consumption and growth rates and RNA to body-mass ratios (RNA:BM) of a dominant amphipod Echinogammarus berilloni. We predicted responses to follow a subsidy-stress pattern and differences between treatments to increase over time. To examine temporal effects of effluent, we performed a laboratory microcosm experiment under a range of effluent concentrations (0, 20, 40, 60, 80 and 100%), taking samples over time (days 8, 15 and 30; 4 and 10 replicates to assess microbe and detritivore performance respectively, per treatment and day). This experiment was combined with a field in situ Before-After Control-Impact Paired (BACIP) experiment whereby we added WWTP effluent poured (10 L s^-1 during 20-40 min every 2 h) into a stream and collected microbial and detritivore samples at days 8 and 15 (5 and 15 replicates to assess the microbe and detritivore performance respectively, per period, reach and sampling day). Responses were clearer in the laboratory experiment, where the effluent caused a general subsidy response. Field measures did not show any significant response, probably because of the high dilution of the effluent in stream water (average of 1.6%). None of the measured variables in any of the experiments followed the predicted subsidy-stress response. Microbial breakdown, respiration rates, exo-enzymatic activities and invertebrate RNA:BM increased with effluent concentrations. Differences in microbial respiration and exo-enzymatic activities among effluent treatments increased with incubation time, whereas microbial breakdown rates and RNA:BM were consistent over time. At the end of the laboratory experiment, microbial respiration rates increased 156% and RN:BM 115% at 100% effluent concentration. Detritivore consumption and growth rates increased asymptotically, and both responses increased with by incubation time. Our results indicate that WWTP effluent stimulates microbial activities and alters detritivore performance, and stream water dilution may mitigate these effects.