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[Departement_IRSTEA]Eaux [ADD1_IRSTEA]Systèmes aquatiques soumis à des pressions multiples; International audience; Eutrophication is one of the most common causes of water quality impairment of inland and marine waters. Its best-known manifestations are toxic cyanobacteria blooms in lakes and waterways and proliferations of green macro algae in coastal areas. The term eutrophication is used by both the scientific community and public policy-makers, and therefore has a myriad of definitions. The introduction by the public authorities of regulations to limit eutrophication is a source of tension and debate on the activities identified as contributing or having contributed decisively to these phenomena. Debates on the identification of the driving factors and risk levels of eutrophication, seeking to guide public policies, have led the ministries in charge of the environment and agriculture to ask for a joint scientific appraisal to be conducted on the subject. Four French research institutes were mandated to produce a critical scientific analysis on the latest knowledge of the causes, mechanisms, consequences and predictability of eutrophication phenomena. This paper provides the methodology and the main findings of this two years exercise involving 40 scientific experts.

Despite harmful impacts on coastal communities and biodiversity for a few decades, eutrophication of marine systems only recently gained public visibility. Representing a major land-based pollution, it is now considered as the most striking symptom of intractable disruption of biogeochemical nutrients cycles at a global scale, due to massive phosphate ore extraction and industrial synthesis of reactive nitrogen. The ambition of this article is to account for the protracted emergence of coastal eutrophication as a public problem. Based on a comprehensive literature review of previously dispersed works in sustainability, social and political sciences, the article analyses multi-scale dynamics of ocean overfertilization. We show that the experience of local people was usually insufficient to trigger stringent public policies, since the lack of effective public action was often presented as the result of local antagonisms and persistent scientific uncertainties, and identify three stages that characterize the social history of marine eutrophication and how it was handled – or not – by public authorities. Although social mobilizations against coastal eutrophication tend to focus on emblematic sites, socio-environmental conflicts directly related to eutrophication symptoms spread in diverse hydro-social configurations. Ultimately, we develop a typology of four configurations associated with enduring land-based nutrient pollution : noisy, overwhelming, silenced and disturbing eutrophication., Malgré les dommages qu’elle occasionne pour les populations humaines et la biodiversité littorales depuis parfois plusieurs dizaines d’années, l’eutrophisation des milieux marins n’a que très progressivement accédé à une visibilité dans l’espace politique et auprès d’un large public. Due à la production industrielle d’azote réactif et de l’extraction massive de minerai de phosphate, en particulier pour la fertilisation des cultures, cette pollution terrigène majeure est aujourd’hui considérée comme le symptôme le plus manifeste de la perturbation anthropique à grande échelle des cycles biogéochimiques des nutriments. Dans ce contexte, l’ambition de cet article est de rendre compte de l’émergence contrariée de l’eutrophisation côtière comme problème public. Fondé sur une revue systématique de la littérature internationale en sciences sociales et en sciences de la soutenabilité, littérature jusqu’à présent très dispersée, il propose une lecture critique des dynamiques multi-échelles de la surfertilisation des océans. Cette analyse met en évidence trois périodes majeures qui caractérisent l’histoire sociale de l’eutrophisation des milieux marins et examine la façon dont elle a été prise en charge, ou pas, par les autorités publiques. Dans la plupart des cas documentés, l’expérience vécue des populations a rarement suffi à déclencher des politiques publiques régulatrices, le manque d’efficacité de l’action publique étant souvent présenté comme résultant d’antagonismes locaux et d’incertitudes scientifiques persistantes. Si les mobilisations sociales contre l’eutrophisation côtière tendent à se cantonner à des sites emblématiques, les conflits socio-environnementaux directement reliés à des symptômes d’eutrophisation se déploient, en revanche, au sein d’une grande diversité de configurations hydro-sociales, dont nous proposons une typologie.

Despite causing harmful impacts on coastal communities and biodiversity for a few decades, eutrophication of marine systems has only recently gained public visibility. Representing a major land-based pollution, eutrophication is now considered the most striking symptom of intractable disruption of biogeochemical nutrient cycles at a global scale. The objective of this article is to analyze multi-scale dynamics of the problematization and regulation of ocean overfertilization. To do so, we build on a comprehensive literature review of previously published works that address the sociopolitical dimension of eutrophication issues and whose visibility we analyze with a critical perspective. We identify three stages that characterize the social history of marine eutrophication and how it was handled by public authorities. Although social mobilizations focus on emblematic sites, conflicts directly related to eutrophication symptoms spread in diverse hydro-social configurations. We conclude with a typology of four configurations associated with enduring nutrient pollution: noisy, overwhelming, silenced, and disturbing eutrophication.

International audience; Active biomonitoring permits the quantification of biological exposure to chemicals through measurements of bioavailable concentrations in biota and biological markers of toxicity in organisms. It enables respective comparison of the levels of contamination between sites and sampling campaigns. Caged gammarids are recently proposed as relevant probes for measuring bioavailable contamination in freshwater systems. The purpose of the present study was to develop a multi-pressure and multiscale approach, considering metallic contamination levels (from data based on active biomonitoring) as a response to pressures (combination of individual stressors). These pressures were anthropogenic land cover, industry density, wastewater treatment plant density, pressures on stream hydromorphological functioning, riverside vegetation and bioavailability factors. A dataset combining active biomonitoring and potentially related pressures was established at the French national scale, with 196 samplings from 2009 to 2016. The links between pressures and metallic contamination were defined and modelled via structural equation modeling (more specifically partial least squares - path modeling). The model enabled the understanding of the respective influences of pressures on metallic bioconcentration in caged sentinel organisms. Beyond validating the local influence of industries and wastewater treatment plants on metallic contamination, this model showed a complementary effect of driving forces of anthropogenic land cover (leading to human activities). It also quantified a significant influence of pressures on stream hydromorphological functioning, presence of vegetation and physico-chemical parameters on metal bioconcentration. This hierarchical multi-pressure approach could serve as a concept on how pressures and contamination (assessed by active biomonitoring) can be connected. Its future application will enable better understanding of environmental pressures leading to contamination in freshwater ecosystems.

