Search

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.

Macrophytes may either stimulate or depress nitrogen-related microbial processes via radial oxygen loss (ROL), production of exudates or uptake of inorganic N. ROL can favor aerobic processes as nitrification, exudates may stimulate denitrification, whereas N assimilation and competition with microbes may depress both processes. We measured rates of potential nitrification (PN) and denitrification (PD) in oligotrophic lacustrine sediments colonized by submersed and emergent macrophytes. Potential rates were also analyzed in adjacent control sediments devoid of vegetation. Aim of the work was to verify if the presence of macrophytes alters the potential activity of nitrifying or denitrifying bacteria. Vertical profiles (0−10 cm depth) of PN and PD rates were measured via oxic (nitrification) and anoxic (denitrification) slurries, where we measured the accumulation of NOx− from added NH4+ and the production of 30N2 from added 15NO3−, respectively. Results suggest that under oligotrophic settings macrophytes produced small effects on potential nitrification and denitrification activities. Despite elevated oxygen release demonstrated for most of the tested macrophytes, nitrification was likely constrained by significant plant-bacteria competition. Potential denitrification was comparatively more stimulated by macrophytes, but we address this result to a general increase of heterotrophic microbial activity in organic-richer vegetated sediments, due to dead root biomass or exudates. The highest PN and PD rates were measured in sediments colonized by Littorella uniflora, likely due to its large underground biomass (root:shoot ratio ∼3.5), root porosity and oxygen leakage.

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.

International audience; Questions: Do natural and anthropogenic physical disturbances equally affect the distribution of aquatic plants communities? Can hydrodynamic and geomorphological features be used to predict the establishment of macrophyte communities at the shoreline scale?. Locations: The study was carried‐out in two large, shallow lakes in southwestern France, the southern hotspots of isoetid distribution in Europe. Methods: Based on field observations (vegetation occurrence and anthropogenic modifications of the shore) and data generated by a geographical information system (wave exposure, wave‐induced sediment resuspension, slope and land cover), we defined sites and community groups using cluster and indicator species analyses. The groups were then analyzed by means of a statistical classifier (Random Forest). These different steps in data treatment enabled us to characterize the importance of each physical factor in determining macrophytes occurrence and distribution. As a result, a predictive map to forecast aquatic plant distribution at the shoreline scale was obtained. Results: Anthropogenic disturbances were less important parameters thanl variable natural physicas in structuring the distribution of lakeshore macrophytes. Within natural factors, wave‐induced sediment resuspension and slope had the most impact; nevertheless, the presence of swimming areas seemed to have a strong impact on aquatic habitats, being correlated with the total absence of aquatic vegetation. The predictive map obtained through the model spatially defined the position and occurrence of suitable sites for the settlement of both invasive, and rare and endangered species. Conclusions: In this study, natural disturbances play a major role in structuring aquatic plant distribution over physical anthropogenic ones. The model contributes to improving knowledge on plant communities with respect to local hydrodynamic and morphological features of lakeshores. Furthermore, the model provides the production of a predictive map as a useful tool for the management of aquatic vegetation in temperate shallow lakes.

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.

Resume Les communautes de macrophytes des lacs medocains (lac de Carcans-Hourtin et lac de Lacanau) ont fait l’objet en 2011 d’observations et de prelevements sur la totalite de la peripherie des rives, ainsi que sur des profils perpendiculaires vers le large de ces plans d’eau. Les communautes vegetales aquatiques comportent diverses especes communes aux milieux lacustres, ainsi que des especes patrimoniales, telles que les especes des pelouses a littorelles, mais egalement des especes exotiques a caractere envahissant. Les Characees sont bien representees sur les lacs medocains. Chara fragifera colonise les deux tiers du lineaire des rives sur les deux lacs, jusqu’a une profondeur de 4,5 m sur le lac de Carcans-Hourtin. Nitella confervacea est moins abondante et moins frequente, elle semble cependant coloniser les secteurs plus profonds des lacs. La repartition de ces deux especes sur les rives des lacs est relativement homogene, les especes n’ont pas ete observees dans les secteurs d’anses protegees de...

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.