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Intro -- Title page -- Table of Contents -- Copyright -- Contributors -- Preface -- Chapter 1: Defining Multiple Stressor Implications -- Abstract -- Acknowledgments -- 1.1 Global Environmental Change: A Source of Stress -- 1.2 Stress, Stressor, Receptor, and Effect: Context and Evolution of the Terms -- 1.3 Types of Stressors According to Their Energy and Frequency -- 1.4 Multiple Stressors in the Literature -- 1.5 Studying the Effects of Multiple Stressors -- 1.6 Ecosystem Responses to Multiple Stressors -- 1.7 Joint Implications of Multiple Stressors for Social and Ecological Systems -- 1.8 Status and Relevance of Multiple Stressors on Water Policies: A Brief Overview -- 1.9 Reading This Book -- Chapter 2: Climate Change and Interactions With Multiple Stressors in Rivers -- Abstract -- Acknowledgments -- 2.1 Introduction -- 2.2 Observations and Predictions of Climate Change in Rivers -- 2.3 Climate Change in Rivers: Similarities to Existing Stressors and Surprises -- 2.4 Vulnerability of River Ecosystems: An Example of Drought as a Manifestation of Climate Change in Addition to Other Stressors -- 2.5 Concluding Remarks -- Chapter 3: Understanding the Nexus Between Hydrological Alteration And Biological Invasions -- Abstract -- Acknowledgments -- 3.1 Altered Flow Regimes and Biological Invasions: Two Facets of Global Change in River Ecosystems -- 3.2 The Alteration-Invasion Nexus -- 3.3 Effects of the Alteration-Invasion Nexus on Riverine Populations and Communities -- 3.4 Effects of the Alteration-Invasion Nexus on Riverine Food Webs -- 3.5 The Alteration-Invasion Nexus as a Freshwater Conservation Challenge -- Chapter 4: Multiple Stressors and Hydromorphological Degradation -- Abstract -- 4.1 Introduction -- 4.2 Interactions Among Hydrological and Morphological Stressors

Spatial heterogeneity along river networks is interrupted by dams, affecting the transport, processing, and storage of organic matter, as well as the distribution of biota. We here investigated the structure of planktonic (free-living, FL), particle-attached (PA) and sediment-associated (SD) bacterial and archaeal communities within a small reservoir. We combined targeted-amplicon sequencing of bacterial and archaeal 16S rRNA genes in the DNA and RNA community fractions from FL, PA and SD, followed by imputed functional metagenomics, in order to unveil differences in their potential metabolic capabilities within the reservoir (tail, mid, and dam sections) and lifestyles (FL, PA, SD). Both bacterial and archaeal communities were structured according to their life-style preferences rather than to their location in the reservoir. Bacterial communities were richer and more diverse when attached to particles or inhabiting the sediment, while Archaea showed an opposing trend. Differences between PA and FL bacterial communities were consistent at functional level, the PA community showing higher potential capacity to degrade complex carbohydrates, aromatic compounds, and proteinaceous materials. Our results stressed that particle-attached prokaryotes were phylogenetically and metabolically distinct from their free-living counterparts, and that performed as hotspots for organic matter processing within the small reservoir ; Tis study was funded by the project of the Spanish Ministry of Economy and Competitiveness SPACESTREAM (CGL2017-88640-C2-1-R). The authors acknowledge the support from the Economy and Knowledge Department of the Catalan Government through Consolidated Research Group (ICRA-ENV 2017 SGR 1124), and to R. Marcé, D.von Schiller, C. Gutierrez, J. P. Casas-Ruiz, L. Gómez-Gener, V. Acuña and B. Obrador for their help during the sampling. ICRA is part of the CERCA program

Attributing changes in river water quality to specific factors is challenging because multiple factors act at different temporal and spatial scales, and it often requires examining long-term series of continuous data. But data consistency is sometimes hindered by the lack of observations of relevant water quality variables and the low and uneven sampling frequency that characterize many water quality monitoring schemes. Nitrate and dissolved phosphate concentration time-series (1980–2011) from 50 sampling stations across a large Mediterranean river basin were analyzed to disentangle the role of hydrology, land-use practices, and global climatic phenomena on the observed nutrient patterns, with the final aim of understanding how the different aspects of global change affected nutrient dynamics in the basin. Dynamic Factor Analysis (DFA) provided the methodological framework to extract underlying common patterns in nutrient time-series with missing observations. Using complementary methods such as frequency and trend analyses, we sought to further characterize the common patterns and identify the drivers behind their variability across time and space. Seasonal and other cyclic patterns were identified, as well as trends of increase or decrease of nutrient concentration in particular areas of the basin. Overall, the impact of global change, which includes both climate change and anthropogenic impacts, on the dynamics of nitrate concentration across the study basin was found to be a multifaceted process including regional and global factors, such as climatic oscillations and agricultural irrigation practices, whereas impacts on phosphate concentration seemed to depend more on local impacts, such as urban and industrial activities, and less on large-scale factors ; This work was supported by Spanish Ministry of Economy and Competitiveness through the project SCARCE (Consolider-Ingenio 2010 CSD2009-00065) and by a Doctoral Grant (FI-DGR 2012) from the Catalan Government

