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The species-area relationship (SAR) has over a 150-year long history in ecology, but how its shape and origins vary across scales and organisms is still not fully understood. This is the first subcontinental freshwater study to examine both properties of the SAR in a spatially explicit way across major organismal groups (diatoms, insects, and fish), differing in body size and dispersal capacity. First, to describe the SAR shape, we evaluated the fit of three commonly used models, logarithmic, power, and Michaelis-Menten. Second, we proposed a hierarchical framework to explain the variability in the SAR shape, captured by the parameters of the SAR model. According to this framework, scale and species group were the top predictors of the SAR shape, climatic factors (heterogeneity and median conditions) represented the second predictor level, and metacommunity properties (intraspecific spatial aggregation, γ-diversity, and species abundance distribution), the third predictor level. We calculated the SAR as a sample-based rarefaction curve using 60 streams within landscape windows (scales) in the US, ranging from 160,000 to 6,760,000 km

La biodiversité actuellement observée dans les milieux naturels est le résultat de processus passés (naturels ou anthropiques) qui ont contraint et sélectionné les espèces présentes sur ces milieux. L’histoire de ces contraintes est pourtant rarement considérée dans l’étude de la diversité des communautés biologiques, en particulier dans les écosystèmes aquatiques tels que les lacs et étangs. En utilisant l’histoire du paysage comme un indicateur de l’influence anthropique passée, nous avons cherché, dans cette étude, à mesurer l’importance relative de ces processus historiques sur la diversité des plantes aquatiques. Ainsi, nous avons reconstitué l’occupation du sol en 1945, 1965, 1985 et 2000 sur 17 bassins versants de lacs et étangs du littoral aquitain, afin d’examiner dans un premier temps la dynamique paysagère sur ces espaces, et dans un second temps, de tester son importance sur la structure et la composition de la diversité des macrophytes. L’influence relative du paysage passé et récent ainsi que des facteurs abiotiques liés à la qualité de l’eau et aux caractéristiques physiques des masses d’eau sur la richesse et la composition des macrophytes a ainsi été évaluée. Nos résultats montrent que les bassins versants étudiés témoignent d’une dynamique d’artificialisation croissante au dépens des zones semi-naturelles, et que la qualité physico-chimique de l’eau et l’occupation du sol passée jouent conjointement un rôle prépondérant dans l’organisation des communautés végétales. Ces résultats soulignent l’importance de l’histoire du paysage dans la structuration de la biodiversité actuelle et suggèrent des délais de réponse importants des communautés de macrophytes suite aux différentes pressions anthropiques. Intégrer les dynamiques temporelles dans l’étude des écosystèmes lacustres se révèle donc indispensable à une meilleure compréhension des mécanismes naturels qui gouvernent l’assemblage des espèces occupant ces milieux, et offre de nouvelles perspectives pour la gestion de sa biodiversité. Biological diversity currently observed in natural areas is the result of past environmental processes (natural or anthropogenic) which have constrained and selected local species occurrence. Environmental history is however rarely considered in biological studies focusing on community diversity, particularly in freshwater aquatic ecosystems such as lakes and ponds. In using landscape history as an indicator of past anthropogenic influence, we aimed to measure the relative importance of historic processes on aquatic plant diversity. Consequently, we determined land-use occupancies in 1945, 1965, 1985 and 2002 on 17 watershed’s lakes and ponds of South-Western France, in order to i) evaluated land-use changes during the last decades and ii) tested the effect of historical land-use on macrophyte diversity and composition. The relative influence of past land-use, recent land-use as well as abiotic factors related to water quality and lakes’ morphological features on macrophyte taxonomic richness and composition was tested. We found that lakes’ watershed landscapes evidenced an increase of anthropization at the expense of semi-natural areas and that water quality jointly with historical land-use strongly structured macrophytes taxonomic richness and composition. These results underlined the importance of past land-use on current diversity patterns and suggested delay responses of macrophytes communities to anthropogenic pressures. Including temporal dynamics in lacustrine ecosystems studies is thus essential for a better understanding of natural processes patterning aquatic species assemblages, and gives new insights for better management of their biodiversity.

Aim The species-area relationship (SAR) is one of the most distinctive biogeographic patterns, but global comparisons of the SARs between island and mainland are lacking for microbial taxa. Here, we explore whether the form of the SAR and the drivers of species richness, including area, environmental heterogeneity, climate and physico-chemistry, differ between islands and similarly sized areas on mainland, referred to as continental area equivalents (CAEs). Location Global. Taxon Stream benthic diatoms. Methods We generated CAEs on six continental datasets and examined the SARs of CAEs and islands (ISAR). Then, we compared CAEs and islands in terms of total richness and richness of different ecological guilds. We tested the factors contributing to richness in islands and CAEs with regressions. We used structural equation models to determine the effects of area versus environmental heterogeneity, climate and local conditions on species richness. Results We found a non-significant ISAR, but a significant positive SAR in CAEs. Richness in islands was related to productivity. Richness in CAEs was mainly dependent on area and climate, but not directly on environmental heterogeneity. Species richness within guilds exhibited inconsistent relationships with island isolation and area. Main conclusions Ecological and evolutionary processes shaping diatom island biogeography do not depend on area at the worldwide scale probably due to the presence of distinct species pool across islands. Conversely, area was an important driver of diatom richness in continents, and this effect could be attributed to dispersal. Continents had greater richness than islands, but this was a consequence of differences in environmental conditions such as specific island climatic conditions. We stress the need for more island data on benthic diatoms, particularly from archipelagos, to better understand the biogeography of this most speciose group of algae.

International audience; Bioassessment is widely used to measure ecological integrity of natural habitats following anthropogenic dis-turbances and modifications. Traditionally, bioassessment has been based exclusively on species-environment interactions, i.e. niche processes. However, dispersal processes, and in particular mass effect, could mask the influence of niche processes and lead to erroneous conclusions about ecosystem health. To circumvent this problem, we identified 40 diatom species with distributions driven primarily by mass effect and propose an alternative version of the Biological Diatom Index (IBD2007) excluding these species. We tested the environmental responses of both the original IBD (IBD2007) and the modified IBD (IBDmod) with a benthic diatom dataset from France, collected between 2007 and 2013 and including 9487 samples from 3913 spatially distinct localities. Our results indicate a better relationship between the IBDmod scores and environmental conditions, compared to the IBD2007 scores, leading to a more accurate determination of river ecological status, especially in conditions of moderate nutrient enrichment. This study supports the idea that mass effect may result in biased evaluation of water quality. It is advocated that this process is considered in other diatom-based indices, and by extension, in any biotic index.