International audience; Aquatic ecosystems are exposed to multiple environmental pressures including chemical contamination. Polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are persistent organic pollutants (POPs) known as preoccupying substances for the environment. Active biomonitoring (ABM) is a surveillance method for polluted aquatic ecosystems measuring bioavailable contamination. In this work, the aim was to quantify the total links between environmental pressures and bioavailable contamination (for PAHs and PCBs) at the French national scale. Based on 245 sites experimented by ABM from 2017 to 2019, environmental pressures (anthropogenic pressures and environmental parameters) were defined (point source landfill density, point source urban density, point source industry density, point source road density, nonpoint source industry density, nonpoint source road density, nonpoint source urban density, nutrients and organic matter, slope, dams, straightness, coarse sediment, summer precipitation, hydrographic network density and watershed size) and characterized by one or a combination of measures called stressor indicators. The links between environmental pressures and bioavailable POPs contamination (ABM measure) at a large spatial scale were defined and quantified via structural equation modeling. Point source urban density, nutrients and organic matter, summer precipitation, straightness and point source industry density are correlated positively with PAH bioavailable contamination. In contrast, nonpoint source urban density, nonpoint source industry density, nonpoint source road density and watershed size are positively correlated with PCB bioavailable contamination. The dominant pressures linked to PAHs and PCBs were different, respectively local and large-scale pressures were linked to PAH bioavailable contamination, and only large-scale pressures were linked to PCB bioavailable contamination.

When investigating the fields of biogeography and macroecology, climate- and productivity-related variables are frequently identified as the strongest correlates of species-diversity patterns. These variables have been usually merged under the climate/productivity hypothesis and describe the direct and indirect actions of climate on species. Being among the most vulnerable ecosystems to climate change, streams and rivers are expected to be influenced both by climatic and trophic (i.e. productivity-related) factors. We propose here to distinguish the relative influence of the two processes on large-scale, long-term changes in the functional diversity of freshwater invertebrate communities over two decades in France. To this end, we designed two functional indices using invertebrate traits to surrogate the respective mechanisms: climate vulnerability and feeding specialisation. Using Geographically Weighted Regression (GWR) models, we showed that trends in both indices, along with the initial regional species-pools, have significantly contributed to the overall long-term increase in functional diversity of invertebrate communities. In addition, we highlighted a strong geographical differentiation in the contribution patterns with the climate vulnerability effect decreasing with latitude and the feeding specialisation effect being higher in headwaters than in large rivers. Finally, taking into account this non-stationarity in the ecological processes and responses using GWR models allowed explaining about 75% of the long-term changes in the community diversity. Consequently, this study offers sound perspectives in predicting the future patterns of trends in functional diversity of communities under different scenarios of environmental changes, like climate and/or land-use. This article is protected by copyright. All rights reserved.

International audience; Although species traits have the potential to disentangle long-term effects of multiple, potentially confounded drivers in ecosystems, this issue has received very little attention in the literature. We aimed at filling this gap by assessing the relative effects of hydroclimatic and water quality factors on the trait composition of invertebrate assemblages over 30 years in the Middle Loire River (France). Using a priori predictions on the long-term variation of trait-based adaptations over the three decades, we evaluated the ability of invertebrate traits to indicate the effects of warming, discharge reduction and water quality improvement. Hydroclimatic and water quality factors contributed to up to 65% of the variation in trait composition. More than 70% of the initial trait response predictions made according to observed long-term hydroclimatic changes were confirmed. They supported a general climate-induced trend involving adapted resistance and resilience strategies. A partial confounding effect of water quality improvement acting on trophic processes was also highlighted, indicating that improved water quality management can significantly help to reduce some adverse effects of climate change. This trait-based approach can have wider implications for investigating long-term changes driven by multiple, potentially confounded factors, as frequently encountered in the context of global change.

The positive effects of water quality improvement on stream biodiversity in the temperate regions are expected to be at risk with the projected climatic changes. However, the processes and mechanisms behind the predicted threats remain uncertain. From long-term series of benthic invertebrate samples from temperate rivers and streams in France, we analyzed diversity and composition shifts over time in relation to geographic elements and human stressors. Mechanisms for community changes were investigated with a trait-based analysis for the entire dataset and for a selected caddisfly community module. We observed a 42% increase in the taxonomic richness of stream invertebrate communities over the last 25 years. A gradual trend induced by water quality improvement was distinguished from a more abrupt climate change-induced shift in communities around the year 2000. Trophic amplification – the intensification of trophic interactions and pathways through the food web – was identified as the mechanism behind the strong community shift. Four lines of evidence for this trophic amplification are highlighted: (i) higher dissolved oxygen concentrations indicated a shift in primary production, (ii) the trait-based analysis of entire communities showed a bottom-up food web amplification, (iii) the trait-based analysis of the community module evidenced feeding strategy shifts and increased food web interactions, and (iv) the abundance analysis of the community module showed a productivity increase. These results lend credit to persistent investments in water quality for improving stream biodiversity, and contrary to expectation, climate change impacts seem so far to have reinforced these positive effects.

