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An organism's body size plays an important role in ecological interactions such as predator–prey relationships. As predators are typically larger than their prey, this often leads to a strong positive relationship between body size and trophic position in aquatic ecosystems. The distribution of body sizes in a community can thus be an indicator of the strengths of predator–prey interactions. The aim of this study was to gain more insight into the relationship between fish body size distribution and trophic position in a wide range of European lakes. We used quantile regression to examine the relationship between fish species' trophic position and their log‐transformed maximum body mass for 48 fish species found in 235 European lakes. Subsequently, we examined whether the slopes of the continuous community size distributions, estimated by maximum likelihood, were predicted by trophic position, predator–prey mass ratio (PPMR), or abundance (number per unit effort) of fish communities in these lakes. We found a positive linear relationship between species' maximum body mass and average trophic position in fishes only for the 75% quantile, contrasting our expectation that species' trophic position systematically increases with maximum body mass for fish species in European lakes. Consequently, the size spectrum slope was not related to the average community trophic position, but there were negative effects of community PPMR and total fish abundance on the size spectrum slope. We conclude that predator–prey interactions likely do not contribute strongly to shaping community size distributions in these lakes. [ABSTRACT FROM AUTHOR]

Aim: We tested whether there is a strong effect of species interactions on assembly of local lake fish communities, in addition to environmental filters and dispersal. Location: Seven hundred and seventy‐two European lakes and reservoirs. Time period: 1993–2012. Major taxa studied: Nineteen species of freshwater fishes. Methods: We applied a latent variable approach using Bayesian Markov chain Monte Carlo algorithms (R package "BORAL"). We compared the contributions of six environmental predictors and the spatial organization of 772 European lakes in 209 river basins on the presence/absence of the 19 most frequent fish species and on the biomass and mean mass of the six dominant species. We inspected the residual correlation matrix for positive and negative correlations between species. Results: Environmental (50%) and spatial (10%) predictors contributed to the presence/absence assembly of lake fish communities, whereas lake size and productivity contributed strongly to the biomass and mean mass structures. We found highly significant negative correlations between predator and prey fish species pairs in the presence/absence, biomass and mean mass datasets. There were more significantly positive than negative correlations between species pairs in all three datasets. In addition, unmeasured abiotic predictors might explain some of the correlations between species. Main conclusions: Strong effects of species interactions on assembly of lake fish communities are very likely. We admit that our approach is of a correlational nature and does not generate mechanistic evidence that interactions strongly shape fish community structures; however, the results fit with present knowledge about the interactions between the most frequent fish species in European lakes and they support the assumption that, in particular, the mean masses of fish species in lakes are modified by species interactions. [ABSTRACT FROM AUTHOR]

Perch Perca fluviatilis is a widespread predator in European reservoirs, frequent in open waters but also known to spend a lot of time in the littoral zones. To get insight into how adult perch used and selected their habitat in an environment subject to water-level (WL) fluctuations, 21 perch were continuously tracked using acoustic telemetry over 2 years in the Bariousses Reservoir (France). The different available habitats were characterized by depth classes and substrate types, presence of emerging trees and presence of tree stumps in the littoral zone. We showed that perch habitat preferences were strongly dependent on the season, except for substrate type, and in line with their habitat use. Surprisingly we did not find any influence of the WL which, however, reduced the structural complexity of the littoral zone when lowering. In spring and summer, whatever the WL, we observed a strong preference for the littoral zone and complex habitats. In autumn and winter, perch migrated into deeper waters. However, the individual variability of the habitat preferences was quite high. This type of research helps to understand the spatial ecology of fish and provides useful guidance to hydromorphological restoration for fish populations. [ABSTRACT FROM AUTHOR]

We reviewed 219 papers and built an inventory of 532 items of ecological evidence on multiple stressor impacts in rivers, lakes, transitional and coastal waters, as well as groundwaters. Our review revealed that, despite the existence of a huge conceptual knowledge base in aquatic ecology, few studies actually provide quantitative evidence on multi-stress effects. Nutrient stress was involved in 71% to 98% of multi-stress situations in the three types of surface water environments, and in 42% of those in groundwaters. However, their impact manifested differently along the groundwater–river–lake–transitional–coastal continuum, mainly determined by the different hydro-morphological features of these ecosystems. The reviewed papers addressed two-stressor combinations most frequently (42%), corresponding with the actual status-quo of pressures acting on European surface waters as reported by the Member States in the WISE WFD Database (EEA, 2015). Across all biological groups analysed, higher explanatory power of the stress-effect models was discernible for lakes under multi-stressor compared to single stressor conditions, but generally lower for coastal and transitional waters. Across all aquatic environments, the explanatory power of stress-effect models for fish increased when multi-stressor conditions were taken into account in the analysis, qualifying this organism group as a useful indicator of multi-stress effects. In contrast, the explanatory power of models using benthic flora decreased under conditions of multiple stress. [ABSTRACT FROM AUTHOR]

The main obstacles to monitoring aquatic ecosystems have always been the lack of data, the complex socio-economic context and the lack of specialised expertise that characterise the West African region. To overcome these challenges, it is necessary to invest in the definition of context-appropriate approaches that allow good ecological assessment, improve understanding of the functioning of these ecosystems and facilitate data collection. This study focused on using benthic macrofauna to assess the risks of moving away from Good Ecological Status towards the functioning of an anthropised system (Nokoué-Benin), based on the definition of reference values for macroinvertebrate metrics, stress thresholds and the responses of selected metrics to stressors. The approach used is a combination of a joint species distribution model and Bayesian networks. We used JSDM to select relevant metrics and generate posterior probabilities. We then converted these posterior probabilities into posterior response probabilities for each of the stress levels and fed them into a Bayesian network. We used the Bayesian network response predictions to define the reference values of the metrics and the stress thresholds derived from the probability density plots for the low pressure levels. An application of this approach was then carried out on a lagoon sampled during high and low water periods for three years, with 33 macroinvertebrate taxa present in all seasons and sampling points, and measurements of 14 environmental parameters used as application data. The relevance of the results, despite the small sample size, supports wider application of the approach in West Africa. [ABSTRACT FROM AUTHOR]