Aim The interaction of land use with local versus regional processes driving biological homogenization (β‐diversity loss) is poorly understood. We explored: (a) stream β‐diversity responses to land cover (forest versus agriculture) in terms of physicochemistry and physicochemical heterogeneity; (b) whether these responses were constrained by the regional species pool, i.e. γ‐diversity, or local assembly processes through local (α) diversity; (c) whether local assembly operated through the regional species abundance distribution (SAD) or intraspecific spatial aggregation; and (d) the dependence on body size, dispersal capacity and trophic level (producer versus consumer). Location USA, Canada and France. Time period 1993–2012. Major taxa studied Stream diatoms, insects and fish. Methods We analysed six datasets totalling 1,225 stream samples. We compared diversity responses to eutrophication and physicochemical heterogeneity in forested versus agricultural streams with regression methods. Null models quantified the contribution of local assembly to β‐diversity (β‐deviance, βDEV) for both types of land covers and partitioned it into fractions explained by the regional SAD (βSAD) versus aggregation (βAGG). Results Eutrophication explained homogenization and more uneven regional SADs across groups, but local and regional biodiversity responses differed across taxa. The βDEV was insensitive to land use. The βSAD largely exceeded βAGG and was higher in agriculture. Main conclusions Eutrophication but not physicochemical heterogeneity of agricultural streams underlay β‐diversity loss in diatoms, insects and fish. Agriculture did not constrain the magnitude of local versus regional effects on β‐diversity but controlled the local assembly mechanisms. Although the SAD fraction dominated in both land covers, it increased further in agriculture at the expense of aggregation. Notably, the regional SADs were more uneven in agriculture, exhibiting excess common species or stronger dominance. Diatoms and insects diverged from fish in terms of biodiversity, SAD shape and βDEV patterns, suggesting an overriding role of body size and/or dispersal capacity compared with trophic position.

Aim To quantify the relative contributions of local community assembly processes versus gamma-diversity to beta-diversity, and to assess how spatial scale and anthropogenic disturbance (i.e. nutrient enrichment) interact to dictate which driver dominates. Location France and the United States. Time period 1993-2011. Major taxa studied Freshwater stream diatoms. Methods beta-diversity along a nutrient enrichment gradient was examined across multiple spatial scales. beta-diversity was estimated using multi-site Sorensen dissimilarity. We assessed the relative importance of specialists versus generalists using Friedley coefficient, and the contribution of local community assembly versus gamma-diversity to beta-diversity across spatial scales, with a null model. Finally, we estimated the response of beta-diversity to environmental and spatial factors by testing the correlations between community, environmental and geographical distance matrices with partial Mantel tests. Results beta-diversity generally increased with spatial scale but the rate of increase depended on nutrient enrichment level. beta-diversity decreased significantly with increasing nutrient enrichment level due to the loss of specialist species. Local assembly was an important driver of beta-diversity especially under low nutrient enrichment. Significant partial Mantel correlations were observed between diatom beta-diversity and pure environmental distances under these conditions, highlighting the role of species sorting in local assembly processes. Conversely, in heavily enriched sites, only spatial distances were significantly correlated with beta-diversity, which indicated a substantial role of dispersal processes. Main conclusions Nutrient concentration mediated the expected increase in beta-diversity with spatial scales. Across spatial scales, beta-diversity was more influenced by local assembly processes rather than by gamma-diversity. Nutrient enrichment was associated with an overall decline in diatom beta-diversity and a shift in assembly processes from species sorting to dispersal, notably due to the elimination of some specialists and their subsequent replacement by generalists.

Resilience in river ecosystems requires that organisms must persist in the face of highly dynamic hydrological and geomorphological variations. Disturbance events such as floods and droughts are postulated to shape life history traits that support resilience, but river management and conservation would benefit from greater understanding of the emergent effects in communities of river organisms. We unify current knowledge of taxonomic‐, phylogenetic‐, and trait‐based aspects of river communities that might aid the identification and quantification of resilience mechanisms. Temporal variations in river productivity, physical connectivity, and environmental heterogeneity resulting from floods and droughts are highlighted as key characteristics that promote resilience in these dynamic ecosystems. Three community‐wide mechanisms that underlie resilience are (a) partitioning (competition/facilitation) of dynamically varying resources, (b) dispersal, recolonization, and recruitment promoted by connectivity, and (c) functional redundancy in communities promoted by resource heterogeneity and refugia. Along with taxonomic and phylogenetic identity, biological traits related to feeding specialization, dispersal ability, and habitat specialization mediate organism responses to disturbance. Measures of these factors might also enable assessment of the relative contributions of different mechanisms to community resilience. Interactions between abiotic drivers and biotic aspects of resource use, dispersal, and persistence have clear implications for river conservation and management. To support these management needs, we propose a set of taxonomic, phylogenetic, and life‐history trait metrics that might be used to measure resilience mechanisms. By identifying such indicators, our proposed framework can enable targeted management strategies to adapt river ecosystems to global change.

In lake ecosystems, phytoplankton communities can be studied by adopting taxonomic-based approaches. However, these approaches suffer from identification issues and are sometimes of limited ecosystem ecological value. The recent development of functional approaches may allow an evaluation of other aspects of ecosystem quality, functions and interactions with abiotic parameters or other communities. Here, our aim was to create a phytoplankton trait database at the French scale. This database will be relevant for the analysis of phytoplankton communities that will lead to a better understanding of phytoplankton functional ecology in lakes of France and other European countries possessing similar biological communities. We used a French national database of phytoplankton occurrences sampled from 384 lakes over the entire French metropolitan territory. A final list of 636 taxa was used to compile 53 morpho-functional traits associated with taxonomic information. The traits encompassed variables such as shape, biovolume, motility, toxin production and Reynolds groups. With this new database, we aim to provide data for morpho-functional analyses of phytoplankton assemblages from local to European scale., Measurement(s) morphological feature • functional • biovolume • life form • geometric cell form • cell size • carbon content • size • cell motility • mucilage • presence of heterocyte • plastid • presence of siliceous skeleton • cyanotoxin • protuberance • chlorophyll b • chlorophyll c • xanthophyll • nutrition mode • reproduction • trophic water class • Tolerance • Reynolds group • presence of lorica • presence of plate • akinete formation • number of flagella Technology Type(s) digital curation Sample Characteristic - Organism freshwater phytoplankton species Sample Characteristic - Environment lake • freshwater lake Sample Characteristic - Location French River Lakes Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.13416659

