Effects of pollution and water diversion on the organization and functioning of stream food webs

216 p.La contaminación y la detracción de agua son dos de los factores de estrés más frecuentes que afectan a los ecosistemas de agua dulces y que a menudo aparecen de forma simultánea. Los múltiples estresores que afectan a un sistema pueden interactuar y generar efectos complejos amplificando o mitigando el efecto individual de cada uno de ellos, por lo que se han convertido en motivo de gran preocupación dadas las alteraciones que pueden inducir en la estructura y el funcionamiento de los ecosistemas. En esta disertación se exploran los efectos de la detracción de agua y la contaminación en las redes tróficas fluviales mediante la combinación de dos estudios de campo. En el experimento, se analizaron los impactos de los dos estresores, en la estructura y complejidad de las redes tróficas fluviales, seleccionando cuatro ríos que difieren en la calidad del agua, y que constan de un esquema de desvío de agua similar. Utilizando dos metodologías diferentes (i.e. análisis de isótopos estables y flujos de energía de las redes tróficas), se compararon las redes tróficas aguas arriba y aguas abajo de las presas teniendo en cuenta el gradiente de contaminación de los ríos. Cada estresor indujo diferentes cambios en la base de la red trófica: la contaminación aumentó la disponibilidad del biofilm y la detracción de agua redujo la cantidad de detritus grueso aguas abajo de laspresas. En ambos capítulos, la vía marrón mostró una respuesta negativa consistente, disminuyendo con la reducción del stock de detritus. Sin embargo, al contrario de lo que esperábamos, la relevancia de la red trófica verde no aumentó a lo largo del gradiente de contaminación. La comunidad de invertebrados no mostró grandes variaciones en lo que a densidad y diversidad se refiere con el aumento de la contaminación, aunque las comunidades se volvieron más homogéneas a lo largo del gradiente. Sin embargo, la diversidad trófica de los taxones comunes en el área de estudio aumentó, indicando la transición a una dieta más generalista. Las alteraciones observadas aumentaron en presencia de ambos estresores, lo que indica que la contaminación exacerbó los efectos de la desviación de agua. En el segundo experimento, se realizó una manipulación de todo el ecosistema para abordar los efectos de la contaminación puntual en las propiedades de las redes tróficas. Mediante un diseño experimental BACI, se detectaron las alteraciones inducidas por un efluente debidamente tratado siguiendo las dos metodologías anteriormente mencionadas. Los cambios en la base de la red trófica fueron evidentes tras la adición del efluente. El aumento de la cantidad de biofilm promovió los flujos hacia la vía verde. Sin embargo, a pesar de éste aumento, se observó una reducción de la vía marrón al mismo tiempo que una disminución de los flujos energéticos totales y de la diversidad trófica. Estas respuestas fueron coherentes con la disminución observada en la densidad de invertebrados y en la riqueza de taxones, lo que también condujo a un aumento de la heterogeneidad. Esta tesis demuestra que los estresores estudiados producen claros cambios en la base de la red trófica, y que la contaminación puede potenciar los efectos de la detracción de agua en ellas. Además, cabe resaltar que un mismo estresor puede producir patrones opuestos en la organización de las redes tróficas, lo que podría deberse a las diferentes fuerzas de interacción entre los componentes de las mismas

Tertiary wastewater treatment combined with high dilution rates fails to eliminate impacts on receiving stream invertebrate assemblages

he amount of wastewater processed in treatment plants is increasing following more strict environmental regulations. Treatment facilities are implementing upgrades to abate the concentrations of nutrients and contaminants and, thus, reduce their effects on receiving systems. Although many studies characterized the chemical composition and ecotoxicological effects of treated wastewater, its environmental effects are still poorly known, as receiving water bodies are often subjected to other stressors. We performed a field manipulative experiment to measure the response of invertebrate assemblages to one year of tertiary-treated wastewater discharges. We poured treated wastewater from an urban wastewater treatment plant into the lower-most 100-m of a previously unpolluted stream (3.6 % daily flow on average) while using another upstream reach as control. The positive correlation between effect sizes of abundance changes and IBMWP scores suggested assemblage modifications were following taxa tolerance to ecological impairment. The treatment increased the temporal variability of SPEARorganic, EPT relative abundance, and invertebrate functional redundancy. Our results show that even in this best-case scenario of tertiary-treated and highly diluted wastewater, the abundance of the most sensitive taxa in the aquatic assemblages is reduced. Further improvements in wastewater treatments seem necessary to ensure these effluents do not modify receiving water ecosystems.We greatly appreciate the kind and continuous support provided by all the Apraitz WWTP operators before and during fieldwork. We also thank many volunteers from the University of the Basque Country (UPV/EHU) and the University Rey Juan Carlos for their assistance with fieldwork and laboratory analyses. This research was part of the 603629-ENV-2013-6.2.1 (GLOBAQUA) project funded by the European Community's Seventh Framework Programme. We also acknowledge the financial support from the Basque Government (Consolidated Research Group: Stream Ecology 7-CA-18/10). Ioar de Guzmán was supported by a pre-doctoral fellowship from the Basque Government

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.This work has been supported by the EU7th Framework Programme Funding under Grant agreement no. 603629-ENV-2013-6.2.1-Globaqua. We also acknowledge financial support in terms of pre doctoral grants from the University of the Basque Country UPV/EHU (L. Solagaistua) and the Basque Government (I. de Guzman, L. Mijangos). The manuscript benefited greatly from the valuable comments of John Kominoski and two anonymous referees. Also SGIker technical and human support (UPV/EHU, MICINN, GV/EJ, ESF) is gratefully acknowledged

Water diversion and pollution interactively shape freshwater food webs through bottom-up mechanisms

[EN] Water diversion and pollution are two pervasive stressors in river ecosystems that often co-occur. Individual effects of both stressors on basal resources available to stream communities have been described, with diversion reducing detritus standing stocks and pollution increasing biomass of primary producers. However, interactive effects of both stressors on the structure and trophic basis of food webs remain unknown. We hypothesized that the interaction between both stressors increases the contribution of the green pathway in stream food webs. Given the key role of the high-quality, but less abundant, primary producers, we also hypothesized an increase in food web complexity with larger trophic diversity in the presence of water diversion and pollution. To test these hypotheses, we selected four rivers in a range of pollution subject to similar water diversion schemes, and we compared food webs upstream and downstream of the diversion. We characterized food webs by means of stable isotope analysis. Both stressors directly changed the availability of basal resources, with water diversion affecting the brown food web by decreasing detritus stocks, and pollution enhancing the green food web by promoting biofilm production. The propagation of the effects at the base of the food web to higher trophic levels differed between stressors. Water diversion had little effect on the structure of food webs, but pollution increased food chain length and trophic diversity, and reduced trophic redundancy. The effects at higher trophic levels were exacerbated when combining both stressors, as the relative contribution of biofilm to the stock of basal resources increased even further. Overall, we conclude that moderate pollution increases food web complexity and that the interaction with water abstraction seems to amplify this effect. Our study shows the importance of assessing the interaction between stressors to create predictive tools for a proper management of ecosystems.Ministerio de Economia, Industria y Competitividad, Gobierno de Espana, Grant/Award Number: GL2016-77487-R; European Social Fund; Diputacion Foral de Bizkaia; Serra Hunter Fellow; Labex, Grant/Award Number: ANR-10-LABX-41; H2020 European Research Council; Eusko Jaurlaritza; Consejo Nacional de Investigaciones Cientificas y Tecnicas; FRAGCLIM Consolidator, Grant/Award Number: 72617