The Water Framework Directive (WFD) is now well established as the key management imperative in river basins across Europe. However, there remain significant concerns with the way WFD is implemented and there is now a need for water managers and scientists to communicate better in order to find solutions to these concerns. To address this, a Science-Policy Interface (SPI) activity was launched in 2010 led by Directorate-General for Research and Innovation and Onema (the French national agency for water and aquatic ecosystems), which provided an interactive forum to connect scientists and WFD end-users. One major aim of the SPI activity was to establish a list of the most crucial research and development needs for enhancing WFD implementation. This paper synthesises the recommendations from this event highlighting 10 priority issues relating to ecological status. For lakes, temporary streams and transitional and coastal waters, WFD implementation still suffers from a lack of WFD-compliant bioassessment methods. For rivers, special attention is required to assess the ecological impacts of hydromorphological alterations on biological communities, notably those affecting river continuity and riparian covering. Spatial extrapolation tools are needed in order to evaluate ecological status for water bodies for which no data are available. The need for more functional bioassessment tools as complements to usual WFD-compliant tools, and to connect clearly good ecological state, biodiversity and ecosystem services when implementing WFD were also identified as crucial issues. [ABSTRACT FROM AUTHOR]

Since a few decades, underwater substrata can be characterized by methods using acoustic technology. Compare to traditional in situ grabbing techniques (bottom grab sampler, video camera, ...), these methods allow a better spatial resolution. Until today, these acoustical techniques required specialized signal processing materials. This document presents works about lakebed classification using single beam echo sounder and a post processing software (Sonar5) dedicated to the estimation of fish biomass. Indeed, an underwater substrate analyzing module based on the theory developed by Orlowski (Oceanologia 19 (1984), pp. 61-78) has just been implemented in the sofware. An optimum configuration have been selected and permitted to classify four different natures of substrata: silt, sand, sand & stones, and rock. A substrata nature map of an area known as an arctic charr (emblematic salmonid of the alpine big lakes) reproduction site has been done and shows the system's performances. [ABSTRACT FROM AUTHOR]

Abstract: 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 EI were 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. [Copyright &y& Elsevier]

The intercalibration (IC) exercise is a key element in the implementation of the Water Framework Directive (WFD) in Europe. Its focus lies on the harmonization of national classification methods to guarantee a common understanding of ‘Good Ecological Status’ in surface waters. This article defines reference conditions and sets class boundaries for deep (mean depth >15 m, IC lake type L-AL3) and moderately deep (mean depth 3–15 m, IC lake type L-AL4) Alpine lakes >0.5 km2. Data were collated from each of the five EU member states included in the Alpine Geographical Intercalibration Group (Alpine GIG: Austria, France, Germany, Italy and Slovenia). Hydro-morphological, chemical and biological data from 161 sites (sampling stations) in 144 Alpine lakes over a period of seven decades were collated in a database. Based on a set of reference criteria, 18 L-AL3 and 13 L-AL4 reference sites were selected. Reference conditions were defined using a combined approach, based on historical, paleolimnological and monitoring data in conjunction with trophic modelling and expert judgement. Reference values and class boundaries were set for annual mean total biomass (biovolume), and then derived for annual mean chlorophyll- a using a regression between the two parameters. In order to allow for geographical differences within the Alpine GIG and to facilitate the inclusion of the broadly defined common IC types and national lake types, ranges were defined for each reference value. Range of reference values are 0.2–0.3 mg l−1 (L-AL3) and 0.5–0.7 mg l−1 (L-AL4) for total biovolume and 1.5–1.9 μg l−1 (L-AL3) and 2.7–3.3 μg l−1 (L-AL4) for chlorophyll- a. Depending on lake type and variable, the ecological quality ratios (EQR) for setting the class boundaries lie between 0.60 and 0.75 for the high/good class boundary and between 0.25 and 0.41 for the good/moderate class boundary. The response of sensitive phytoplankton taxa along a nutrient gradient and the occurrence of ‘undesirable conditions and secondary effects’ as defined in the WFD was used to validate the class boundary values, which are thus considered to be compliant with the requirements of the WFD. [ABSTRACT FROM AUTHOR]

To analyse the importance of local and regional influences on the patterns of species richness in natural and man-made lakes and to infer the impacts of human-mediated introductions on these patterns. France. Species occurrence data were gathered for 25 natural and 51 man-made lakes. Analysis is based on regression models of local richness against their related regional richness and lake environmental variables. Local native richness was mostly controlled by the regional richness. Conversely, local total richness was mainly explained by local variables. These statements apply to both natural and man-made lakes. Lacustrine systems displayed weak resistance to invaders. Species introductions have apparently contributed to saturate fish communities in these systems even if no clear negative effect on the survival of native species (i.e. species extinction) is detectable so far. [ABSTRACT FROM AUTHOR]

Fish community feeding and production rates may differ between lakes despite similar fish biomass levels because of differences in size structure and local temperature. Therefore, across‐lake comparisons of the strength and direction of top‐down and bottom‐up fish–phytoplankton relationships should consider these factors. We used the metabolic theory of ecology to calculate size‐ and temperature‐corrected community energy demand (CEDom) and community production (CP) of omnivorous fishes in 227 European lakes from major habitat types (MHTs) of polar freshwaters, temperate floodplain rivers and wetlands, and temperate coastal rivers. We related CEDom with total phosphorus (TP)‐corrected chlorophyll a (Chl a) concentrations to evaluate a potential top‐down directed trophic cascade from fish to phytoplankton. Furthermore, we related Chl a with CP to demonstrate potential bottom‐up effects of phytoplankton on fish. For both analyses, we added the CED of piscivorous fishes (CEDpi) as a predictor to account for potential predation effects on the omnivorous fish community. CEDom was weakly positively related with TP‐corrected Chl a, but the strength of the relationship differed between MHTs. In contrast, CP was consistently positively related with Chl a in the entire dataset. CEDpi did not contribute to top‐down or bottom‐up relationships. The application of metabolic variables characterizing fish community feeding and production rates makes these results robust because the approach accounted for the usually neglected effects of fish size and temperature in across‐lake comparisons. Our results suggest that bottom‐up effects from phytoplankton on fish secondary production in lakes are substantially stronger than top‐down effects from fish on phytoplankton biomass. [ABSTRACT FROM AUTHOR]