International audience; We investigated the relationships between the ecological status of wadeable rivers and the intensity of various stressors related with hydromorphological alterations, nutrient and organic matter contaminations. The French invertebrate-based multimetric index (I2M2), which efficiently responds to the effects of both physical and chemical and hydromorphological stressors, was used as descriptor of river reach ecological status. We developed a model focusing on the effects of hydromorphological and physical and chemical stressor gradients on the I2M2 in different physiographic contexts. The potential confounding effects of natural geographic conditions and watershed scale pressure gradients were taken into account and neutralized by gathering watersheds into homogeneous clusters integrated as an interaction factor in the model. Whatever effects were considered (general or within-spatial clusters), the I2M2 was impaired similarly by the same stressor types, being negatively influenced by an increase in BOD5, ammonium, nitrite, nitrate and total phosphorus concentrations. The I2M2 was also negatively influenced by variables describing hydromorphology at the reach scale, especially by the 'loss of sinuosity', 'increasing rates of bank erosion', 'flow reductions' and 'alteration of pool/riffle succession'. The I2M2 was generally more strongly impaired by physical and chemical pressures than by hydromorphological alterations.

International audience; The purpose of our approach was to take into account the nested spatial scales driving stream functioning in the description of pressures/ecological status links by analysing the results of a hierarchical model. The development of this model has allowed us to answer the following questions: Does the consideration of the indirect links between anthropogenic pressures and stream ecological status modify the hierarchy of pressure types impacting benthic invertebrates? Do the different nested scales play different roles in the anthropogenic pressures/ecological status relationship? Does this model lead to better understanding of the specific role of hydromorphology in the evaluation of stream ecological status? To achieve that goal, we used the Partial Least Square (PLS) path modelling method to develop a structural model linking variables describing (i) land use and hydromorphological alterations at the watershed scale, (ii) hydromorphological alterations at the reach scale, (iii) nutrients-organic matter contamination levels at the site scale, and (iv) substrate characteristics at the sampling site scale, to explain variation in values of a macroinvertebrate-based multimetric index: the French I2M2. We have highlighted the importance of land use effects exerted on both hydromorphological and chemical characteristics of streams observed at finer scales and their subsequent indirect impact on stream ecological status. Hydromorphological alterations have an effect on the substrate mosaic structure and on the concentrations of nutrients and organic matter at site scale. This result implies that stream hydromorphology can have a major indirect effect on macroinvertebrate assemblages and that the hierarchy of impacts of anthropogenic pressures on stream ecological status generally described in the literature - often determining strategic restoration priorities - has to be re-examined. Finally, the effects of nutrients and organic matter on macroinvertebrate assemblages are lower than expected when all the indirect effects of land use and hydromorphological alterations are taken into account.

Water temperature and discharge are fundamental to lotic ecosystem function, and both are strongly affected by climate. In large river catchments, however, climatic effects might be difficult to discern from background variability and other cumulative sources of anthropogenic change arising from local land and water management. Here, we use trend analysis and generalised linear modelling on the Loire, the longest river in France to test the hypotheses that i) long-term trends in discharge and river temperature have arisen from climate change and ii) climatic effects on water quality have not been overridden by local effects. Over 32 years (1977–2008), discharge in the Middle Loire fell by about 100 m 3 /s while water temperature increased by 1.2 °C with greatest effects during the warm period (May–August). Although increasing air temperature explained 80% of variations in water temperature, basin-wide precipitation showed no long-term trend and accounted for only 18% of inter-annual fluctuations in flow. We suggest that trends in abstraction coupled with a potential increase in evapo-transpiration at the catchment scale could be responsible for the majority of the long-term discharge trend. Discharge and water temperature explained only 20% of long-term variations in major water quality variables (conductivity, dissolved oxygen, pH, suspended matter, biochemical oxygen demand, nitrate, phosphate and chlorophyll-a), with phosphate and chlorophyll declining contrary to expectations from global change probably as a consequence of improved wastewater treatment. These data partially support our first hypothesis in revealing how warming in the Loire has been consistent with recent atmospheric warming. However, local management has had larger effects on discharge and water quality in ways that could respectively exacerbate (abstraction) or ameliorate (reduced point-source pollution) warming effects. As one of the first case-studies of its kind, this multi-parametric study illustrates the potential for complex interactions between climate change and other environmental factors in large rivers.

1. Assuming that recruitment variation is one of the main sources of fish population and assemblage changes, it is necessary to understand how natural variations in the environment influence 0+ fish abundance. Temperature regimes play an important role in enhancing both spawning activity and survival during early larval fish development. Flow regime variation, which is a powerful source of stream disturbance, is another factor to be taken into account. 2. Responses to these variables need to be assessed using long-term datasets, since standard statistical approaches fail to provide a causal structure or to quantify the different effects. We therefore used a 26-year dataset to evaluate the respective effects of seven derived independent variables describing the effects of temperature and flow regimes on the 0+ juvenile abundance of eight fish species in the River Rhone. 3. A clustering procedure using the Kendall tau rank correlation coefficient was implemented and identified three groups of fish according to their synchronic variations in juvenile abundance; i.e. varying with decreasing juvenile abundance, slightly increasing juvenile abundance and increasing juvenile abundance. These clusters provided the basis for building hierarchical log-Poisson generalized linear models. The Bayesian paradigm gives a reliable framework for model selection, and the best model was determined using the Bayes Factor. 4. The posterior distribution of the regression parameters was coherent with what was expected based on knowledge of the biology of the different species. It indicates that temperature regime drives 0+ juvenile abundance but that flow regime also plays an important regulating role. The models thus detected evidence of the consequences of specific flow events such as larval drift and an increase in available habitat during higher flow discharges. 5. Our study illustrates the advantages of using a hierarchical modelling approach to quantify ecological effects by improving discrimination between the different sources of uncertainty, leading to better precision when estimating regression parameters.

