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Lacs et étangs du littoral aquitain abritent des plantes aquatiques visibles à l’œil nu (macrophytes) inféodées aux hydrosystèmes oligotrophes sableux peu profonds, incluant des espèces rares, endémiques ou présentes habituellement dans les lacs d’Europe du Nord. Depuis une quarantaine d’années, ces plans d’eau sont également le lieu de développements plus ou moins importants de macrophytes exotiques à caractère envahissant. Ces plantes peuvent former des herbiers denses susceptibles de créer localement des envahissements importants dès que les conditions environnementales leurs sont favorables, menant parfois à la colonisation quasi totale de la superficie de certains petits étangs. L’obstacle que représentent les herbiers denses dans la pratique de certains usages récréatifs et les interrogations liées à leurs impacts potentiels sur les espèces indigènes et le fonctionnement écologique des limnosystèmes, ont mené gestionnaires et hydrobiologistes à établir une stratégie commune visant à acquérir des connaissances sur ces "nouvelles" espèces, et à mettre en œuvre des solutions de gestion. À partir de la fin des années 1980, le Syndicat Mixte Géolandes et l’Irstea-Cemagref (désormais l’INRAE) ont collaboré pour établir des diagnostics des espèces présentes et des sites colonisés, pour élaborer ensuite un plan de gestion dédié à la régulation adaptée de chaque espèce en fonction des enjeux identifiés, et à l’entretien régulier des zones traitées pour éviter toute recolonisation massive. Depuis, cette collaboration perdure, le dialogue s’opérant sur la politique de gestion adoptée par le Syndicat Mixte, s’appuyant sur un réseau de référents locaux coordonnant les opérations de surveillance et d’entretien confiées aux collectivités locales avec le soutien technique et financier du Syndicat. Elle s’appuie également sur l’analyse à long terme des dynamiques des plantes et sur le développement de techniques d’échantillonnage et de prédiction de leur distribution potentielle à fine échelle. Ainsi, les questions de gestion alimentent en continu les questions de recherche scientifique, pour tenter de répondre aux interrogations actuelles et émergentes liées à la présence des végétaux dans les plans d’eau. Natural lakes and ponds located along the Aquitaine coastline shelter large aquatic plants (macrophytes), inhabiting shallow sandy oligotrophic habitats and including rare and endemic species usually present in northern European lakes. Since forty years, these lakes are also the place of exotic plant growth. These plants are able to colonize a large part of the lake area (especially in small ponds) when local environmental conditions are favourable to them. Large and dense beds induce difficulties in the practice of traditional and recreational use of lakes. The questions related to their potential impact on native species and the ecological functioning of hydrosystems have led managers and hydrobiologists to establish a common strategy, in order to improve knowledge on these "new" species and to implement management solutions. Since the late 1980s, Géolandes (managers) and Irstea-Cemagref (researchers, from now INRAE) worked together to establish a diagnosis of invasive species distribution and abundance. This common work aims to draw up a management plan dedicated to the regulation of exotic species and the monitoring of potential recolonization following management actions. Since then, this collaboration between managers and researchers has continued. Now, discussions are focused on the management policy adopted by Géolandes based on a local referent network, who coordinate monitoring operations entrusted to the local authorities with the technical and financial support of Géolandes. This collaboration is also based on a long-term analysis of plant dynamics, the development of sampling techniques and the prediction of their potential distribution on a fine scale. In this way, management issues are continuously feeding scientific research questions related to the presence and the trajectory of plants in the lakes.

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.

The spread of invasive aquatic plants (i.e., aquatic weeds) requires a broader knowledge of the factors determining their settlement at the lake scale, in order to improve management practices and biomonitoring. Among hydrodynamic pressures, wave action might influence submerged vegetation distribution in shallow lakes and potentially engender modifications of plant morphological traits. We here report a field survey conducted between 2014 and 2015 in French Atlantic Lakes to assess the spatial distribution and biomass production of two rooted aquatic weeds, Egeria densa Planch. and Lagarosiphon major (Ridl.) Moss, in relation to wind-induced sediment resuspension, water depth and sedimentary features. Moreover, we explored the relation between plant morphological traits and hydrodynamic disturbance under in situ conditions. At the lake scale, E. densa and L. major formed extensive monospecific stands, and occurred in the same areas only at low biomass. Both monospecific and mixed stands preferentially developed in shallow-sheltered or deep-exposed areas. Plant morphological traits showed different patterns in function of sediment resuspension according to the species and the lake. The influence of resuspension was however not systematic, with many cases where morphological traits were not affected at all. Maximum stem length appeared to be the most correlated trait towards sediment resuspension. Moreover, we found a significant correlation between the biomass and the sedimentary organic matter, indicating an interaction between the organic substrate preference of the plants and the effect of the canopy on fine particles sedimentation. On the whole, we highlighted the link between plant distribution, phenotypic plasticity and sediment resuspension, with calm-water zones favouring the settlement of dense vegetated stands. Our study could thus contribute to improve prediction models for identifying suitable areas for potential colonization by aquatic weeds. Further research is needed to better understand the role played by hydraulic forces in structuring the habitats in shallow lakes.

Eleven countries participated in an intercalibration exercise to harmonise diatom-based methods used for status assessment in lakes. Participating countries extended from Sweden and Finland in the north to Italy and Slovenia in the south, and from Ireland in the west to Hungary in the east. Intercalibration followed standard procedures developed for the European Commission which ensures that outcomes of this exercise are compatible with those for other types of biota in both freshwater and marine water bodies throughout Europe. Lakes were divided into low, medium and high alkalinity types for this exercise. However, it was not possible to perform a full intercalibration on the low alkalinity lakes due to the short gradient length and confounding influences of pH and humic substances. Values of the Trophie Index of Rott et al. (1999) were computed for all samples in order that national datasets could all be expressed on a common scale. Not all participating countries had reference sites against which national methods could be standardised and, therefore, a Generalised Linear Modelling approach was used to control the effect of national differences in datasets. These steps enabled the position of high/good and good/moderate status boundaries for all participating countries to be expressed on a common scale and for deviations beyond  0.25 class widths ( 0.05 Ecological Quality Radio (EQR) units, assuming equal distances between all class boundaries) to be identified. Those countries which had relaxed boundaries were required to adjust these to within  0.25 class widths whilst the intercalibration rules allowed those countries with more stringent boundaries to retain these. Despite biogeographical and typological differences between participating countries, there was broad agreement on the characteristics of high, good and moderate status diatom assemblages, and the intercalibration exercise has ensured a consistent application of Water Framework Directive assessments around Europe., JRC.H.1-Water Resources

International audience; Uncertainty is an important factor in ecological assessment, and has important implications for the ecological classification and management of lakes. However, our knowledge of the effects of uncertainty in the assessment of different ecological indicators is limited. Here, we used data from a standardized campaign of aquatic plant surveys, in 28 lakes from 10 European countries, to assess variation in macrophyte metrics across a set of nested spatial scales: countries, lakes, sampling stations, replicate transects, and replicate samples at two depth-zones. Metrics investigated in each transect included taxa richness, maximum depth of colonisation and two indicators of trophic status: Ellenberg’s N and a metric based on phosphorus trophic status. Metrics were found to have a slightly stronger relationship to pressures when they were calculated on abundance data compared to presence/absence data. Eutrophication metrics based on helophytes were found not to be useful in assessing the effects of nutrient pressure. These metrics were also found to vary with the depth of sampling, with shallower taxa representing higher trophic status. This study demonstrates the complex spatial variability in macrophyte communities, the effect of this variability on the metrics, and the implications to water managers, especially in relation to survey design.