International audience; The historical composition of landscapes is recognized as an important factor for explaining plant diversity patterns, because current species assemblages are not only patterned by current ecological conditions, but also represent legacies of the past. Indeed, historical landscapes can represent past anthropogenic impact on the environment whose effects could still be observed nowadays. However, the influence of history on diversity patterns has mostly been investigated for terrestrial ecosystems, but has rarely been considered for aquatic ones. Here, we examined the effect of historical landscapes and land-use changes on the taxonomic richness and composition of aquatic plant communities in French freshwater shallow lakes and ponds. We aimed to test the relative influence of environmental variables and landscapes legacy on macrophyte species assemblages. We surveyed the macrophytes communities in 100m sections of the shoreline of each lake (N=17) up to 1m depth. We used water chemistry and physical features of lakes as environmental variables, and reconstructed historical land-use of lakes' watershed with aerial pictures of 1945, 1965, 1985 and 2000 for examining historical variables. Landscape changes were examined with transition matrices and principal component analysis (PCA) and species composition with non-metrical multidimensional scaling (NMDS). The relative influence of physical, chemical and historical factors on species richness and composition were tested with variation partitioning methods. We found that the landscape of all lakes' watershed exhibit a trend toward an increase of urban and arable areas at the expense of semi-natural ones. Environmental variables and historical factors are strongly correlated with macrophyte diversity but current landscape composition seems to have only a weak influence. Especially, historical landscape factors appeared to be strongly associated with species macrophyte composition. These results underline the importance of historical factors in explaining aquatic plant diversity and suggest delayed responses of plant communities to anthropogenic pressures. Integrating historical factors in future analyses of aquatic ecosystems would thus greatly contribute to understand ecological processes governing their species assemblages, and would be crucial for their conservation and management.

Questions: Forest fragmentation affects biodiversity locally (alpha diversity) and beyond - at relatively larger scales (gamma diversity) - by increasing dispersal and recruitment limitations. Yet, does an increase in fragmentation affect the relationship between alpha and gamma diversity and what can we learn from it? Location: Northern France. Methods: We surveyed 116 forest patches across three fragmentation levels: none (continuous forest); intermediate (forest patches connected by hedgerows); and high (isolated forest patches). Plant species richness of both forest specialists and generalists was surveyed at five nested spatial resolutions across each forest patch: 1 m(2); 10 m(2); 100 m(2); 1,000 m(2); and total forest patch area. First, we ran log-ratio models to quantify the alpha-gamma relationship. We did that separately for all possible combinations of fragmentation level (none vs intermediate vs high) x spatial scale (e.g., alpha-1 m(2) vs gamma-10 m(2)) x species type (e.g., alpha-specialists vs gamma-specialists). We then used linear mixed-effects models to analyze the effect of fragmentation level, spatial scale, species type and all two-way interaction terms on the slope coefficient extracted from all log-ratio models. Results: We found an interaction effect between fragmentation level and species type, such that forest specialists shifted from a linear (i.e., proportional sampling) to a curvilinear plateau (i.e., community saturation) relationship at low and high fragmentation, respectively, while generalists shifted from a curvilinear to a linear pattern. Conclusions: The impact of forest fragmentation on the alpha-gamma relationship supports generalist species persistence over forest specialists, with contrasting mechanisms for these two guilds. As fragmentation increases, forest specialists shift from proportional sampling towards community saturation, thus reducing alpha diversity likely due to dispersal limitation. Contrariwise, generalists shift from community saturation towards proportional sampling, thus increasing alpha diversity likely due to an increase in the edge:core ratio. To ensure long-term conservation of forest specialists, one single large forest patch should be preferred over several small ones.

International audience; 1. Mass effect, allowing species to persist in unfavorable habitats, and dispersal limitation, preventing species from reaching favorable habitats, are the two major dispersal processes. While dispersal limitation can be detected by experimental or modeling techniques, mass effect is more challenging to evaluate, which hampers our ability to disentangle the influence of the environment vs. dispersal on species distribution. This is undesirable for biomonitoring programs built on known species-environment relationships.2. We developed an innovative method for detection of species influenced by mass effect. We tested it on stream diatoms, a widely used taxonomic group for stream biomonitoring, from four French watersheds. This method combined (1) an appropriate spatial framework, the Asymmetric Eigenvector Map (AEM), used in species distribution modeling to measure the relative influence of dispersal vs. niche processes, (2) an analysis of negative co-occurrence patterns to separate mass effect from dispersal limitation, and (3) a measurement of niche breadths to distinguish between non-spatially structured generalists and species influenced by mass effect.3. We propose that species characterized by low negative co-occurrence values, a high correlation to spatial factors and average to low niche breadths are sensitive to mass effect.4. Synthesis and applications: We suggest that the sensitivity of species towards mass effect should represent a new ecological trait to be considered for fundamental and applied issues concerning ecology and water quality assessment. Almost all of the species identified here as influenced by mass effect are contributing to the calculation of different diatom-based indices (e.g., Biological Diatom Index or Specific Pollution-sensitivity Index) and should be treated with caution when assigning ecological status classes to water bodies.

International audience; Niche conservatism (NC) describes the scenario in which species retain similar characteristics or traits over time and space, and thus has potentially important implications for understanding their biogeographic distributions. Evidence consistent with NC includes similar niche properties across geographically distant regions. We investigated whether NC was evident in stream diatom morphospecies by modeling species responses to environmental and climatic variables in a set of calibration sites (from the US) and then evaluated the models with test sets (from France, Finland, New Zealand, Antilles and La Réunion). We also examined whether diatom species showed congruency in environmental niche optima and niche breadths between the study regions, and whether species occupancy and functional traits influenced the observed patterns. We used boosted regression tree models with local environmental variables and climatic variables as predictors. We detected low NC in both environmental and climate models and a lack of consistent differences in NC between widely distributed and regionally rare species and among functional groups. For all species, diatom environmental and climatic optima varied clearly between the regions but showed some positive relationships especially for pH and total phosphorus. Diatom niche breadths were only weakly correlated between the US and the other regions. We demonstrated that diatoms showed overall relatively little NC globally, and NC was especially low for climatic variables. Collectively, these findings suggest that there may exist locally adapted lineages within the diatom morphospecies or diatoms possess some adaptation potential for differences in temperature. We argue that in diatoms, environmental and especially climate models may not be transferrable in space globally but need regional diatom data for calibration because species niches seem to differ among geographical regions.