International audience; We built a corpus of models capable of explaining the variability of the biological indices used in the French surveillance monitoring network and also predict the ecological status of non-monitored water bodies. Benthic macroinvertebrates, diatoms and fish indices have been used to determine the ecological status of 1100 sites of the monitoring network distributed homogeneously over national territory.The pressures taken into account to explain and predict ecological status cover three spatial scales: catchment, reach, site. The set of predictive data cover three types of pressure: land use pressure, hydromorphological pressure and physico-chemical pressure measured at catchment, reach and site scale, respectively.We showed that the parameters characterising the load of nutrients and organic matter had a predominant effect on the three biological compartments, and that land use variables played an integrating role of the different pressures acting on rivers and explained a major part of their degradation. On the contrary, we also showed that it was more difficult to characterise the role of the hydromorphological descriptors measured at the intermediate scale of the reach due to the difficulty of characterising the links between scales.The three predictive models developed demonstrated good performances to evaluate biological condition and are of great interest for managers as it permits using a set of pressure data to successively predict the status of water bodies for which biological monitoring data are unavailable.

International audience; Rivers are complex systems for which it is hard to make reliable assessments of causes and responses to impairments. We present a holistic risk-based framework for river ecosystem assessment integrating all potential intervening processes and functions. Risk approaches allow us to deal with uncertainty both in the construction of indicators for magnitude of stressors and in the inference of environmental processes and their impairment. Yet, here we go further than simply replacing uncertainty by a risk factor. We introduce a more accurate and rigorous notion of risk with a transcription of uncertainty in causal relationships in probability distributions for the magnitude of impairment and the weight of different descriptors, with an associated confidence in the diagnostic. We discuss how Bayesian belief networks and Bayesian hierarchical inference allow us to deal with this risk concept to predict impairments and potential recovery of river ecosystems. We developed a comprehensive approach for river ecosystem assessment, which offers an appealing tool to facilitate diagnosis of the likely causes of impairment and predict future conditions. The ability of the risk approaches to integrate multi-scale quantitative and qualitative descriptors in the identification of multiple stressor sources and pathways in the stream network, and their impairment of specific processes and structures is illustrated for the national-level risk analysis for hydromorphology and pesticide pollution. Not only does the risk-based framework provide a more complete picture of environmental impairments, but it also offers a comprehensive, user-friendly tool to instruct the decision process.

1. In the context of a generalised modification of hydraulic conditions in medium to large streams, modelling the impacts of stream regulation on fish communities in multiple streams is an important challenge for basic and applied freshwater ecology. Conventional instream habitat models such as PHABSIM link a hydraulic model with preference curves for various species to estimate habitat value changes with discharge in stream reaches. Despite world-wide applications, they have been scarcely used in multiple sites with multiple species. 2. We assigned 21 size classes of European fish species to four habitat guilds (cluster analysis grouping size classes with comparable microhabitat preference curves). Then, we ran a conventional instream habitat model on 28 French stream reaches belonging to the `barbel zone', to estimate habitat values versus discharge curves for the 21 size classes. We summarised the outputs as mean habitat values for guilds, and tested if they were predictable from average characteristics of reaches (discharge, depth, width, particle size). 3. As was obtained elsewhere for populations, habitat values for guilds were strongly related to average, dimensionless characteristics of reaches. The Reynolds number of reaches, equivalent to a discharge per width unit, reflected most of the discharge-dependent changes in habitat values (within reaches). In particular, habitat values of species preferring bank (respectively midstream) microhabitats decreased (respectively increased) with increasing Reynolds number. The Froude number at median discharge was the major predictor of reach-dependent but discharge-independent variations in habitat values. Habitat values of species preferring riffle versus pool or bank microhabitats were higher in reaches with high Froude numbers. These relationships were consistent with existing knowledge on the different species. 4. Such results suggest that the input variables required to estimate habitat values for fish communities can be greatly simplified, as illustrated by a general estimation of the sensitivity of species preferring midstream habitats to discharge changes in any reach. Cost-efficient alternatives to conventional instream habitat models should facilitate their validation in multiple sites, a point that remains critical in instream habitat modelling of fish communities.

Hydrological and morphological characteristics of streams and rivers influence a wide array of ecological processes and patterns ; they define the aquatic habitat template. Interdisciplinary approaches are required to study physical-biological interactions ; increased collaboration between ecologists and experts in morphology, hydrology and hydraulic engineering is particularly desirable. In this paper, we present our experience of aquatic habitat modelling. In particular, we indicate how statistical instead of mechanistic hydraulic descriptions simplify the input field data required to estimate instream habitat suitability. We have also conducted significant biological validations of habitat model predictions expressed at the fish community level. A better quantification of instream habitat suitability is meaningful when associated with the understanding of the large-scale processes governing stream equilibrium. In this context, the knowledge of bankfull discharge, at which the self-maintenance of the river morphology occurs, is of particular importance. The identification of key hydraulic variables and periods for population dynamics and vital processes are challenging for the near future. A greater understanding of physical-biological interactions will not only yield deeper insights into the ecological organization of streams and rivers, it will also improve our ability to predict how flow alterations caused by various human activities affect these vital ecosystems. Our progresses depend mainly on of our ability to produce biologically validated models.

