Rôle de l’histoire du paysage sur la diversité des macrophytes dans les lacs du littoral Aquitain

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

Front. Plant Sci.

International audienc

Invasive Aquatic Plants as Ecosystem Engineers in an Oligo-Mesotrophic Shallow Lake

Exotic hydrophytes are often considered as aquatic weeds, especially when forming dense mats on an originally poorly colonized environment. While management efforts and research are focused on the control and on the impacts of aquatic weeds on biodiversity, their influence on shallow lakes’ biogeochemical cycles is still unwell explored. The aim of the present study is to understand whether invasive aquatic plants may affect the biogeochemistry of shallow lakes and act as ecosystem engineers. We performed a multi-year investigation (2013–2015) of dissolved biogeochemical parameters in an oligo-mesotrophic shallow lake of south-west of France (Lacanau Lake), where wind-sheltered bays are colonized by dense mats of exotic Egeria densa Planch. and Lagarosiphon major (Ridl.) Moss. We collected seasonal samples at densely vegetated and plant-free areas, in order to extrapolate and quantify the role of the presence of invasive plants on the biogeochemistry, at the macrophyte stand scale and at the lake scale. Results revealed that elevated plant biomass triggers oxygen (O2), dissolved inorganic carbon (DIC) and nitrogen (DIN) stratification, with hypoxia events frequently occurring at the bottom of the water column. Within plants bed, elevated respiration rates generated important amounts of carbon dioxide (CO2), methane (CH4) and ammonium (NH4+). The balance between benthic nutrients regeneration and fixation into biomass results strictly connected to the seasonal lifecycle of the plants. Indeed, during summer, DIC and DIN regenerated from the sediment are quickly fixed into plant biomass and sustain elevated growth rates. On the opposite, in spring and autumn, bacterial and plant respiration overcome nutrients fixation, resulting in an excess of nutrients in the water and in the increase of carbon emission toward the atmosphere. Our study suggests that aquatic weeds may perform as ecosystem engineers, by negatively affecting local oxygenation and by stimulating nutrients regeneration

The three Rs of river ecosystem resilience : Resources, recruitment, and refugia

This review article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived VersionResilience 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

Stream diatom community assembly processes in islands and continents: a global perspective

[EN] Understanding the roles of deterministic and stochastic processes in community assembly is essential for gaining insights into the biogeographical patterns of biodiversity. However, the way community assembly processes operate is still not fully understood, especially in oceanic islands. In this study, we examine the importance of assembly processes in shaping diatom communities in islands and continents, while also investigating the influence of climate and local water chemistry variables on species distributions. Location Global. Taxon Stream benthic diatoms. Methods We used diatom datasets from five continents and 19 islands and applied beta diversity analyses with a null model approach and hierarchical joint species distribution modelling. To facilitate comparisons with continents, we used continental area equivalents (CAEs), which represent continental subsets with comparable areas and the same number of study sites as their corresponding islands counterparts. Results We found that homogeneous selection (i.e., communities being more similar than the random expectation) was the dominant assembly process within islands whereas stochastic processes tended to be more important within continents. In addition, assembly processes were influenced by study scale and island isolation. Climatic variables showed a greater influence on species distribution than local factors. However, in islands, local environmental variables had a greater impact on the distributions of unique taxa as opposed to non-unique taxa. Main Conclusions We observed that the assembly processes of diatom communities were complex and influenced by a combination of deterministic and stochastic forces, which varied across spatial scales. In islands, there was no universal pattern of assembly processes, given that their influence depends on abiotic conditions such as area, isolation, and environmental heterogeneity. In addition, the sensitivity of species occurring uniquely in islands to local environmental variables suggests that they are perhaps less vulnerable to climatic changes but may be more influenced by changes in local physicochemistrySIFor financial support, the authors thank the Academy of Finland (grant nr. 346812 to JS); the Institut Francais de Finlande; the Embassy of France to Finland; the French Ministry of Education and Higher Education; Finnish Society of Sciences and Letters. J.J. Wang was further supported by the National Natural Science Foundation of China (91851117, 41871048), CAS Key Research Program of Frontier Sciences (QYZDB-SSW-DQC043), and The National Key Research and Development Program of China (2019YFA0607100