Attributing changes in river water quality to specific factors is challenging because multiple factors act at different temporal and spatial scales, and it often requires examining long-term series of continuous data. But data consistency is sometimes hindered by the lack of observations of relevant water quality variables and the low and uneven sampling frequency that characterize many water quality monitoring schemes. Nitrate and dissolved phosphate concentration time-series (1980–2011) from 50 sampling stations across a large Mediterranean river basin were analyzed to disentangle the role of hydrology, land-use practices, and global climatic phenomena on the observed nutrient patterns, with the final aim of understanding how the different aspects of global change affected nutrient dynamics in the basin. Dynamic Factor Analysis (DFA) provided the methodological framework to extract underlying common patterns in nutrient time-series with missing observations. Using complementary methods such as frequency and trend analyses, we sought to further characterize the common patterns and identify the drivers behind their variability across time and space. Seasonal and other cyclic patterns were identified, as well as trends of increase or decrease of nutrient concentration in particular areas of the basin. Overall, the impact of global change, which includes both climate change and anthropogenic impacts, on the dynamics of nitrate concentration across the study basin was found to be a multifaceted process including regional and global factors, such as climatic oscillations and agricultural irrigation practices, whereas impacts on phosphate concentration seemed to depend more on local impacts, such as urban and industrial activities, and less on large-scale factors ; This work was supported by Spanish Ministry of Economy and Competitiveness through the project SCARCE (Consolider-Ingenio 2010 CSD2009-00065) and by a Doctoral Grant (FI-DGR 2012) from the Catalan Government

Attributing changes in river water quality to specific factors is challenging because multiple factors act at different temporal and spatial scales, and it often requires examining long-term series of continuous data. But data consistency is sometimes hindered by the lack of observations of relevant water quality variables and the low and uneven sampling frequency that characterize many water quality monitoring schemes. Nitrate and dissolved phosphate concentration time-series (1980–2011) from 50 sampling stations across a large Mediterranean river basin were analyzed to disentangle the role of hydrology, land-use practices, and global climatic phenomena on the observed nutrient patterns, with the final aim of understanding how the different aspects of global change affected nutrient dynamics in the basin. Dynamic Factor Analysis (DFA) provided the methodological framework to extract underlying common patterns in nutrient time-series with missing observations. Using complementary methods such as frequency and trend analyses, we sought to further characterize the common patterns and identify the drivers behind their variability across time and space. Seasonal and other cyclic patterns were identified, as well as trends of increase or decrease of nutrient concentration in particular areas of the basin. Overall, the impact of global change, which includes both climate change and anthropogenic impacts, on the dynamics of nitrate concentration across the study basin was found to be a multifaceted process including regional and global factors, such as climatic oscillations and agricultural irrigation practices, whereas impacts on phosphate concentration seemed to depend more on local impacts, such as urban and industrial activities, and less on large-scale factors ; This work was supported by Spanish Ministry of Economy and Competitiveness through the project SCARCE (Consolider-Ingenio 2010 CSD2009-00065) and by a Doctoral Grant (FI-DGR 2012) from the Catalan Government

4 páginas, 2 tablas, 1 figura. ; In situ denitrification (DNT) and denitrification enzyme activity (DEA) were measured in a Mediterranean riparian forest soil during two periods under contrasting soil moisture conditions in order to investigate the factors that affect denitrification through the year. Results showed that in summer, soil moisture limited denitrification throughout the entire soil profile, whereas in winter, anaerobic conditions in the soil were more favourable for denitrifiers. The potential for denitrification was larger at shallow depths (<30 cm), and neither nitrate nor organic carbon limited denitrification significantly. Some denitrification was measured during winter at depths below 30 cm, suggesting that a reduction of groundwater nitrate could occur in some areas of this riparian forest during the wet period. In summer, low denitrification, together with high mineralization rates, brought about an increase of soil N, which could be leached to the stream channel during rainfall events. This study suggests that Mediterranean riparian soils act as sources or sinks of dissolved nitrogen depending on the period of the year. ; This study was supported by funds provided by the European Community (NICOLAS Project ENV4-CT97-0395), and S.B.'s work was supported by a Formación de Personal Investigador (FPI) grant from the Spanish Government. ; Peer reviewed