1. Understanding the mechanisms that drive beta diversity (i.e. beta-diversity), an important aspect of regional biodiversity, remains a priority for ecological research. beta-diversity and its components can provide insights into the processes generating regional biodiversity patterns. We tested whether environmental filtering or dispersal related processes predominated along the stream watercourse by analysing the responses of taxonomic and functional diatom beta-diversity to environmental and spatial factors. 2. We examined the variation in total beta-diversity and its components (turnover and nestedness) in benthic diatom species and ecological guilds (motile, planktonic, low-and high profile) with respect to watercourse position (up-, mid-and downstream) in 2,182 sites throughout France. We tested the effects of pure environmental and pure spatial factors on beta-diversity with partial Mantel tests. Environmental factors included eight physicochemical variables, while geographical distances between sites were used as spatial factors. We also correlated a and c-diversity, and the degree of nestedness (NODF metric) with environmental variables. 3. Total beta-diversity and its turnover component displayed higher values upstream than mid-and downstream. The nestedness component exhibited low values, even when NODF values increased from up-to downstream. Pure environmental factors were highly significant for explaining total beta-diversity and turnover regardless of watercourse position, but pure spatial factors were mostly significant mid-and downstream, with geographical distance being positively correlated with beta-diversity. Across sites, nutrient enrichment decreased turnover but increased the degree of nestedness. Motile and low profile diatoms comprised the most abundant guilds, but their beta-diversity patterns varied in an opposite way. The lowest guild beta-diversity was observed upstream for low profile species, and downstream for motile species. 4. In conclusion, environmental filtering seemed to play a major role in structuring metacommunities irrespective of site watercourse position. Filtering promoted strong turnover patterns, especially in disconnected upstream sites. The greater role of spatial factors mid-and downstream was consistent with mass effects rather than neutral processes because these sites had lower total beta-diversity than upstream sites. Motile species were most strongly affected by mass effects processes, whereas low profile species were primarily influenced by environmental conditions. Collectively, these findings suggest that partitioning of total beta-diversity into its components and the use of diatom ecological guilds provide a useful framework for assessing the mechanisms underlying metacommunity patterns along the stream watercourse.

In this intercontinental study of stream diatoms, we asked three important but still unresolved ecological questions: (1) What factors drive the biogeography of species richness and species abundance distribution (SAD)? (2) Are climate-related hypotheses, which have dominated the research on the latitudinal and altitudinal diversity gradients, adequate in explaining spatial biotic variability? and (3) Is the SAD response to the environment independent of richness? We tested a number of climatic theories and hypotheses (i.e., the species-energy theory, the metabolic theory, the energy variability hypothesis, and the climatic tolerance hypothesis) but found no support for any of these concepts, as the relationships of richness with explanatory variables were nonexistent, weak, or unexpected. Instead, we demonstrated that diatom richness and SAD evenness generally increased with temperature seasonality and at mid- to high total phosphorus concentrations. The spatial patterns of diatom richness and the SADmainly longitudinal in the United States but latitudinal in Finlandwere defined primarily by the covariance of climate and water chemistry with space. The SAD was not entirely controlled by richness, emphasizing its utility for ecological research. Thus, we found support for the operation of both climate and water chemistry mechanisms in structuring diatom communities, which underscores their complex response to the environment and the necessity for novel predictive frameworks.

Aim To examine species and trait composition in stream diatoms along environmental, climatic and spatial gradients and to ascertain if the use of different levels of biological organization is beneficial for investigating global environmental changes and the role of history in structuring communities. Location Global cover with datasets from the Antilles, France, Finland, New Zealand, La Reunion and the United States. Methods We related diatom species composition, guild composition, total richness and richness across guilds to environmental, climatic and spatial variables. We used non-metric multidimensional scaling (NMDS) with environmental variable fitting, redundancy analysis (RDA) with variation partitioning, analysis of similarities and linear mixed models as statistical tools. Results Species composition differed significantly among the study regions, while the differences in guild composition were less pronounced. US and French streams shared a large number of species, whereas islands shared only a few species with continents. For species composition, all predictors showed significant relationships with diatoms but pH, longitude, annual temperature and precipitation had the strongest impact. Variation partitioning revealed that the local environment outperformed climatic and spatial variables. For guild composition, there was a substantial overlap across regions in NMDS. The results from RDA demonstrated, however, that guild composition was better explained than species composition, especially by environmental variables. Both species and guild richness were significantly correlated with most predictors. Notably, species richness scaled positively with latitude. Main conclusions Diatom species and guild composition varied substantially in response to local environment and climatic and spatial variables indicating both environmental and historical effects. Species composition discriminated the geographical regions better, while guild composition detected the environmental gradients better. This emphasizes the need to examine different levels of organization to gain a deeper understanding of the roles of environment versus history in structuring communities. These findings suggest that diatom species distributions are under strong microevolutionary constraints. Conversely, guild distributions are less dependent on historic factors and are driven primarily by the environment, which makes them better suited for research on global environmental change.

Ecological niches of organisms vary across geographical space, but niche shift patterns between regions and the underlying mechanisms remain largely unexplored. We studied shifts in the pH niche of 42 temperate forest plant species across a latitudinal gradient from northern France to boreo-nemoral Sweden. We asked 1) whether species restrict their niches with increasing latitude as they reach their northern range margin (environmental constraints); 2) whether species expand their niches with increasing latitude as regional plant species richness decreases (competitive release); and 3) whether species shift their niche position toward more acidic sites with increasing latitude as the relative proportion of acidic soils increases (local adaptation). Based on 1458 vegetation plots and corresponding soil pH values, we modelled species response curves using Huisman-Olff-Fresco models. Four niche measures (width, position, left and right border) were compared among regions by randomization tests. We found that with increasing latitude, neutrophilic species tended to retreat from acidic sites, indicating that these species retreat to more favorable sites when approaching their range margin. Alternatively, these species might benefit from enhanced nitrogen deposition on formerly nutrient-poor, acidic sites in southern regions or lag behind in post-glacial recolonization of potential habitats in northern regions. Most acidophilic species extended their niche toward more base-rich sites with increasing latitude, indicating competitive release from neutrophilic species. Alternatively, acidophilic species might benefit from optimal climatic conditions in the north where some have their core distribution area. Shifts in the niche position suggested that local adaptation is of minor importance. We conclude that shifts in the pH niche of temperate forest plants are the rule, but the directions of the niche shifts and possible explanations vary. Our study demonstrates that differentiating between acidophilic and neutrophilic species is crucial to identify general patterns and underlying mechanisms.