Highlights • The WFD recommends estimating the confidence related to the ecological assessment. • We develop a method to assess the uncertainty associated with reference values. • The method can be transposed to multimetric indices that use statistical models. • We use the European Lake Fish Index as an illustration. • The uncertainty associated with index values vary among ecological classes. Abstract The publication of the Water Framework Directive by the European commission in 2000 has promoted the development of many multimetric biological indices to assess the ecological status of European waterbodies. These ecological assessments are based on the measurement of deviations between a metric's (characteristic of assemblages) observed values (obtained by sampling) and a metric's expected values in the absence of anthropogenic stressors (reference conditions). In addition, the confidence in the ecological status evaluation provided by the different biological indices is required. Numerous sources of uncertainty due to sampling variability or operator bias, for example, are often considered on observed metric values, whereas uncertainty associated with expected metric values are seldom discussed. In this study, we developed a methodology based on Monte-Carlo methods to assess the uncertainty associated with the establishment of reference values for multimetric predictive indices. This was done by randomly generating reference values and propagating the uncertainty throughout the computation of the index. This methodology can be applied to a wide variety of indices as long as it is possible to make assumptions about the statistical distributions of some of the index's numerical components (e.g. coefficients of the statistical models, metric values). The European Lake Fish Index was used to illustrate the methodology and show how this method can provide valuable information on the confidence in the ecological status defined by the index. These results also revealed that the degree of uncertainty varied between the ecological classes, which were highest for the "Moderate" class and lowest for the "Poor" and "High" classes for the ELFI. [ABSTRACT FROM AUTHOR]

Invasion of non-native species is considered a major threat to global biodiversity. Here we present a comprehensive overview of the occurrence, richness and biomass contribution of non-native fish species in 1943 standing water bodies from 14 countries of the Western Palearctic, based on standardised fish catches by multi-mesh gillnetting. We expected strong geographical gradients to emerge in the occurrence of non-natives. We further hypothesised that the contribution by non-natives to the local fish community biomass was correlated with local richness and the trophic level of native and non-native species. Non-native fish species occurred in 304 of 1943 water bodies (16%). If the average number of occupied water bodies per country was weighted by number of water bodies per country, the grand mean occurrence of non-natives in Western Palearctic water bodies was 10%. Exotic (non-native to the Palearctic) and translocated (non-native only to parts of the Palearctic) species were found in 164 (8.4%) or 235 (12.1%) of the water bodies, respectively. The occurrence and local richness of non-native fish species increased with temperature, precipitation and lake area and were substantially higher in reservoirs than in natural lakes. High local biomass contributions of non-native species were strongly correlated with low richness of native species and high richness of non-native species, whereas the trophic level of the fish species had only a weak effect. Single non-native species rarely dominated community biomass, but high biomass contributions and thus strong community and ecosystem impacts can be expected if several non-native species accumulate in a water body. [ABSTRACT FROM AUTHOR]

Biological indicators are frequently used to assess the effects of anthropogenic stressors on freshwater ecosystems. The structure of fish communities and their response to stressors have been commonly described by taxonomic richness, diversity and evenness. More recently, functional structure of communities has also been suggested to be a reliable indicator of disturbance. This article aimed at testing whether taxonomic and functional diversity metrics can provide comparable or complementary information on the response of fish communities to eutrophication and abundance of non-native species in reservoirs. Comparison of the responses of taxonomic and functional diversities to biogeography, habitat and stressors was made in 112 French reservoirs. Widely observed effects of biogeographic and habitat variables on taxonomic and functional diversities were identified. Taxonomic and functional richness metrics notably increased with lake area and temperature respectively. Taxonomic diversity metrics did not respond to any stressor, while all functional diversity metrics were found to be impacted by non-native species. Eutrophication was further found to decrease the impact of non-native species on two functional diversity metrics: evenness and divergence. Our study therefore reveals that functional metrics are more sensitive than taxonomic metrics to anthropogenic stressors in the studied reservoirs. Still, the multiple linear regressions tested had overall low explanatory power. Further refinements will thus be needed to use this type of metrics in an impact assessment scheme. [ABSTRACT FROM AUTHOR]

Lakes in Europe are subject to multiple anthropogenic pressures, such as eutrophication, habitat degradation and introduction of alien species, which are frequently inter-related. Therefore, effective assessment methods addressing multiple pressures are needed. In addition, these systems have to be harmonised (i.e. intercalibrated) to achieve common management objectives across Europe. Assessments of fish communities inform environmental policies on ecological conditions integrating the impacts of multiple pressures. However, the challenge is to ensure consistency in ecological assessments through time, across ecosystem types and across jurisdictional boundaries. To overcome the serious comparability issues between national assessment systems in Europe, a total anthropogenic pressure intensity (TAPI) index was developed as a weighted combination of the most common pressures in European lakes that is validated against 10 national fish-based water quality assessment systems using data from 556 lakes. Multi-pressure indices showed significantly higher correlations with fish indices than single-pressure indices. The best-performing index combines eutrophication, hydromorphological alterations and human use intensity of lakes. For specific lake types also biological pressures may constitute an important additional pressure. The best-performing index showed a strong correlation with eight national fish-based assessment systems. This index can be used in lake management for assessing total anthropogenic pressure on lake ecosystems and creates a benchmark for comparison of fish assessments independent of fish community composition, size structure and fishing-gear. We argue that fish-based multiple-pressure assessment tools should be seen as complementary to single-pressure tools offering the major advantage of integrating direct and indirect effects of multiple pressures over large scales of space and time. [ABSTRACT FROM AUTHOR]