Many rivers in the Mediterranean region are regulated for urban and agricultural purposes. Reservoir presence and operation results in flow alteration and sediment discontinuity, altering the longitudinal structure of the fluvial system. This study presents a 3-year sediment budget of a highly dammed Mediterranean river (the Muga, southern Pyrenees), which has experienced flow regulation since the 1969 owing to a 61-hm3 reservoir. Flow discharge and suspended sediment concentration were monitored immediately upstream and downstream from the reservoir, whereas bedload transport was estimated by means of bedload formulae and estimated from regional data. Results show how the dam modifies river flow, reducing the magnitude of floods and shortening its duration. At the same time, duration of low flows increases. The downstream flow regime follows reservoir releases that are mostly driven by the irrigation needs in the lowlands. Likewise, suspended sediment and bedload transport are shown to be notably affected by the dam. Sediment transport upstream was mainly associated with floods and was therefore concentrated in short periods of time (i.e.,> 90% of the sediment load occurred in< 1% of the time). Downstream from the dam, sediments were transported more constantly (i.e., 90% of the load was carried during 50% of the time). Total sediment load upstream from the dam equalled 23,074 t, while downstream it was< 1000 t. Upstream, sediment load was equally distributed between suspension and bedload (i.e., 10,278 and 12,796 t respectively), whereas suspension dominated sediment transport downstream. More than 95% of the sediments transported from the upstream basins were trapped in the reservoir, a fact that explains the sediment deficit and the river bed armouring observed downstream. Overall, the dam disrupted the natural water and sediment fluxes, generating a highly modified environment downstream. Below the dam, the whole ecosystem shifted to stable conditions owing to the reduction of water and sediment loads. ; This research was developed in the frame of the project 'SCARCE Consolider Ingenio 2010 CSD2009-00065(2009-2014)' funded by the Spanish Ministry of Economy and Competitiveness. Authors acknowledge the support from the Economy and Knowledge Department of the Catalan Government through the Consolidated Research Groups: Fluvial Dynamics Research Group (2014 SGR 645) and the Catalan Institute for Water Research (2014 SGR 291). The authors would like to thank the Catalan Water Agency for allowing the installation of measuring equipment in the Muga gauging stations and for providing hydrological data, as well as the Meteorological Service of Catalonia and two private owners for supplying precipitation data. Part of the water stage and turbidity probes was funded jointly by the European Regional Development Fund and the Spanish Ministry of Science and Innovation.

We analyzed the conjoint effects of sewage inputs and hydrological alteration on the occurrence of teratological forms and on the assemblage composition of stream benthic diatoms. The study was performed in 11 Mediterranean streams which received treated or untreated urban sewage (Impact sites, I), whose composition and morphological anomalies were compared to upstream unaffected (Control, C) sites. The impact sites had high concentrations of ammonium, phosphorus, and pharmaceutical compounds (antibiotics, analgesics, and anti-inflammatories), particularly in those receiving untreated sewage. Impact sites had a higher proportion of teratological forms as well as a prevalence of diatom taxa tolerant to pollution. The differences in the diatom assemblage composition between the paired C and I sites were the largest in the impacted sites that received untreated sewage inputs as well as in the systems with lower dilution capacity. In these sites, the diatom assemblage was composed by a few pollution-tolerant species. Mediterranean river systems facing hydrological stress are highly sensitive to chemical contamination, leading to the homogenization of their diatom assemblages. Hydrologically stressed streams are highly sensitive to chemical contamination, causing the homogenization of diatom assemblages ; This study has been financially supported by the EU through the FP7 project GLOBAQUA (Grant agreement No 603629). Authors acknowledge the support from the Economy and Knowledge Department of the Catalan Government through Consolidated Research Group (ICRA-ENV 2017 SGR 1124)

Pollutants' dynamics in rivers flowing through industrial areas is linked to the entrainment and transport of contaminants attached to solid particles. The transport of sediment is mainly associated to high discharges and flood episodes, and these events constitute one of the main factors causing fluxes of buried pollutants in rivers. We performed a field study in the lower River Cinca (Ebro basin, Northeast Spain) to quantify the mobilization and transfer of several contaminants present in the river bed sediments. We focused on contaminants previously identified (PCBs, DDXs, PBDEs, HBCDs and PFRs) to occur in the river. River bed sediment samples were collected during low flows and a subsequent sampling campaign was used to capture sediment borne contaminants during a flood event. Water samples were taken at the same locations as the static sediment samples and used to determine the suspended sediment concentrations and the contaminants content (i.e. mass of contaminant per sediment mass unit) during the event. We estimated mass fluxes for both sediment and pollutants, and determined that sediment transport followed a clockwise hysteresis. This is typically observed in high flow events after dry summer periods. With sediments there was a large mobilization of PFRs (36 kg in 48 h in one of the main tributaries) and PCBs not previously observed in the static sediment. Observed contaminant load ranges during the two-day sampling campaign were: PCB (34–152 g), DDT (12–213 g), PBDE (50–1740 g), HBCD (0–2.2 g) and PFR (2410–35,895 g). An environmental risk assessment was carried out by comparing the pollutant concentrations found in the sediments with the Canadian quality guidelines (ISQC), showing a significant noncompliance for PCBs in dynamic sediments. Our results point out to the need of a regular assessment of the downstream transfer of the sediment-borne pollutants in drainage basins historically affected by intense industrial activities and associated contamination. ; The authors are grateful to the reviewers, whose valuable suggestion contributed to improving the present manuscript. This work was funded by the EU through the FP7 Project GLOBAQUA (grant agreement 603629) and the Redes de Excelencia project NET-SCARCE (CTM2015-69780-REDC). Authors acknowledge the support from the Economy and Knowledge Department of the Catalan Government through Consolidated Research Groups ICRA-ENV 2017 SGR 1124 and 2017 SGR 459 Fluvial Dynamics Research Group (RIUS). Authors thank Efren Muñoz who helped during the sampling campaign and provided turbidity data.