International audience; The restoration and the preservation of aquatics ecosystems is a critical issue in our contemporary society. In lake ecosystems, phytoplankton taxonomic-based indicators have been developed to evaluate water quality, but suffer of limited ecosystem ecological value. The recent development of functional approaches may allow to evaluate other aspects of ecosystem quality, and to develop new trait-based indicators responding to different environmental conditions. Here, our aim was to analyze the response of phytoplankton traits to numerous environmental variables and to identify relevant traits for the development of future indicator metrics. We used a French national database of 469 lakes, consisting in phytoplankton biovolumes and physicochemical values. The response of 84 morpho-functional traits towards environmental variables was tested with Machine Learning models, taking into account lake typology. We identified 21 traits significantly related to environmental variables. Dissolved Organic Carbon, Nitrates and Total Suspend Solids were the physiochemical parameters which had the higher influence on our traits selection. However the response of phytoplanktonic traits to environmental variables did not change according to lake typology, advocating for a consistent response at the whole national scale contrary to the classical taxonomic approach. We finally identified several candidate traits that could be used for the development of new metrics for French lakes in a context of bio-assessment programs.

International audience; Questions How does plant community diversity influence variation in plant biomass? There are two competing hypotheses: the ‘biomass‐ratio hypothesis’, where biomass is influenced by the abundance and traits of the most dominant species, and the ‘diversity hypothesis’, where the diversity of organisms influences biomass through mechanisms such as niche complementarity. However, no studies have tested which one of these two hypotheses better explains the variation in plant biomass in the forest understorey. Location Temperate deciduous forests in Northern France. Methods For the forest understorey, we assessed species diversity and biomass as well as soil and light conditions in 133 forest plots of 100m² each. Using mixed‐effect models and after controlling for 2 Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. potential confounding factors, we tested the ‘biomass‐ratio hypothesis’ by relating the relative abundance of the most dominant species across our study sites and the community‐weighted mean values (CWM) of plant traits (leaf area and plant height) to biomass. The ‘diversity hypothesis’ was tested by relating biomass to various measures of taxonomic, functional and phylogenetic diversity. Results Biomass of the forest understorey was mainly related to the relative abundance and the trait values of the most dominant species, supporting the ‘biomass‐ratio hypothesis’. In contrast with the ‘diversity hypothesis’, functional diversity indices had a negative impact on biomass. We found no contribution of taxonomic or phylogenetic diversity indices. Conclusion The abundance and traits of the most dominant species matter more than taxonomic, functional or phylogenetic diversity of the forest understorey in explaining its biomass. Thus, there is a need for experiments that aim to fully understand keystone species’ responses to ongoing changing biotic and abiotic conditions and to predict their effects on ecosystem functioning and processes.

Le pourcentage de déformations des diatomées est de plus en plus utilisé comme un indicateur de stress toxique dans les cours d'eau. Cependant, ce pourcentage ne permet pas de quantifier directement l'importance de ce stress, et certains auteurs suggèrent que la quantification de la « sévérité » des déformations pourrait améliorer les relations de cause à effet. Dans cette optique, nous avons testé l'applicabilité de la morphométrie géométrique pour évaluer l'intensité de déformation chez différents spécimens de diatomées déformées. Cette méthode a pour but d'étudier des variations subtiles de formes et a été utilisée dans le but de mettre en évidence un gradient de déformations chez les diatomées. Sur la base de microphotographies d'individus normaux et déformés, le positionnement des points-repères a été optimisé, puis la méthode de superposition de Procruste a été utilisée pour mettre en évidence les variations morphologiques, en s'affranchissant des différences de taille, d'orientation mais aussi de position. Les résultats de ces analyses montrent l'efficacité de la méthode pour discriminer les individus normaux et déformés, avec un nombre limité de points-repères (6). A partir des caractéristiques morphométriques des individus déformés, la méthode permet de mettre en évidence un gradient de déformations au sein des individus déformés, en concordance avec une classification préalable de la sévérité des déformations réalisée visuellement. De plus, elle permet de mettre en évidence une relation entre l'intensité d'une pression toxique du milieu et les déformations des diatomées face à ce stress.

Estimating the available surface area of plants is of crucial importance for ecologists interested in epiphytic flora and fauna. However, surface area measurements can be highly time-consuming, especially in the case of heavy-branched macrophytes. The aim of this study is to investigate the relationship between biomass and plant surface, in order to use biomass as a proxy for surface area, and to search for eventual seasonal variability. We quantified the surface-biomass relationship (SBR) as well as the surface/biomass ratio (RSA) and tested the effect of season for five macrophytes: Chara fragifera , Juncus bulbosus , Lagarosiphon major , Lobelia dortmanna and Phragmites australis . Furthermore, we tested the influence of the sampling position along the stem for the submerged macrophyte Egeria densa . We found a significant correlation between surface area and biomass for all species, suggesting that biomass could be a good surrogate of surface area. However, the SBR and RSA of C. fragifera , L. dortmanna and P. australis varied significantly with the season. For the caulescent species E. densa, the position of the sampling shoot along the stem resulted significant, with a non-significant SBR in the basal section of the stem. We conclude that biomass can be used as a proxy of surface area, but recommend the calibration of models for each investigated species, sampling season and shoot fragment.

The effects of habitat fragmentation on species richness and composition have been extensively studied. However, little is known about how fragmentation affects functional diversity patterns. Fragmentation can indeed affect functional diversity directly (e.g. by promoting traits associated to long-distance dispersal when fragment isolation increases) or indirectly (e.g. by decreasing species richness, hence trait diversity, when fragment area decreases). Here, we used structural equation modeling to determine whether factors associated to forest fragmentation, namely area, habitat heterogeneity, spatial isolation and age have a direct effect on forest herb functional diversity. Using occurrence data from 243 forest fragments located in northern France and six plant life-history traits, we estimated species richness and calculated functional diversity in each of these 243 forest fragments. We found that species richness was the primary driver of functional diversity in these fragments, with a strong positive and direct relationship between species richness and functional diversity. Interestingly, both fragment isolation and age had a direct negative effect on functional diversity independent of their effects on species richness. Isolation selected life-history traits associated with long-distance dispersal, while age selected for life-history traits typical of forest habitat specialists. Isolated and/or older forest fragments are thus at greater risk of local species and functional extinctions, and hence making these forest fragments particularly vulnerable to future global changes.