While many efforts have been devoted to understand variations in food web structure among terrestrial and aquatic ecosystems, the environmental factors influencing food web structure at large spatial scales remain hardly explored. Here, we compiled biodiversity inventories to infer food web structure of 67 French lakes using an allometric niche‐based model and tested how environmental variables (temperature, productivity, and habitat) influence them. By applying a multivariate analysis on 20 metrics of food web topology, we found that food web structural variations are represented by two distinct complementary and independent structural descriptors. The first is related to the overall trophic diversity, whereas the second is related to the vertical structure. Interestingly, the trophic diversity descriptor was mostly explained by habitat size (26.7% of total deviance explained) and habitat complexity (20.1%) followed by productivity (dissolved organic carbon: 16.4%; nitrate: 9.1%) and thermal variations (10.7%). Regarding the vertical structure descriptor, it was mostly explained by water thermal seasonality (39.0% of total deviance explained) and habitat depth (31.9%) followed by habitat complexity (8.5%) and size (5.5%) as well as annual mean temperature (5.6%). Overall, we found that temperature, productivity, and habitat characteristics collectively shape lake food web structure. We also found that intermediate levels of productivity, high levels of temperature (mean and seasonality), as well as large habitats are associated with the largest and most complex food webs. Our findings, therefore, highlight the importance of focusing on these three components especially in the context of global change, as significant structural changes in aquatic food webs could be expected under increased temperature, pollution, and habitat alterations. [ABSTRACT FROM AUTHOR]

Water resources globally are affected by a complex mixture of stressors resulting from a range of drivers, including urban and agricultural land use, hydropower generation and climate change. Understanding how stressors interfere and impact upon ecological status and ecosystem services is essential for developing effective River Basin Management Plans and shaping future environmental policy. This paper details the nature of these problems for Europe's water resources and the need to find solutions at a range of spatial scales. In terms of the latter, we describe the aims and approaches of the EU-funded project MARS (Managing Aquatic ecosystems and water Resources under multiple Stress) and the conceptual and analytical framework that it is adopting to provide this knowledge, understanding and tools needed to address multiple stressors. MARS is operating at three scales: At the water body scale, the mechanistic understanding of stressor interactions and their impact upon water resources, ecological status and ecosystem services will be examined through multi-factorial experiments and the analysis of long time-series. At the river basin scale, modelling and empirical approaches will be adopted to characterise relationships between multiple stressors and ecological responses, functions, services and water resources. The effects of future land use and mitigation scenarios in 16 European river basins will be assessed. At the European scale, large-scale spatial analysis will be carried out to identify the relationships amongst stress intensity, ecological status and service provision, with a special focus on large transboundary rivers, lakes and fish. The project will support managers and policy makers in the practical implementation of the Water Framework Directive (WFD), of related legislation and of the Blueprint to Safeguard Europe's Water Resources by advising the 3rd River Basin Management Planning cycle, the revision of the WFD and by developing new tools for diagnosing and predicting multiple stressors. [ABSTRACT FROM AUTHOR]

The diversity of predatory species plays a key role in ecosystem functioning but our understanding of the mechanisms underlying their coexistence is limited, particularly in freshwater ecosystems. Northern pike Esox lucius, European perch Perca fluviatilis and pikeperch Sander lucioperca are three widespread predatory species in European lakes, where they often coexist. As potential competitors, we hypothesised that partitioning habitat is a determinant of species coexistence. This was tested by quantifying the variability of their habitat use in tracking adult individuals in the Bariousses reservoir (France, 86.6 ha, mean depth 7.1 m). Specifically, we investigated their distribution along the littoral–pelagic and depth axes along the daily cycle and across seasons. From littoral to pelagic waters were first found pike, then perch and finally pikeperch. Pike was the closest to the surface while pikeperch was the deepest. This general pattern was, however, variable across seasons with the three species located in the upper layer in summer during reservoir stratification. Individuals were more evenly distributed along the littoral–pelagic axis and closer to the bottom when water was mixing (autumn, winter). In summer, perch used more intensively in the pelagic zone during the daytime. Other species did not show any diel change of habitats. Our results highlighted that species coexistence is associated with habitat partitioning among these three predators, with perch showing a more variable behaviour regarding habitat characteristics. Now more than ever, in the context of global change which modifies habitats, it is of crucial importance to understand the coexistence mechanisms of species that shape ecosystems. [ABSTRACT FROM AUTHOR]

Fish stocking and regulation of fishing activities are widely implemented by freshwater fisheries authorities who have to protect aquatic communities. The effects of fish management on fish communities have not received sufficient attention. As a result of two datasets, one from EU Water Framework Directive (WFD) monitoring surveys, and one from a French survey targeting angler's associations, this study investigated the impacts of both fishing intensity and fish stocking on fish communities in French lakes. Both co‐inertia analysis and community size spectra analysis, showed that: (i) stocking does not contribute to the standing stocks for five of the six most stocked fish species in France; and (ii) neither fishing intensity nor fish stocking lead to a change in the proportion of large and small fish. The results support alternative methods being promoted to enhance native fish populations. [ABSTRACT FROM AUTHOR]

This assesses features of otoliths from laboratory‐reared embryos, larvae and juvenile European chub Squalius cephalus from hatching to 180 days post‐hatching (dph). We observed the development of the three pairs of otoliths (lapilli, sagittae and asterisci) and more precisely shape changes, as well as timing and deposition rate of increments of the lapilli. The lapilli and the sagittae were present at hatching, whereas the asterisci formed between 20 and 30 dph. The lapillus and sagitta shapes were round until 20 dph. From 60 dph the anterior and the posterior rostra of the sagittae were well developed, but very thin, making this otolith too fragile to manipulate for further studies of shape and validation of otolith increment deposition rate. The lapilli provided reliable age estimates for free embryos, larvae and juveniles up to 120 dph. However, caution should be taken when ageing fish older than 150 dph as an underestimation was noticeable. The regression of the number of otolith increments on age showed a slope and an intercept not significantly different from 1 and 0, respectively, which indicated that otolith growth increments were deposited on a daily basis, with the first microincrement occurring at hatching. Increment counts were consistent between three interpreters, indicating a consistent and reliable age estimate. This study validates that the otolith increment deposition rate can be used to assess hatching dates and daily growth of wild S. cephalus under 150 dph and in environments similar to the conditions used in this study. [ABSTRACT FROM AUTHOR]