International audience; Recent progress in very high spatial resolution imagery (VHSRI) has increased the availability of fine-scale land cover data over extensive areas. This new spatial information might improve our understanding of how land cover affects stream ecosystems.Land cover information was investigated in whole catchments and riparian areas in Normandy (France) and related to stream macroinvertebrates at 155 sites. The first model was based on the land cover data obtained via moderate spatial resolution imagery (MSRI) at the catchment scale. The second and the third models were additionally based on riparian land cover data both locally (at the site) and upstream. The riparian land cover data were derived from the MSRI for the second and from the VHSRI for the third model.Riparian vegetation (at a distance from the channel of 5–100 m) was poorly detected by MSRI when compared to VHSRI. The model based on higher resolution data at the riparian scale (i) ensured that relationships identified at the riparian corridor scale were independent from those at whole catchment scale, (ii) identified the influence of riparian vegetation on the aquatic environment as being most pronounced when using data from a 20-m wide strip (i.e. 20 m on each bank for a total of 40 m) and (iii) disentangled the effects of roads from other urban infrastructure.This study confirms the advantages of using very high resolution data in landscape approaches to the study of aquatic ecosystems. It provides accurate spatial information at the riparian scale that improves interpretation of the influence of land cover on the ecological condition of streams and allows identification of relevant features for management.

1. Relationships between fish and their habitat over whole geographic regions, which are evident from studies of many streams and species, can improve understanding of lotic communities and provide reliable management tools. Nevertheless, most habitat preference studies have been based on single sites, and confined to small streams and to game species. 2. Regional habitat preference models, based on local velocity, depth and roughness, were developed for twenty-four species and their size classes commonly found in large European streams. Fish surveys were conducted in six large streams in southern France over an 8-year period. To limit the influences of habitat variables other than those studied, we estimated fish preferences within each survey and averaged this information across surveys. Preferences were fitted with confidence intervals and their sensitivity to field uncertainty was evaluated. 3. Most species and size classes had significant preferences for local habitat conditions which were consistent across the region. Habitat preferences predominant in the region overall were not always observed at any one site, but habitat conditions preferred on average in the region were never actually avoided locally. These results support the use of regional preference models for fish and the development of similar models for other lotic groups whose sensitivity to local habitat conditions has been reported elsewhere.

1. One current approach to the prediction of community characteristics is to use models of key local-scale processes (e.g. niche dimensions) affecting individuals and to estimate the effects of these attributes over larger scales. We tested this approach, focusing on how the hydraulic habitat structures fluvial fish communities. 2. We used a recent statistical habitat model to predict fish community characteristics in eleven reaches in the Rhone river basin in France. Predictions were made ‘blindly’ since most reaches were not used to calibrate the model. The model reflects species preferences for local hydraulics. We made predictions of the fish community from the local hydraulic conditions found in the reaches under low flow conditions. The overall abundance and the relative abundance (both as indices) of fish species, specific size classes and species traits (i.e. reproductive, trophic, morphological and others) were predicted. We summarized our predictions of the relative abundance of species as two ‘community structure indices’ using Principal Component Analysis. 3. Our predictions from low-flow hydraulics were compared with long-term observations of fish communities. The relative abundance of species actually observed depended largely on zoogeographic factors within the Rhone basin which could not be predicted by the model. The model predicted 13% of the variance in the indices of relative abundance at the species level and 23% of this variance at the trait level for all zoogeographic regions combined. However, when focused on reaches within a geographic region, the model explained up to 47% of the same variance. Therefore, geographic regions act as ‘filters’ on the relative abundance of species, but hydraulics do affect fish communities within a given geographical context. 4. For the synthetic ‘community structure indices’, we obtained good predictions from hydraulics independently of the geographical context (variance explained up to 95%). These indices were linked to simple key hydraulic characteristics of river reaches (Froude and/or Reynolds number). The indices enabled interpretations of the links between hydraulics, geomorphology, discharge and community patterns. These links were consistent with existing knowledge of species and their traits. 5. In addition to the above validations, the habitat model partly explained the observed effects of impoundment on fish communities. 6. The present results show that stream hydraulics strongly impact fish community structure. Consequently, our findings confirm that community characteristics can be predicted using models of the local-scale habitat requirements of the species forming the community.

Global change calls for an understanding of how temperature and flow regimes influence aquatic ecosystems. Fish assemblages are a major component of river ecosystems and are thought to exhibit more integrative informative responses than single species to environmental variations, whether rare and sudden or gradual and continuous. The use of long-term datasets is thus of primary importance, allied to statistical modeling. For each of three previously identified species clusters, we performed Bayesian variable selection and inference within a hierarchical log Poisson Generalized Linear Model using a spike and slab normal prior to pinpoint which subset of environmental variables is of importance for each fish assemblage. Fish counts from electrofishing experiments are known to provide overdispersed data and, not surprisingly, the contribution of recorded environmental effects is found to be weak compared with those of other intra-assemblage sources of variation. The posterior distribution of the regression parameters is in coherence with what was expected from biological knowledge of the three species clusters. In particular, thermophilic species tend to benefit from warmer waters, whereas the recruitment of cold water species decreases due to global warming effects. Our study provides an example of the advantages of hierarchical modeling for quantifying interspecies ecological effects and selecting common environmental variables of importance.