Aim In response to environmental changes and to avoid extinction, species may either track suitable environmental conditions or adapt to the modified environment. However, whether and how species adapt to environmental changes remains unclear. By focusing on the realized niche (i.e. the actual space that a species inhabits and the resources it can access as a result of limiting biotic factors present in its habitat), we here examine shifts in the realized-niche width (i.e. ecological amplitude) and position (i.e. ecological optimum) of 26 common and widespread forest understorey plants across their distributional ranges. Location Temperate forests along a ca. 1800-km-long latitudinal gradient from northern France to central Sweden and Estonia. Methods We derived species' realized-niche width from a β-diversity metric, which increases if the focal species co-occurs with more species. Based on the concept that species' scores in a detrended correspondence analysis (DCA) represent the locations of their realized-niche positions, we developed a novel approach to run species-specific DCAs allowing the focal species to shift its realized-niche position along the studied latitudinal gradient while the realized-niche positions of other species were held constant. Results None of the 26 species maintained both their realized-niche width and position along the latitudinal gradient. Few species (9 of 26: 35%) shifted their realized-niche width, but all shifted their realized-niche position. With increasing latitude, most species (22 of 26: 85%) shifted their realized-niche position for soil nutrients and pH towards nutrient-poorer and more acidic soils. Main conclusions Forest understorey plants shifted their realized niche along the latitudinal gradient, suggesting local adaptation and/or plasticity. This macroecological pattern casts doubt on the idea that the realized niche is stable in space and time, which is a key assumption of species distribution models used to predict the future of biodiversity, hence raising concern about predicted extinction rates.

International audience; L'une des questions essentielle en écologie est de comprendre les mécanismes responsables des patrons d'assemblages des espèces. Pour les milieux terrestres, l'histoire du milieu et de son environnement est reconnu comme ayant un impact majeur sur sa composition biologique (temps de réponse des espèces, caractéristiques physiques du milieu modifié sur le long terme, capacité de colonisation). Cependant, pour les milieux aquatiques, cette thématique n'a que rarement été étudié. A travers l'étude de photographie aériennes anciennes, nous nous sommes intéressés à l'histoire du bassin versant de 8 lacs et étangs du littoral Aquitain. Nous avons ainsi reconstitué l'occupation du sol depuis 1945 jusqu'à la période actuelle et mis en relation ces changements paysagers avec la structure et la composition de la diversité macrophytique actuelle. Sur l’ensemble des bassins versant, les principaux changements paysagers concernent les typologies liés à l'urbanisation et au couvert forestier, se faisant principalement au détriment des prairies. Ces deux principaux changements paysagers sont corrélés à la structure de la diversité (modèles linéaires) et sa composition. Ces résultats suggèrent une influence de l'occupation du sol du bassin versant, à la fois sur la qualité du milieu mais aussi sur les processus gouvernant le fonctionnement des métacommunautés. Ils apportent également de nouvelles perspectives en termes de gestion et de conservation de ces milieux humides.

Questions (i) Do species richness and turnover across forest edges change with edge age and management intensity of adjacent lands? (ii) Does edge species composition respond to aging and landscape management and what are the environmental factors explaining this response? Location Agricultural landscapes of the Picardy region, N France. Methods We sampled forest edges differing in age (from a few decades to several centuries) and embedding landscape matrix (from slightly managed ‘bocages’ to intensively cultivated open fields). We recorded vascular plant species and a set of environment, landscape and historical variables along transects oriented perpendicularly to forest edges. We used mixed models to assess the impact of edge age and landscape type on edge species richness and turnover. We investigated the relationship between edge community composition and explanatory variables using redundancy analyses and a split-plot design. Results Species richness decreased with both increased edge age and increased landscape management intensity, while species turnover was not influenced by any of these factors. Edge maturation was characterized by a specialization of the flora over the entire transect, which is likely a response to increased shade, litter layer thickness and soil acidity. As landscape management was more intensive, true forest species were replaced by nitrophilous and/or calciphilous non-forest species, which might be more tolerant of agrochemical and lime drift and are able to disperse through a hostile matrix. Conclusion Although edge aging was associated with the progressive development of environmental gradients, especially light availability and litter thickness, plant communities poorly reflect these gradients under the constraint of neighbouring landscape management. On the contrary, the stronger the management intensity, the sharper the edge–interior gradient.

e relative importance of local, regional and historical factors in controlling the spatial patterns of plant species distribution is still poorly known and challenging for conservation ecology. We conducted an empirical study to link the spatial variation of species and environments among forest patches embedded in contrasted agricultural matrices. We compared how forest herb communities responded to spatial environmental gradients and past forest cover. We found low values of B-diversity in both unfragmented and highly fragmented systems, independently from local and regional diversities. As fragmentation increased, the spatial structure of local plant communities was more complex and spatial e"ects explained an increasing proportion of B-diversity, suggesting that the importance of dispersal limitations increased and played out at broad spatial scales. However, where spatio-temporal isolation of forest patches was the highest, local species assemblages could not be explained, suggesting that the metacommunity functioning was disrupted. Where the historical continuity was high, local environmental characteristics explained a significant amount of species assemblages within metacommunities, suggesting habitat-selection processes. Beta-diversity and variations in presence‐absence of species were also influenced by the intensity of landscape management, via the permeability of both forest edges and the matrix. !is spatially-explicit analysis of metacommunities revealed that forest fragmentation impacts beta-diversity by altering not only the relative importance of deterministic and stochastic processes, but also the spatial scales at which they act. !ese results provide empirical support for the conservation of ancient forests and the maintenance of a high connectedness between fragments within agricultural landscapes.