The hierarchical branching nature of river networks can have a strong influence on the assembly of freshwater communities. This unique structure has spurred the development of the network position hypothesis (NPH), which states that the strength of different assembly processes depends on the community position in the river network. Specifically, it predicts that 1) headwater communities should be exclusively controlled by the local environment given that they are more isolated and environmentally heterogeneous relative to downstream reaches. In contrast, 2) downstream communities should be regulated by both environmental and dispersal processes due to increased connectivity given their central position in the riverscape. Although intuitive, the NPH has only been evaluated on a few catchments and it is not yet clear whether its predictions are generalizable. To fill this gap, we tested the NPH on river dwelling fishes using an extensive dataset from 28 French catchments. Stream and climatic variables were assembled to characterize environmental conditions and graph theory was applied on river networks to create spatial variables. We tested both predictions using variation partitioning analyses separately for headwater and downstream sites in each catchment. Only 10 catchments supported both predictions, 11 failed to support at least one of them, while in 7 the NPH was partially supported given that spatial variables were also significant for headwater communities. We then assembled a dataset at the catchment scale (e.g. topography, environmental heterogeneity, network connectivity) and applied a classification tree analysis (CTA) to determine which regional property could explain these results. The CTA showed that the NPH was not supported in catchments with high heterogeneity in connectivity among sites. In more homogeneously connected catchments, the NPH was only supported when headwaters were more environmentally heterogeneous than downstream sites. We conclude that the NPH is context dependent even for taxa dispersing exclusively within streams. [ABSTRACT FROM AUTHOR]

Body size, coupled with abundance and taxonomy, may help to understand the mechanisms shaping community structure. Since the body size of fish is closely related to their trophic niche, size diversity (based on individual body size) of fish communities may capture intraspecific variations in fish trophic niches that are not detected by species diversity. Thus, the relationship between size diversity and species diversity may help to integrate variation at both intraspecific and interspecific levels. We studied the relationship between species diversity and size diversity as a measure of the degree of overlap in size among species and thereby the potential overlap in niches in a community. We hypothesized that the relationship between size diversity and species would be different across the European continent due to different levels of size overlap in fish communities. The data were derived from samplings of fish communities using standardised benthic gill nets in 363 lakes. At the continental scale, size diversity increased with species diversity; at the ecoregion scale, the slope of the relation changed across the continent, with the greatest mismatch occurring in northern Europe where communities comprised only one or a few species, but each of which exhibited a great range in size. There was an increase in slope towards the south with significant relations for four out of six ecoregions. The steeper size diversity‐species diversity slope at lower latitudes is attributable to a lower overlap in fish size and thus likely to finer niche separation. Our results also suggest that size diversity is not a strong surrogate for species diversity in European lake fish communities. Thus, particularly in fish communities composed of few species, measuring size diversity may help to detect potential functional variation which may be neglected by measuring species diversity alone. [ABSTRACT FROM AUTHOR]

Planktivorous and benthivorous fish have been documented to influence the density and size structure of their prey communities in lakes. We hypothesized that piscivorous fish modify their prey fish communities in the same way and sought to find evidence for such predation effects from a comparison across 356 lakes located in nine European ecoregions. We categorized individual fish as being piscivore, nonpiscivore, or prey of piscivores, depending on species and individual size. We calculated piscivore, nonpiscivore, and piscivore prey densities, respectively, and fit linear abundance size spectra (SS) on lake-specific piscivore, nonpiscivore, and piscivore-prey size distributions. Multiple linear regressions were calculated to quantify the effect of piscivore density and SS slopes on nonpiscivore and piscivore-prey densities and SS slopes by accounting for potentially confounding factors arising from lake morphometry, productivity, and local air temperature. Piscivore density correlated positively with piscivore-prey density but was uncorrelated with density of nonpiscivores. Across a subset of 76 lakes for which SS slopes of piscivores were statistically significant, SS slopes of piscivores were uncorrelated with SS slopes of either nonpiscivores or piscivore prey. However, densities of piscivores, nonpiscivores, or piscivore prey were a significant negative predictor of SS slopes of the respective groups. Our analyses suggest that direct predation effects by piscivorous fish on density and size structure of prey fish communities are weak in European lakes, likely caused by low predator-prey size ratios and the resulting size refuges for prey fish. In contrast, competition may substantially contribute to between-lake variability in fish density and size. [ABSTRACT FROM AUTHOR]

We studied fish size structure by using mean size, size diversity, and the slope of linear size spectra of six common European fish species along large-scale environmental gradients. We further analyzed the response of these three size metrics to environmental variables and to density-dependent effects, i.e., relative estimates of abundance (catch per unit effort, CPUE). We found differences in the strength of main predictors of size structure between the six species, but the direction of the response was relatively similar and consistent for most of the size metrics. Mean body size was negatively related to temperature for perch ( Perca fluviatilis), roach ( Rutilus rutilus), and ruffe ( Gymnocephalus cernuus). Lake productivity (expressed as total phosphorus concentration) and lake depth were also predictors of size structure for four of six species. Moreover, we found a strong density dependence of size structure for all species, resulting in lower mean body size and size diversity and steeper size spectra slopes when density dependence increases. This suggests that density dependence is a key driver of fish size structure. [ABSTRACT FROM AUTHOR]