Scientists’ Warning to Humanity: Rapid degradation of the world\u27s large lakes

Large lakes of the world are habitats for diverse species, including endemic taxa, and are valuable resources that provide humanity with many ecosystem services. They are also sentinels of global and local change, and recent studies in limnology and paleolimnology have demonstrated disturbing evidence of their collective degradation in terms of depletion of resources (water and food), rapid warming and loss of ice, destruction of habitats and ecosystems, loss of species, and accelerating pollution. Large lakes are particularly exposed to anthropogenic and climatic stressors. The Second Warning to Humanity provides a framework to assess the dangers now threatening the world\u27s large lake ecosystems and to evaluate pathways of sustainable development that are more respectful of their ongoing provision of services. Here we review current and emerging threats to the large lakes of the world, including iconic examples of lake management failures and successes, from which we identify priorities and approaches for future conservation efforts. The review underscores the extent of lake resource degradation, which is a result of cumulative perturbation through time by long-term human impacts combined with other emerging stressors. Decades of degradation of large lakes have resulted in major challenges for restoration and management and a legacy of ecological and economic costs for future generations. Large lakes will require more intense conservation efforts in a warmer, increasingly populated world to achieve sustainable, high-quality waters. This Warning to Humanity is also an opportunity to highlight the value of a long-term lake observatory network to monitor and report on environmental changes in large lake ecosystems

Relations entre les diversités alpha, béta et gamma de la flore vasculaire de fragments forestiers inclus dans des paysages agricoles contrastés

Whereas habitat fragmentation is considered as one of the main cause of biodiversity loss, the underlying mechanisms are still poorly understood. In this thesis I aim at identifying the main factors controlling forest species assemblages, at several spatial and temporal scales within contrasted agricultural landscapes. Vascular plant species have been surveyed in all forest patches that were present in 9 5x5km landscape windows located in the Picardy region (N-France). First, a regressive analysis of the landscape was conducted to reconstruct forest cover changes over the last three centuries; it revealed that the forest of Picardy has become more and more fragmented. The influence of local, landscape and historical factors on species richness and composition was then analysed using structural equation modelling; patch size was found to be the main driver, but landscape and historical factors were also influential when only forest herb species were considered. A principal coordinates of neighbour matrices analysis, coupled with a redundancy analysis with variation partitioning was used to analyse beta (inter-patches) diversity along the landscape gradient. I found that diversity appeared to be controlled by local factors in the oldest, less fragmented systems (habitat selection), whereas landscape spatial configuration was more important in the most fragmented and/or recent woodlands (dispersal limitation). A “pseudo-diachronic” approach, studying species assemblages along the temporal gradient clarified the relative importance of neutral vs. niche processes in local species assemblages. When analysing diversity at a finer scale, using a nested plot design, I showed that at the plot scale (1000m2), plant diversity was governed not only by the size of the regional species pool but also by edaphic properties; conversely, inter-specific competition became significant at a very local scale (1m2). Finally, field sowing and transplanting experiments were conducted, which confirmed that the presence of ancient forest species was limited by their dispersal abilities, whilst the presence of recent forest species was limited by habitat quality. By clarifying the multiscalar and interactive influence of local, landscape, historical and spatial factors, this work increases our knowledge of forest plant communities organisation and of their functioning as metacommunities.Bien que la fragmentation des habitats soit reconnue comme une cause majeure de l'érosion de la biodiversité, les mécanismes sous-jacents sont encore mal connus. L'objectif de ce travail est d'identifier les principaux facteurs qui contrôlent l'organisation des communautés végétales forestières, aux différentes échelles spatiales et temporelles et dans différents contextes paysagers. Les espèces végétales vasculaires ont été inventoriées dans les fragments forestiers présents dans 9 fenêtres de 5x5km situées en Picardie (France). Une analyse régressive du paysage a d'abord permis de reconstituer l'histoire du couvert forestier pendant les trois derniers siècles, révélant que le paysage forestier picard était dans une dynamique de fragmentation croissante. L'influence des facteurs locaux, paysagers et historiques sur la richesse et la composition forestière des fragments a ensuite été analysée à l'aide de modèles d'équations structurales ; si la taille du fragment est le principal déterminant, les facteurs paysagers et historiques influencent fortement la présence des espèces herbacées forestières sensu stricto. Une analyse en coordonnées principales de matrices de voisinage couplée à une analyse de redondance avec partition de la variance a permis d'analyser la diversité inter-fragments le long du gradient paysager. Celle-ci apparaît contrôlée par les facteurs locaux en système ancien et peu fragmenté (partition des niches), tandis que la configuration spatiale du paysage devient essentielle en système fragmenté et/ou récent (neutralisme). Une approche « pseudo-diachronique » étudiant l'assemblage des espèces le long du gradient temporel a précisé l'importance relative des mécanismes neutralistes et de partition des niches dans les assemblages locaux des espèces. Une approche plus fine, utilisant des relevés emboîtés, a ensuite montré qu'à une échelle stationnelle (1000m2), la diversité végétale était surtout sous la double dépendance du réservoir régional d'espèces et des caractéristiques édaphiques, tandis qu'à une échelle très locale (1m2), la compétition interspécifique devenait significative. Une étude expérimentale a finalement confirmé que la présence des espèces de forêt ancienne était limitée par leurs faibles capacités de dispersion, alors que celle des espèces de forêt récente apparaissaient fortement limitée par les conditions d'habitat. En précisant l'influence multiscalaire et interactive des facteurs locaux, paysagers, historiques et spatiaux, ce travail contribue à une meilleure connaissance de l'organisation des communautés végétales forestières locales et de leur fonctionnement en métacommunauté