Summary1. Al th o ug h h ab itat frag me n tatio n is reco gn ized as a ma jor th reat to bio dive rsity, few stu dies h av eex am ined th e relative imp o rtan ce o f loca l, land scap e an d histo rica l facto rs in co nt ro lling loca lsp ec ies as semb lage s, an d h o w th ese facto rs intera ct,in pa tchy ec o sy stem s. We q ua n tiÞe d th e directan d ind irec t effects o f pa tch size ,p atch h etero ge n eity,ag ricu ltu ra linten sity an d p atch ag e o n plan tsp ec ies rich n ess an d co mp o sitio n o ffo rest pa tche s em b ed d ed in ag ricu ltu ra lland scap es.2. In six 5 a 5 k m- sa mp ling lan ds ca pe wi n do ws , we su rv ey ed ea ch fo rest p atch fo r va scular p lantsp ec ies an d co llec ted th ree sets of ind ep en d en t va riab les, d escribing p atch size an d h etero ge n eity,lan ds ca pe co mp o sition an d h isto ry .T he six wi n do ws w ere ar ra ng ed alon g a gr ad ien t o f ag ricu ltu reint en sity in ru ra llan ds ca pe s of th e P icar d y region (N Fr an ce ).3. We u sed n on -m etric mu ltidim en sion al scaling (N MS ) to d etec t ma jor en viro n me n tal gr ad ient su n de rlying va riation in sp ec ies co mp o sition am on g pa tche s. We th en co n stru cted stru ctu ra l eq u a-tion mo de ls (S E M) to q ua nt ify th e direct an d ind irec t effects of th e th ree sets of va riab les o n loca lp lant d iversity,wh ich wa s su cc essive ly inco rp or ated as pa tch scor es alon g th e Þ rst th ree NM S ax es,w oo dy sp ec ies rich ne ss,fo rest he rb sp ec ies rich ne ss,an d no n- fo rest h erb sp ec ies rich ne ss.4. A ma jor insu en ce o f th e lan ds ca pe ma trix on loca l sp ec ies co mp os ition wa s reve aled b y NM San d su bs eq ue nt S EM ,ma inly th ro u gh no n -for esthe rb sp ec ies, w hich ex plain ed mo st of th e be tw ee np atch s or istic d issimi larity.S pe cies rich ne ss increa sed wi th pa tch he tero ge n eity,wh erea s pa tch ar ean ev er h ad a direct effect. Fo rest he rb s we re mo re resp o ns ive to pa tch ag e an d co nn ec tivity tha no th er sp ec ies, wh erea s n on -for est an d wo o d y sp ec ies we re mo re insu en ce d by ag ricu ltu re inten sityin th e su rro u nd ing ma trix.5. S ynthesis.We us ed o ne o f th e larg est da ta sets ev er co llec ted in temp erate frag me n ted fo rests tob u ild, fo r th e Þ rsttime ,a stru ctur al mo de linco rp or ating allsu sp ec ted dr ivers of loca lplan t co mm u -n ities. We sh o we d th at th e n u mb er an d iden tity of loca lsp ec ies co ex istin g in su cc essio na lfrag me n tso f a fo rest me taco mm un ity at a give n time is co nt ro lled by a u n iqu e co mb ination o f int erac tin gloc al, lan ds ca pe an d histo rica l factor s. Pr eservin g the larg est, old est frag me n ts an d fav o ur ing sp e-cies mo ve me n ts in th e su rro un d ing ma trix is th e be st wa y to co ns erve fo rest sp ec ialists in ch an gingru ra llan ds ca pe s.Key-wor ds: agricu ltural intens ity, disper sal lim itation, habita t fragm entat ion, he dgerow net-work, metacommun ity, plant communi ty divers ity, specie sÐarea relationshi p, specie sÐtimerelationshi p, struc tural equati on modellingIntroduc tion

Background Invasive species can be a major threat to native biodiversity and the number of invasive plant species is increasing across the globe. Population genetic studies of invasive species can provide key insights into their invasion history and ensuing evolution, but also for their control. Here we genetically characterise populations of Impatiens glandulifera, an invasive plant in Europe that can have a major impact on native plant communities. We compared populations from the species’ native range in Kashmir, India, to those in its invaded range, along a latitudinal gradient in Europe. For comparison, the results from 39 other studies of genetic diversity in invasive species were collated. Results Our results suggest that I. glandulifera was established in the wild in Europe at least twice, from an area outside of our Kashmir study area. Our results further revealed that the genetic diversity in invasive populations of I. glandulifera is unusually low compared to native populations, in particular when compared to other invasive species. Genetic drift rather than mutation seems to have played a role in differentiating populations in Europe. We find evidence of limitations to local gene flow after introduction to Europe, but somewhat less restrictions in the native range. I. glandulifera populations with significant inbreeding were only found in the species’ native range and invasive species in general showed no increase in inbreeding upon leaving their native ranges. In Europe we detect cases of migration between distantly located populations. Human activities therefore seem to, at least partially, have facilitated not only introductions, but also further spread of I. glandulifera across Europe. Conclusions Although multiple introductions will facilitate the retention of genetic diversity in invasive ranges, widespread invasive species can remain genetically relatively invariant also after multiple introductions. Phenotypic plasticity may therefore be an important component of the successful spread of Impatiens glandulifera across Europe. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

1. Worldwide, the floristic composition of temperate forests bears the imprint of past land use for decades to centuries as forests regrow on agricultural land. Many species, however, display signi ...