Global biodiversity in freshwater and the oceans is declining at high rates. Reliable tools for assessing and monitoring aquatic biodiversity, especially for rare and secretive species, are important for efficient and timely management. Recent advances in DNA sequencing have provided a new tool for species detection from DNA present in the environment. In this study, we tested whether an environmental DNA ( eDNA) metabarcoding approach, using water samples, can be used for addressing significant questions in ecology and conservation. Two key aquatic vertebrate groups were targeted: amphibians and bony fish. The reliability of this method was cautiously validated in silico, in vitro and in situ. When compared with traditional surveys or historical data, eDNA metabarcoding showed a much better detection probability overall. For amphibians, the detection probability with eDNA metabarcoding was 0.97 ( CI = 0.90-0.99) vs. 0.58 ( CI = 0.50-0.63) for traditional surveys. For fish, in 89% of the studied sites, the number of taxa detected using the eDNA metabarcoding approach was higher or identical to the number detected using traditional methods. We argue that the proposed DNA-based approach has the potential to become the next-generation tool for ecological studies and standardized biodiversity monitoring in a wide range of aquatic ecosystems. [ABSTRACT FROM AUTHOR]

In this study we developed a novel methodology based on fish communities to assess the ecological potential of central European reservoirs. Using the hindcasting approach, our index predicts values that could be observed in the absence of pressures for each reservoir depending on their environmental characteristics. Fish data were collected from 144 French and Czech reservoirs between 2005 and 2013 by standardized benthic gillnet sampling and transformed to functional and taxonomical metrics. After all validation by multiple testing of models redundancy and pressure-response, the final index was composed of three metrics: total biomass of fish, abundance of invertivores/piscivores, and abundance of planktivorous fish. The index accurately identifies reservoirs that are lightly, moderately and heavily affected by eutrophication. In addition to French and Czech reservoirs, this index could be a useful tool for countries with few reservoirs and the basis for further collaborative studies. [ABSTRACT FROM AUTHOR]

Aim Our aim was to document geographical patterns of variation in the body-size structure of European lake fish assemblages along abiotic gradients, and any differences in fish assemblage structure. We hypothesized that patterns in the body-size structure of entire lake fish assemblages are primarily temperature driven and consistent with the dominant pattern of the temperature-size rule, which suggests a decrease in adult body size with increasing developmental temperature for many ectothermic species. Location 356 European lakes. Methods Variation in the body-size structure of fish assemblages was explored on a continental scale along gradients of temperature, morphometry, productivity and fish assemblage structure for 356 European lakes. The mean fish assemblage body-size and individual body-size distributions were selected as size metrics. Separate analyses were conducted for lakes located within five ecoregion subsets (Borealic Uplands/Tundra, FennoScandian Shield, Central Plains, Western Plains and Western Highlands) and for lakes with different functional fish classifications (cold-, cool- and warmwater fish assemblages). Results Geographical patterns of variation in the body-size structure of European lake fish assemblages could be clearly discerned along a temperature gradient for both the continental dataset (356 lakes) and the smaller geographical (ecoregion) subsets. We found systematic changes in fish assemblage body-size structure across temperature gradients in correspondence with the dominant thermal fish guild. The majority of the lakes, mainly located in the warmer European lowlands, were dominated by eurythermic cool- and warmwater fish assemblages, with smaller sized individuals characterized by linear individual body-size distributions. Lakes located in colder regions and dominated by stenothermic coldwater salmonids with larger sized individuals were characterized by unimodal or bimodal size distributions. The mean body size of cold-, cool- and warmwater fish assemblages changed uniformly along the temperature gradient. Main conclusions Patterns of variation in the body-size structure of European lake fish assemblages are consistent with the temperature-size rule. Temperature modifies fish assemblage size structure uniformly within the thermal fish guilds and in different ecoregions. Furthermore, our results indicate an increasing predictive power of temperature to explain variability in body-size structure when moving from warmer to colder geographical regions. [ABSTRACT FROM AUTHOR]

We aimed to distinguish the relative contributions of natural and anthropogenic local factors on patterns of fish diversity in European lakes at different geographical scales., We compiled data from standardised fish monitoring using multimesh benthic gill nets, information on lake morphometry and on geographical, climatic and anthropogenic pressure variables from 1632 lakes in 11 European countries. By means of regression trees, we determined those natural and anthropogenic factors and their thresholds that best predicted local fish diversity, density and mean size. Generalised linear models were used to assess the influence of anthropogenic factors at smaller geographical and morphometric scales., Local fish species richness and diversity were related mainly to morphometric and (bio)geographical/climatic variables. Larger and deeper lakes in warm areas tended to be the most species rich and diverse. Fish density was related mainly to anthropogenically driven productivity but also was sensitive to geographical/climatic factors. Thus, warmer and shallower lower-altitude European lakes, which are usually more eutrophic, had higher fish densities than cold and deeper higher-altitude lakes. Fish size increased with altitude and declined with increasing seasonality and temperature., After controlling for the natural factors, productivity had a positive effect on fish species richness and diversity, whereas it negatively influenced fish size., Our results suggest that macroecological patterns of lake fish diversity across Europe are best predicted by natural factors. The contribution of anthropogenic factors to fish diversity was evident only via the effect of eutrophication at smaller geographical scales, whereas no effect could be found from hydromorphological pressures. From an applied perspective, these results suggest that bioassessment and biodiversity evaluation might be most effectively conducted and interpreted locally, where anthropogenic effects on biodiversity become more apparent. At a macroecological scale, the strong effect of environmental temperature on most components of fish diversity suggests future changes in fish diversity as a consequence of climate change. [ABSTRACT FROM AUTHOR]

Data on phytoplankton, macrophytes, benthic invertebrates and fish from more than 2000 lakes in 22 European countries were used to develop and test metrics for assessing the ecological status of European lakes as required by the Water Framework Directive. The strongest and most sensitive of the 11 metrics responding to eutrophication pressure were phytoplankton chlorophyll a, a taxonomic composition trophic index and a functional traits index, the macrophyte intercalibration taxonomic composition metric and a Nordic lake fish index. Intermediate response was found for a cyanobacterial bloom intensity index (Cyano), the Ellenberg macrophyte index and a multimetric index for benthic invertebrates. The latter also responded to hydromorphological pressure. The metrics provide information on primary and secondary impacts of eutrophication in the pelagic and the littoral zone of lakes. Several of these metrics were used as common metrics in the intercalibration of national assessment systems or have been incorporated directly into the national systems. New biological metrics have been developed to assess hydromorphological pressures, based on aquatic macrophyte responses to water level fluctuations, and on macroinvertebrate responses to morphological modifications of lake shorelines. These metrics thus enable the quantification of biological impacts of hydromorphological pressures in lakes. [ABSTRACT FROM AUTHOR]