Existing methods for connectivity analysis still encounter difficulties in explaining functional relationships between network structure and ecological patterns over larger territories or complex structures like dendritic river networks. We propose a method that addresses the problem of scale and resolution in the connectivity analysis of dendritic network structures, illustrated here for the re-colonization of the French Loire river basin by the European otter. The ecological niche factor approach is applied to infer favourable habitat in the river network based on large scale data of land use and hydro-morphology of river segments for the entire river basin. These analyses identified the stressors to the riparian zone of channel straightening, urbanisation and forest fragmentation as the principal factors explaining otter occurrence. Using this estimate of habitat favourability, we used the Integral Index of Connectivity to quantify habitat availability and connectivity in the dendritic river network. When we calculate the integral index of connectivity over different spatial extents by constraining network distances, the scale-sensitivity of the network’s connectivity emerges. Accounting for high mobility by entering larger network distances in the analysis identifies conservation networks and priorities mainly in downstream parts of the river basin, whereas with smaller network distances, more restricted high quality areas in central and upstream parts are highlighted. The presented approach performed better than distribution modelling approaches in explaining species occurrence over the river network and confirms the crucial aspect of connectivity in otter re-colonization.

International audience; Aquatic species living in running waters are widely acknowledged to be vulnerable to climate-induced, thermal and hydrological fluctuations. Climate changes can interact with other environmental changes to determine structural and functional attributes of communities. Although such complex interactions are most likely to occur in a multiple-stressor context as frequently encountered in large rivers, they have received little attention in such ecosystems. In this study, we aimed at specifically addressing the issue of relative long-term effects of global and local changes on benthic macroinvertebrate communities in multistressed large rivers. We assessed effects of hydroclimatic vs. water quality factors on invertebrate community structure and composition over 30years (19792008) in the Middle Loire River, France. As observed in other large European rivers, water warming over the three decades (+0.9 degrees C between 19791988 and 19992008) and to a lesser extent discharge reduction (80m3s1) were significantly involved in the disappearance or decrease in taxa typical from fast running, cold waters (e.g. Chloroperlidae and Potamanthidae). They explained also a major part of the appearance and increase of taxa typical from slow flowing or standing waters and warmer temperatures, including invasive species (e.g. Corbicula sp. and Atyaephyra desmarestii). However, this shift towards a generalist and pollution tolerant assemblage was partially confounded by local improvement in water quality (i.e. phosphate input reduction by about two thirds and eutrophication limitation by almost one half), explaining a significant part of the settlement of new pollution-sensitive taxa (e.g. the caddisfly Brachycentridae and Philopotamidae families) during the last years of the study period. The regain in such taxa allowed maintaining a certain level of specialization in the invertebrate community despite climate change effects.

The well-documented re-colonisation of the French large river basins of Loire and Rhone by European otter and beaver allowed the analysis of explanatory factors and threats to species movement in the river corridor. To what extent anthropogenic disturbance of the riparian zone influences the corridor functioning is a central question in the understanding of ecological networks and the definition of restoration goals for river networks. The generalist or specialist nature of target species might be determining for the responses to habitat quality and barriers in the riparian corridor. Detailed datasets of land use, human stressors and hydro-morphological characteristics of river segments for the entire river basins allowed identifying the habitat requirements of the two species for the riparian zone. The identified critical factors were entered in a network analysis based on the ecological niche factor approach. Significant responses to riparian corridor quality for forest cover, alterations of channel straightening and urbanisation and infrastructure in the riparian zone are observed for both species, so they may well serve as indicators for corridor functioning. The hypothesis for generalists being less sensitive to human disturbance was withdrawn, since the otter as generalist species responded strongest to hydro-morphological alterations and human presence in general. The beaver responded the strongest to the physical environment as expected for this specialist species. The difference in responses for generalist and specialist species is clearly present and the two species have a strong complementary indicator value. The interpretation of the network analysis outcomes stresses the need for an estimation of ecological requirements of more species in the evaluation of riparian corridor functioning and in conservation planning.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]GEUSI; Social and scientific factors are deeply enmeshed in each other within the development and the use of ecological indicators (EI). Yet low research has assessed which factors contribute to selecting ecological indicators on the long-term. This article proposes to study the historical construction of EI by examining ecological, political, and social background of specific places where EIwere developed, in France on lakes and rivers. Our major findings in France were that ecological indicators were never optimised for the present market or political arena. Instead EI development was typically recycling previous tools that were elaborated for other purposes by environmentally committed outsiders, without regular funding. We found that regular funding for monitoring an EI was only provided when it matched an institution's goal. Beyond the geographically limited relevance of the case studies, these results therefore improve the theoretical framework we deploy when constructing or relying on indicators.