International audience

In lake ecosystems, biological taxa are usually studied either through trophic relationships (without considering environmental factors) or distinctly for each taxonomical group. In the last case, studying the response of communities to environmental abiotic factors is usually the main objective and biotic interactions are therefore rarely considered. This is the objective of the present study which aims to investigate, synchronously, the response of different biological taxa in the Lacanau's lake (South-West of France) to abiotic and biotic factors. More precisely, the effect of season, spatial variation, physico-chemical parameters and physical factors (e.g. wind, slope) on macrophytes, phytoplankton, epiphytic diatoms and micro-meiofauna communities and biomass will be tested. The aim is also to construct global models of the ecosystem to determine the relative effect of environmental factors and biotic relationships. At the same time, another goal is to investigate, for the first time, epiphytic micro-meiofauna taxa living in Lacanau's lake. We thus studied 6 stations located around the littoral shoreline that were sampled during the four temperate seasons. Permanent plots (24 per station) were installed to study macrophyte composition and phytoplankton biomass. Water samples were taken (3 per station) and water composition was analyzed through more than 20 parameters. Five macrophytes species were also sampled in each station to measure the composition and biomass of the epiphytic flora and fauna. Results revealed a strong seasonal variation of water composition. Taxa did not exhibit a synchronic response through time and, within taxa group, we often evidenced different variation of diversity and biomass with time. Microscopic organisms appeared to be strongly dependent of seasonal variation but it was almost impossible to separate the influence of physico-chemical parameters from those of seasonal succession. Weak influence of physico-chemical parameters on macrophytes communities was found (except for biomass) but these communities strongly depends on hydrodynamical and hydromorphological conditions. Physical factors also appeared to strongly impact phytoplankton diversity and biomass. Nature and architecture of plant host was found to greatly influence the biomass and diversity of diatoms and micro-meiofauna epiphytic communities. Considering biotic relationships, competition processes with macrophytes species seems to structure phytoplankton communities and the micro-meiofauna biomass and diversity appeared to be controlled by epiphytic diatoms. All these processes were correctly modeled in final models which allowed to test the relative influence of chemical, physical and biological factors. This fundamental study bring better knowledge on processes patterning biological communities in the Lacanau's lake and also contribute to a better understanding of the whole functioning of this lacustrine ecosystem. Some scientific and management perspectives were drawn from these results, but supplementary studies are still needed to better understand mechanisms patterning these biological communities.; Dans les écosystèmes lacustres, les différents maillons biologiques sont souvent étudiés via le réseau trophique de l'écosystème ou alors de manière complètement indépendante. La réponse des communautés aux différents facteurs abiotiques est alors le principal objectif de ces études mais les interactions biotiques sont très rarement considérés dans l'analyse des assemblages d'espèces. C'est l'objectif de cette étude qui vise à étudier de manière synchrone différents taxons biologiques au sein du lac de Lacanau. Plus précisément, l'influence de la saison, de la variabilité spatiale, de la physico-chimie de l'eau et des facteurs physiques (e.g. vent, pente) sera testé sur les communautés de macrophytes, de phytoplancton, de diatomées épiphytes et sur la micro-méiofaune épiphyte. L'objectif est également de construire des modèles d'interaction entre ces différentes communautés biologiques et les paramètres environnementaux mesurés. Parallèlement, un autre objectif est aussi de dresser pour la première fois la liste des taxons de la micro-méiofaune présent dans le lac de Lacanau. Nous avons pour ceci étudié 6 stations réparties autour du lac de Lacanau que nous avons échantillonnées pendant les 4 saisons. Des quadrats fixes (24 par stations) ont été placés pour quantifier la composition en macrophytes et la biomasse phytoplanctonique. Des prélèvements d'eaux (3 par stations) ont également été prélevés sur chaque station pour mesurer la composition de plus de 20 paramètres physico-chimiques. Cinq espèces de macrophytes ont également été prélevées sur chaque station afin de quantifier la composition et la biomasse de la flore et de la faune épiphyte. Les résultats ont montré une forte variation saisonnière de la composition physico-chimique de l'eau. Les différents taxons ne montrent pas une réponse synchrone au cours du temps, et au sein de chaque groupe taxonomique, la réponse de la diversité et de la biomasse est elle aussi rarement similaire au cours du temps. Les organismes microscopiques se révèlent être fortement dépendant des variations saisonnières mais il est difficile déconvoluer la part lié aux paramètres physico-chimiques de celle lié à la successions saisonnière. Les macrophytes apparaissent en revanche être peu influencés par les paramètres physico-chimiques/saison (à l'exception de leur biomasse) mais dépendent surtout des conditions hydrodynamiques et hydromorphologiques. Les facteurs physiques se révèlent également être particulièrement structurants pour la diversité et la biomasse phytoplanctonique. La nature et l'architecture de la plante hôte semble aussi jouer un rôle majeur sur la diversité et la biomasse de la faune et de la flore épiphyte. Des processus de compétition avec les macrophytes semblent structurer la communauté phytoplanctonique et la micro-méiofaune apparait être particulièrement dépendante des fluctuations de la micro-flore épiphyte. L'ensemble de ces processus semblent correctement modélisés dans les modèles globaux qui permettent de synthétiser et de mieux tester l'influence relative des facteurs chimiques saisonniers, physiques et biologiques. Cette étude, essentiellement fondamentale, apporte des connaissances nouvelles sur le fonctionnement des communautés biologiques au sein du lac de Lacanau et permet de mieux comprendre son fonctionnement et son organisation. Elle offre cependant quelques perspectives de gestion mais nécessite aussi des études complémentaires pour mieux comprendre les principaux mécanismes structurant les communautés biologiques.

International audience; Shallow lakes provide multiple ecosystem services, and macrophytes are key elements of these systems. Understanding the processes behind their species assemblages is thus essential for lake management. In this study, we surveyed macrophyte communities of 14 lakes and ponds in South-Western France since 1984 and explored their structure and composition change with time. We used a set of physical, anthropogenic and landscape variables to understand processes driving local species assemblages. Abundance of invasive species was also included as explanatory variables. Species richness, rank-abundance curves and non-metrical multidimensional scaling were used to investigate macrophyte diversity, and the coefficients of these metrics were correlated with previous explanatory variables. We did not find a common temporal response of communities. However, some variables, such as residence time, watershed and lake areas were found to be important factors in explaining community changes across time and led to suggest either an important influence of physico-chemistral variation or regional-dispersion processes. Also, the presence of invasive species and their abundance explained some patterns of macrophyte diversity underlying the importance of biotic interactions. These results suggest that both biotic and abiotic processes influence local macrophyte assemblages and provide new insights in terms of lake management for biodiversity conservation.

International audience; Shallow-lakes ecosystems play a major role in European landscapes in providing multiple ecosystem services, such as biodiversity, human amenities and resource supplying. Macrophytes are key elements of these ecosystems and understanding the processes behind their species assemblages is essential for lake management. In this study, we followed macrophytes communities of 14 lakes in South-Western France since 1984 and explored their structure and composition change with time. We used a set of physical, anthropogenic and landscape variables to understand processes driving local species assemblages. Abundance of invasive species was also included as explanatory variables. Species richness, rank-abundance curves and non-metrical multidimensional scaling were used to investigate macrophyte diversity, and coefficients of these metrics were correlated with previous explanatory variables. We did not find a common response of communities with time, each lake experiencing its own path. However, some variables, such as lake renewal, area of the watershed and lake area were found to be important factors in explaining community changes across time and lead to suggest either an important influence of physic-chemistry variation or regional-dispersion processes. Also, the presence of invasive species and their abundance explained some patterns of macrophytes diversity underlying the importance of biotic interactions. These results suggest that both biotic and abiotic processes influence local macrophytes assemblages and provide new insights in terms of lake management for biodiversity conservation.