1. Sampling of lake fish assemblages is a challenging task in fish science, and the information obtained strongly depends on the choice of sampling gear. The use of more than one sampling technique is generally preferred in order to achieve a comprehensive view on fish assemblage structure. Therefore, the knowledge of whether catches between fishing gears are comparable is crucial. 2. We compared catches in benthic multi-mesh gillnets with fish biomass estimates obtained by vertical hydroacoustics in 18 European lakes strongly varying in morphometry and trophic status. Separate analyses were conducted for different depth strata and for several fish length thresholds to account for depth- and size-selective gillnet catches. 3. Gillnet catches and hydroacoustically obtained fish biomass estimates were significantly correlated. The strength of correlations was independent of the fish length thresholds applied, but varied across different depth strata of the lakes, with the strongest correlations occurring in the shallow strata. 4. The results support the applicability of vertical hydroacoustics for the quantification of fish biomass in stratified lakes. Survey designs combining hydroacoustics with limited gillnetting at sampling dates shortly one after the other, the latter for the purpose of inventory sampling only, are a cost-effective strategy for sampling fish assemblages in lakes. However, gillnet sampling does not provide reliable fish density estimates in very deep lakes with separate, pelagic-dwelling fish assemblages. [ABSTRACT FROM AUTHOR]

ABSTRACT Aim In times of biodiversity crisis, it is extremely important to understand diversity gradients. In particular, the study of the diversity of ecological functions is a key issue for the management of ecosystem integrity. Here we identify areas of low functional diversity of the native fish fauna in European drainage basins and we determine the relative importance of three underlying mechanisms: environmental filtering, geographic isolation and climatic history. Location The European continent. Methods Based on 14 morphological traits that are closely related to fish function (habitat and dietary niches), three independent functional diversity indices [functional richness (FR), functional evenness (FE), functional divergence (FD)] were calculated for 128 European drainage basins with a total of 230 fish species. The indices were standardized for species richness using null models. The patterns of the standardized indices are described and three potentially underlying mechanisms are tested using variance partitioning and multi-linear regression models. Results FR and FD were highest in eastern European drainage basins and in Great Britain and lowest in the Mediterranean. FE patterns were less pronounced. All observed patterns were mainly governed by geographic isolation and present environmental conditions. Within the environmental conditions, average annual temperature and precipitation were good predictors for functional diversity. The role of habitat diversity and size was negligible. Main conclusions Geographic isolation coupled with harsh environmental conditions such as extreme temperatures and low precipitation, as in Mediterranean regions, can lead to low FR and FD. This can be explained by extinction that could not be compensated by re-colonization and high speciation. Due to their high functional redundancy, communities in these areas might better withstand further species extinctions on a small scale. Over the short term, however, their often extremely low FR suggests a less flexible functioning that can hinder their ability to withstand today's rapid environmental and anthropogenic threats. [ABSTRACT FROM AUTHOR]

Abstract: The European Water Framework Directive clearly indicates that fish is one of the “quality elements” to be considered for the assessment of lentic systems. However, few fish-based indices (FBIs) of biotic integrity have been developed for lakes so far. Hence, the aim of our study was to develop such a tool for French lakes. Fish surveys, lakes natural environmental parameters, catchment-scale anthropogenic pressures, and local pressures were collected for 67 reservoirs and 24 natural lakes throughout France. After assigning fish species into trophic, reproductive, and tolerance guilds, we derived a set of metrics reflecting complementary aspects of community functioning and condition. Other community-level traits such as richness and evenness were added. These metrics were modeled vs. natural environmental variables and pressures. Reference conditions at each site were then assessed using hindcasting modeling. Separate indices were eventually obtained for natural and artificial lakes by combining selected metrics. Fifteen out of 73 candidate fish metrics, covering all three groups of functional traits, displayed a significant response to anthropogenic pressures. After removal of the redundant traits, the final indices for natural lakes and reservoirs included three and six metrics, respectively. Agricultural-related impacts were prominent for reservoirs, whereas for natural lakes urban and local pressures displayed the most significant effects. [Copyright &y& Elsevier]

1. The mechanisms that structure biological communities hold the key to understanding ecosystem functioning and the maintenance of biodiversity. Patterns of species abundances have been proposed as a means of differentiation between niche-based and neutral processes, but abundance information alone cannot provide unequivocal discrimination. 2. We combined species niche information and species’ relative abundances to test the effects of two opposing structuring mechanisms (environmental filtering and niche complementarity) on species’ relative abundances in French lacustrine fish communities. The test involved a novel method comparing the abundance-weighted niche overlap within communities against that expected when relative abundances were randomized among species within the community. 3. Observed overlap was consistently significantly lower than expected at random for two (swimming ability and trophic status) of four primary niche axes across lakes of differing physical environments. Thus, for these niche axes, pairs of abundant species tended to have relatively low niche overlap, while rare species tended to have relatively high niche overlap with abundant species. 4. This suggests that niche complementarity may have acted to enhance ecosystem function and that it is important for species coexistence in these fish communities. The method used may be easily applied to any sort of biological community and thus may have considerable potential for determining the generality of niche complementarity effects on community structure. [ABSTRACT FROM AUTHOR]