Biofilm sur macrophytes : Quid de la Micro-méiofaune ?

National audienceThe epiphytic biofilm is composed of many organisms such as micro-algae, fungi, bacteria but also among others protozoa and metazoans belonging to the micro-meiofauna community. Little is known about this community and those living on aquitain's lakes macrophytes are completely unknown. The aim of this study is first to list the main taxa living on Lacanau lake's macrophytes and then to test the influence of macrophyte host on micro-meiofauna community. During one year, we sampled 5 macrophytes species (Chara fragifera, Juncus bulbosus, Lagarosiphon major, Lobelia dortmanna and Phragmites australis) in littoral stations located around the Lacanau's lake and we identified the main taxa of the micro-meiofauna community. We observed that the micro-meiofauna consists of 15 major taxonomic groups and that taxonomic richness and density significantly vary according to macrophyte's host. Whereas individual density was the lowest on L. dortmanna, the species richness was higher on this host compared to J. bulbosus and L. major.Le biofilm épiphytique est composé de nombreux organismes tels que des micro-algues, des champignons, des bactéries mais aussi entre autres de protozoaires et de métazoaires appartenant à la communauté de la micro-méiofaune. Ces organismes sont relativement peu connus, et ceux vivants sur les macrophytes des lacs aquitains n'ont jusqu'à présent jamais été étudiés. C'est l'objectif de cette étude qui vise dans un premier temps à inventorier les principaux taxons de la micro-méiofaune présents sur les macrophytes du lac de Lacanau, et dans un second temps à tester l'influence de la plante hôte sur ces communautés. Au cours des 4 saisons nous avons ainsi échantillonné 5 espèces de macrophytes (Chara fragifera, Juncus bulbosus, Lagarosiphon major, Lobelia dortmanna et Phragmites australis) sur différentes stations littorales du lac de Lacanau et identifié les principaux taxons de la micro-méiofaune. Nous avons ainsi observé que la micro-méiofaune était constituée de 15 grands groupes taxonomiques et que la richesse et la densité taxonomique variait significativement en fonction de l'espèce de macrophytes support. Alors que la densité d'individus est significativement plus faible sur L. dortmanna, la richesse estimée apparait être plus importante sur cette espèce que sur J. bulbosus et L. major

Genetic and taxonomic diversity of Aquitaine coast lakes isoetid communities

Southwestern freshwater lakes of the Aquitaine coast host a unique aquatic plant diversity, which maintain vital ecosystem functions. However, anthropogenic activities and global change pose significant threats to these communities, leading to extinctions. We first aim to investigate the genetic diversity of nine macrophytes species composing isoetids communities from five lakes. To this end, we will study the genetic variation applying sequence-based nuclear microsatellite genotyping across 4000 sampled individuals. The distribution of genetic diversity within and between lakes will help understanding populationlevel processes as well as identifying ancient and contemporary factors structuring populations and communities. These results will provide insights into the adaptation capacity, demographic history, dispersal abilities and structure of macrophytes populations. In addition, we aimed to deepen our understanding of ecological process to a larger biological scale in examining the correlation between genetic and taxonomic diversity using the SGDC (species-gene diversity correlation) framework. This method aims to determine if the same ecological processes occur at different biological scales as well as to establish if taxonomic diversity can be a good proxy for genetic diversity. In essence, the aim of this project is to provide valuable information for conservation management plans in the Aquitaine region and to contribute to our understanding of how freshwater plant communities are structured and how they respond to threats. The ultimate goal of this project is to work with the stakeholders to ensure the conservation of these sensitive but fundamental populations