International audience; A sociologist's and an ecologist's critical view of the Rhone river restoration programme. A ten-year physical and ecological restoration programme was defined for the Rhone River in 1998. Initially, one of its main objectives was to increase the minimum flow in several bypassed sections, with the clear goal of improving the fluvial habitat and the general ecological functioning of the river. A complex consultation process which brought together the hydropower industry, regional civil organisations, and local civil representatives was adopted to define more precisely the local measures to be taken. In the course of this process, the programme underwent a progressive change from purely ecological to more socially motivated outcomes such as improving amenity values and flood defence. We studied how increasing the minimum flow is a topic that can or cannot be discussed by the actors during the consultation process. For example, there is less discussion about the ecological value than about the economic value of the restoration programme between the hydropower industry and the local elected members. But, the various actors readily debate with the scientists about floods and sedimentary transport, linked with local knowledge. We applied a multidisciplinary approach, involving sociology and ecological sciences, to question the restoration programme and to analyse its hybrid results in terms of both the ecological and amenity values and at the same time to understand the governance process.; Il est assez commun que dans un processus de politique publique donnant large place à la concertation, l'objet initial soit déplacé. Il en est ainsi de ce cas d'étude qui, partant de la question des débits réservés, aboutit à celle de la lutte contre les inondations. Pour saisir cette trajectoire tracée aussi bien par la résistance du milieu aquatique que par les interactions humaines que celui-ci construit, il a fallu le regard croisé de l'écologue et du sociologue. L'hybridation est réussie, mais son efficacité tient aussi à une mise en critique des « utopies gestionnaires et scientifiques » qui voudraient un alignement pur et simple des différents types de savoirs et une heuristique propre à la dynamique des apprentissages sociaux. Asymétries, contradictions et ambivalences sont toujours là et la question de l'efficacité environnementale d'une gouvernance vertueuse reste posée. La Rédaction Mots-clés : restauration écologique ; concertations ; hydrosystème fluvial ; écologie et sociologie ; Rhône Résumé – Acté en 1998, le Programme décennal de restauration hydraulique et écologique du Rhône s'est fixé comme objectif principal d'augmenter les débits réservés dans les bras court-circuités du fleuve. Cet objectif était initialement pensé comme une action de restauration écologique de milieux naturels fortement perturbés. Un processus concomitant de concertations a plusieurs fois déplacé l'intérêt initial vers des projets d'élaboration d'un territoire fluvial, puis d'un outil de gestion contre les crues. L'approche interdisciplinaire appliquée, entre sociologie et écologie, interroge la capacité d'une telle action environnementale à aboutir à des résultats hybrides, relevant à la fois de l'amélioration d'un écosystème fluvial et d'un processus de gouvernance.

Changes in a brown trout (Salmo trutta L.) population result from interaction among various mechanisms which are dependent on environmental conditions and biotic processes. In reaches influenced by the presence of dams, the instream flow in the bypassed section is not the only parameter which affects the population. Flood episodes, the general connectivity of the bypassed section, and the characteristics of the substrate which define the availability and quality of spawning grounds may also have a crucial impact. The design and fine-tuning of tools which take environmental parameters into account can improve our understanding of the dynamics of such influenced populations. In this perspective, a deterministic model (MODYPOP) has been developed in an attempt to integrate all these factors and to test the effect of different long-term scenarios of influenced flow regimes on the structure of trout populations. MODYPOP was applied to three populations and three reaches (on the Roizonne, Neste d'Aure and Lignon du Forez rivers in France). For each stream, experiments were carried out on a bypassed section downstream of a hydropower station, before and after an increase in the minimum instream flow due to relicensing. These experiments allowed integrating into MODYPOP local phenomena (impact of flood episodes, impact of flushing, impact of downstream migration of juveniles and adults) affecting the populations during the study period and then calibrating them. To estimate the change in the population due to the increase in minimum instream flow, different long-term simulations were run, selecting discharge patterns at random. These scenarios help to evaluate the time required for the population to return to a range close to habitat saturation after an improvement in the hydraulic habitat or following a flood event. These applications have enabled determining the relative importance of changes in population density due to different types of events. Copyright © 2008 John Wiley & Sons, Ltd.

The conclusions of numerous stream restoration assessments all around the world are extremely clear and convergent: there has been insufficient appropriate monitoring to improve general knowledge and expertise. In the specialized field of instream flow alterations, we consider that there are several opportunities comparable to full-size experiments. Hundreds of water management decisions related to instream flow releases have been made by government agencies, native peoples, and non-governmental organizations around the world. These decisions are based on different methods and assumptions and many flow regimes have been adopted by formal or informal rules and regulations. Although, there have been significant advances in analytical capabilities, there has been very little validation monitoring of actual outcomes or research related to the response of aquatic dependent species to new flow regimes. In order to be able to detect these kinds of responses and to better guide decision, a general design template is proposed. The main steps of this template are described and discussed, in terms of objectives, hypotheses, variables, time scale, data management, and information, in the spirit of adaptive management. The adoption of such a framework is not always easy, due to differing interests of actors for the results, regarding the duration of monitoring, nature of funding and differential timetables between facilities managers and technicians. Nevertheless, implementation of such a framework could help researchers and practitioners to coordinate and federate their efforts to improve the general knowledge of the links between the habitat dynamics and biological aquatic responses.

International audience; Despite the many habitat simulations that have been undertaken around the world, not enough biological monitoring has been performed following flow manipulations. It is difficult, however, to refine flow management decisions without a better understanding of the links between amounts, durations and seasonality of flow deliveries and population dynamics. Trout populations were monitored before and after flow alterations in five trout streams, involving 17 study sites over a 4- to 12-year period, depending on the sites. A comparison of the trout populations observed to theoretical Habitat/population models pointed up several helpful lessons. Various factors slow increases in population size, including the availability and quality of spawning grounds, the general connectivity of the bypass section (BPS) and severe spate events. In addition to these site-dependent factors, hydrological dynamics may explain why it is so difficult to clearly identify relationships between habitat availability and real fish stocks. Moreover, opportunities to observe population changes are improved when the pre-enhancement instream flow value is very low, and when there is a considerable difference between pre- and post-enhancement values. A population dynamics model that incorporates different habitat limitations and demographic background can be a very precious tool to improve understanding of the different situations and to build scenarios of population recovery.