1. Interspecific niche differences have long been identified as a major explanation for the occurrence of species-rich communities. However, much fieldwork studying variation in local species richness has focused upon physical habitat attributes or regional factors, such as the size of the regional species pool. 2. We applied indices of functional diversity and niche overlap to data on the species niche to examine the importance of interspecific niche differentiation for species richness in French lake fish communities. We combined this information with environmental data to test generalizations of the physiological tolerance and niche specialization hypotheses for species–energy relationships. 3. We found evidence for a largely non-saturating relationship (relative to random expectation) between species richness and functional evenness (evenness of spacing between species in niche space), while functional richness (volume of niche space occupied) peaked at moderate levels of species richness and niche overlap showed an initial decrease followed by saturation. This suggests that increased niche specialization may have allowed species to coexist in the most species-rich communities. 4. We tested for evidence that increased temperature, local habitat area, local habitat diversity and immigration affected species richness via increased niche specialization. Temperature explained by far the largest amount of variation in species richness, functional diversity and niche overlap. These results, combined with the largely non-saturating species richness–functional evenness relationship, suggest that increased temperature may have permitted increased species richness by allowing increased niche specialization. 5. These results emphasize the importance of niche differences for species coexistence in species-rich communities, and indicate that the conservation of functional diversity may be vital for the maintenance of species diversity in biological communities. Our approach may be applied readily to many types of community, and at any scale, thus providing a flexible means of testing niche-based hypotheses for species richness gradients. [ABSTRACT FROM AUTHOR]

Functional characters have the potential to act as indicators of species turnover between local communities. Null models provide a powerful statistical approach to test for patterns using functional character information. A combined null model/functional character approach provides the ability to distinguish between the effect of competition and environmental filtering on species turnover. We measured 13 functional characters relating directly to resource use for the fish species found in French lakes. We combined this functional character data with a null model approach to test whether co-occurring species overlapped more or less than expected at random for four primary niche axes. We used an environmentally constrained null model approach to determine if the same mechanisms were responsible for species turnover at different sections of the altitudinal gradient. Functional diversity indices were used to examine the variation in functional character diversity with altitude, as a test of the hypothesis that competitive intensity decreases with increasing environmental adversity. The unconstrained null model showed that environmental filtering was the dominant influence on species turnover between lakes. In the constrained null model, there was much less evidence for environmental filtering, emphasising the strong effect of altitude on turnover in functional character values between local communities. Different results were obtained for low-altitude and high-altitude lake subsets, with more evidence for the effect of environmental filtering being found in the high-altitude lakes. This demonstrates that different processes may influence species turnover throughout an environmental gradient. Functional diversity values showed a slight decrease with altitude, indicating that there was only weak evidence that competitive intensity decreased with increasing altitude. Variation resource availability and environmental stress probably cause the observed turnover in functional characters along the altitudinal gradient, though the effects of dispersal limitation and species introductions in high-altitude lakes cannot be ruled out. [ABSTRACT FROM AUTHOR]

Aim To assess whether eight traits of fish communities (species richness, three reproductive traits and four trophic traits) respond similarly to environmental gradients, and consequently display convergence between the lakes of France and north-east USA (NEUSA). Location 75 French and 168 north-east USA lakes. Methods The data encompass fish surveys, the assignment of species into reproductive and trophic guilds, and environmental variables characterizing the lakes and their catchments. The analytical procedure was adapted from the recommendations of Schluter (1986 ) [ Ecology, 67, 1073–1085]. Results The comparison of the regional pools of lacustrine fishes indicated that NEUSA was about twice as speciose as France, mostly due to higher species turnover across lakes, although NEUSA lakes were consistently about 20% more speciose than French lakes for a given surface area. Warmer environments were consistently inhabited by a higher proportion of phytophilous and guarder species than were colder lakes. Hence there was convergence in community reproductive traits. Conversely, there was no evidence of convergence in the trophic structure of lacustrine fish communities between regions. Main conclusions The influence of temperature on the availability and quality of spawning substrates appears to be a major constraint on present-day lacustrine fish communities. In parallel, phylogenetic constraints, past events such as the diversification of the North American fish fauna, and selective extinctions during Pleistocene glaciations and subsequent recolonizations contribute to explaining the dissimilarities between the communities of the two regions and differences in their relationship to the environment. [ABSTRACT FROM AUTHOR]

Pikeperch ( Sander lucioperca L.) is a broadly distributed fish species in Europe but little is known about its ecology in the southern part of its distribution area in warm climatic conditions. The aim of this study was to analyse pikeperch rate of movement and to assess whether it displayed a diel pattern related to temperature. Thus acoustic telemetry was used to track adult pikeperch in a drainage canal located in south of France. The survey was carried out in spring, during the spawning period. The results showed that females were more active than males. This is in accordance with previous data on the nest guardian behaviour of the males. For both genders, the activity rates increased during the study period as water temperature rose. Males and females displayed the same diel activity with a maximum at dusk, thereby confirming many indirect observations. Nevertheless, inter-individuals variations were observed. Thus, these results on diel activity are rather a general trend than a strict rule and suggest the involvement of other factors than light intensity in the control of diel activity. This diel rhythm is positively correlated to water temperature for females. Pikeperch activity may be the result of a trade-off between physiological requirements of temperature and light, satisfaction of energy needs and avoidance of predators. [ABSTRACT FROM AUTHOR]

Biodiversity has reached a critical state. In this context, stakeholders need indicators that both provide a synthetic view of the state of biodiversity and can be used as communication tools. Using river fishes as model, we developed community indicators that aim at integrating various components of biodiversity including interactions between species and ultimately the processes influencing ecosystem functions. We developed indices at the species level based on (i) the concept of specialization directly linked to the niche theory and (ii) the concept of originality measuring the overall degree of differences between a species and all other species in the same clade. Five major types of originality indices, based on phylogeny, habitat-linked and diet-linked morphology, life history traits, and ecological niche were analyzed. In a second step, we tested the relationship between all biodiversity indices and land use as a proxy of human pressures. Fish communities showed no significant temporal trend for most of these indices, but both originality indices based on diet- and habitat- linked morphology showed a significant increase through time. From a spatial point of view, all indices clearly singled out Corsica Island as having higher average originality and specialization. Finally, we observed that the originality index based on niche traits might be used as an informative biodiversity indicator because we showed it is sensitive to different land use classes along a landscape artificialization gradient. Moreover, its response remained unchanged over two other land use classifications at the global scale and also at the regional scale. [ABSTRACT FROM AUTHOR]