The real impact on downstream fish populations of a change in management of a hydropower installation, in particular, the impact of an increase in instream minimum flow, can be difficult to assess. Other factors, in addition to the simple habitat limitations in low-flow periods, can influence the evolution of a fish population over time. It has often observed in French streams that the 0+ individuals appear limited by strong discharge in the first months after their emergence (March to May). A dynamic model of a trout populations (MODYPOP) has been applied to the river Roizonne in the French Alps, upstream and downstream of a hydropower station, in order to estimate the effect of different discharge time series on trout population dynamics. Trout populations were monitored by means of annual inventories at two study sites, one with minimum flow and the other with natural flow. At each site, available habitat was described and simulated as it changed with discharge, according to the ‘microhabitat’ method (EVHA, derived from PHABSIM). Habitat time series were built on the basis of discharge time series. MODYPOP simulations, integrating habitat and discharge time series, were compared with observed fish numbers. Results indicated that fluctuations in discharge during the post-emergence period played a major role in the dynamics of the total population. Predictions were significantly improved when the impact of strong discharge on emerging fry was integrated in the model. Copyright © 2003 John Wiley & Sons, Ltd.

The quantification of management impacts on fish populations requires a reliable estimation of the local hydraulic variability. In the case of complex natural flows, deterministic hydaulic models are expensive and not adapted to the description of local velocities. Statistical descriptions of the velocity distribution as a function of easily obtained input variables are therefore an attractive alternative. Existing velocity data on several French stream segments with intermediate -to largescale roughness were analysed statistically to define a shape parameter of the point velocity frequency distributions. Dimensional analysis was used to model this shape parameter, and thereby the velocity distributions, as a function of simple average descriptors of stream reaches (discharge, mean roughness, mean depth, and mean width). Such predictive models of the physical habitat availability for fish in the probability domain can provide stream managers with cost-effective tools., La quantification des impacts de la gestion sur les populations de poisson nécessite de disposer d'estimations fiables de la variabilité hydraulique locale de l'habitat. Dans le cas de débits naturels complexes, les modèles hydrauliques déterminants sont chers et non adaptés à la description des vitesses locales. Des descriptions statistiques de la distribution des vitesses en fonction de variables plus aisément accessibles présentent une alternative interessante. Des données disponibles de vitesse sur plusieurs tronçons de cours d'eau français avec des rugosités médianes à grandes mailles ont été analysées statistiquement pour définir un paramètre de forme des distributions de fréquence des vitesses ponctuelles. Une analyse dimensionnelle a été utilisée pour modéliser ce paramètre de forme, ainsi que les distributions de vitesse, en fonction de simples descripteurs moyens du tronçon (débit, rugosité moyenne, hauteur d'eau moyenne et largeur moyenne). De tels modèles prédictifs de la disponibilité de l'habitat physique des poissons dans le domaine des probabilités est de nature à doter le gestionnaire des cours d'eau d'outils de décision d'un coût acceptable.

The actual main result of habitat simulation procedures is a static relationship between an index of potential habitat, e.g. Weighted Usable Area (WUA), versus discharge in a study reach representative of a stream. This paper presents a new methodology to analyse timing and magnitude of physical habitat variations. Three options are presented : the habitat time series, the habitat duration curves, and the continuous under threshold habitat duration curves (CUT curves). The last option is a new procedure to interpret habitat chronicles. It determines continuous durations during which total WUA in a study reach was lower than a given threshold. The assumption according to which some durations/thresholds values could represent limiting events for fish population dynamics was illustrated with surveys of two wild brown trout populations. We studied the relationship between spawning habitat conditions and the relative density of 0+ the year after. In this case a continuous duration of more than 20 days with spawning habitat conditions lower than 80 % of the optimum conditions, seemed to limit the number of 0+. Finally, this new procedure was presented like a potential helping tool to interpret natural discharge chronicle in a management point of view.; Le principal résultat des procédures de simulation de l'habitat est une relation statique entre un indice d'habitat potentiel, c'est-à-dire la Surface Utilisable Pondérée (WUA) et le déversement dans un cours d'eau représentatif du point de vue de l'étude. Cet article représente une nouvelle méthodologie d'analyse de la durée et de l'étendue des variations physiques de l'habitat. Trois options sont présentées : (i) la série de durée de l'habitat, (ii) les courbes de durée de l'habitat et (iii) les courbes de durée continue d'habitat (courbes CUT) sous le seuil (courbes CUT). La dernière option est une nouvelle procédure d'interprétation des chroniques de l'habitat. Elle permet de déterminer les durées continues pendant lesquelles la WUA totale d'un cours d'eau représentatif du point de vue de l'étude était inférieure à un seuil donné. La supposition selon laquelle certaines valeurs de durée/seuil freineraient la dynamique de la population des poissons est illustrée par les études de deux populations de truites brunes sauvages. Une étude a été faite de la relation entre les conditions de l'habitat de frai et la densité relative de 0+ l'année suivante. Une durée continue de plus de 20 jours avec des conditions d'habitat de frai inférieures à 80 % des conditions optimales semblaient limiter le nombre de truites 0+. Cette nouvelle procédure a été présentée comme un outil d'interprétation utile de l'évolution du déversement naturel du point de vue de la gestion.