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Malgré les nombreux usages dont ils font l’objet et leur rôle avéré pour la biodiversité aquatique et terrestre, les plans d’eau et les estuaires français ont été historiquement peu étudiés, et leur état écologique peu renseigné. L’évaluation et le suivi de leur qualité écologique dans le cadre de la directive cadre européenne sur l’eau ont relevé d’un véritable challenge dans les années 2000. Aujourd’hui, l’acquisition de données de qualité au cours des deux dernières décennies et l’arrivée de nouvelles méthodes de surveillance, comme la télédétection spatiale ou le recours à l’ADN environnemental, laissent présager de nouvelles perspectives de suivi de ces précieux écosystèmes.

En mettant en avant l’hydromorphologie comme un critère clé de la qualité écologique des masses d’eau, la directive cadre européenne sur l’eau s’est révélée être un moteur important de nouveaux travaux en Europe. Pour les plans d’eau, plusieurs méthodes ont été développées en France pour répondre à ce nouveau contexte réglementaire, par le biais d’une approche multi-échelles spatiales couplant informations numériques géolocalisées et mesures de terrain.

The European Water framework directive requires observation and monitoring of certain biological communities to assess the ecological status of aquatic environments. How does the WFD contribute to knowledge and evaluation of aquatic biodiversity? What may be the results in terms of monitoring?

La directive cadre sur l'eau (DCE) impose l'observation et la caractérisation de certaines communautés biologiques pour évaluer le bon état écologique des milieux aquatiques. En quoi la DCE a-t-elle contribué à la connaissance et à la mesure de la biodiversité aquatique ? Que peut-on en attendre en termes de suivi ?

Lakes host a high level of biodiversity, especially in their littoral zone, and provide various ecological and socio‐economic services. Lake ecosystems are impacted by a combination of pressures from different human activities (industry, hydroelectricity, agriculture, recreational activities, domestic and urban uses). Anthropic pressures can lead to alterations and dysfunctions in lakes, particularly in the littoral zone.The 2000 EU Water Framework Directive led to considerable efforts to improve the ecological status of European water bodies. However, in France, measures implemented by lake managers to counteract, limit or suppress anthropogenic impacts are still poorly identified. Moreover, their effects on hydromorphology, physico‐chemistry and biotic communities are often only partially reported (the monitoring being incomplete or heterogeneous, if not missing).This paper proposes a framework to harmonize the monitoring of habitat improvement actions conducted in the littoral zone of lakes. The monitoring method is presented as well as the sampling protocols and techniques set out to survey both the restored and control zones at the European French territory scale.Long‐term monitoring of hydromorphological, biological and physicochemical features is expected to improve our understanding of recolonization mechanisms following restoration. The proposed approach will therefore guide practitioners to identify the best restoration options and ultimately contribute to the conservation of recipient populations and communities. [ABSTRACT FROM AUTHOR]

Hydromorphology provides a physical framework for aquatic biocenoses. Its condition directly affects the quality of habitats available for fauna and flora, and its assessment is therefore useful for biodiversity and water quality conservation, as well as for restoration programmes required by European and national directives.In this study, a site‐specific index of Lake HYdroMOrphology, LHYMO, was developed to provide a quantitative assessment of the hydromorphological status of natural and non‐natural lakes that is consistent with European Committee for Standardization (CEN) standards and that can be used for different purposes, including the implementation of the Water Framework Directive (WFD) and of the Habitats Directive.This new LHYMO index includes nine metrics related to the morphological WFD quality elements (QEs) and six metrics related to the hydrological WFD QEs, all of which are considered to support biological elements. The reference conditions were defined for each metric using an original approach: the degree of alteration is measured in relation to the natural characteristics of each lake, relative to a state that 'would be expected in the absence of disturbances'.Besides its use for regulatory purposes, this index is also a suitable tool for monitoring the efficacy of hydromorphological restoration projects or to help target effective conservation measures on lakes.Application of this index to 72 French lakes provides the first quantitative and homogeneous assessment of the hydromorphological quality of lakes over a whole territory and gives the first overview of the hydromorphological status of lakes in France, with classification into five classes ranging from 'high' to 'bad'.LHYMO is already operational for large French lakes as it relies mostly on reference datasets available at a national scale, and it may also be used in a wider scope through gathering or completing data from other sources. [ABSTRACT FROM AUTHOR]

We studied fish size structure by using mean size, size diversity, and the slope of linear size spectra of six common European fish species along large-scale environmental gradients. We further analyzed the response of these three size metrics to environmental variables and to density-dependent effects, i.e., relative estimates of abundance (catch per unit effort, CPUE). We found differences in the strength of main predictors of size structure between the six species, but the direction of the response was relatively similar and consistent for most of the size metrics. Mean body size was negatively related to temperature for perch (Perca fluviatilis), roach (Rutilus rutilus), and ruffe (Gymnocephalus cernuus). Lake productivity (expressed as total phosphorus concentration) and lake depth were also predictors of size structure for four of six species. Moreover, we found a strong density dependence of size structure for all species, resulting in lower mean body size and size diversity and steeper size spectra slopes when density dependence increases. This suggests that density dependence is a key driver of fish size structure. Nous avons etudie la structure de tailles des poissons en utilisant la taille moyenne, la diversite des tailles et la pente des distributions lineaires des tailles de six especes communes de poissons europeens sur des gradients environnementaux a large echelle. Nous avons ensuite analyse la reponse de ces trois metriques de tailles aux variables environnementales et a des mesures de densite-dependances, i.e. estimations relatives de l'abondance (captures par unite d'effort, CPUE). Nous avons decele des differences quant a l'importance des principales variables explicatives de la structure de taille entre les six especes, mais le sens de la reponse etait relativement comparable et coherent pour la plupart des parametres associes a la taille. La taille moyenne du corps presentait une relation negative avec la temperature pour la perche (Perca fluviatilis), le gardon (Rutilus rutilus) et la gremille (Gymnocephalus cernuus). La productivite du lac (representee par la concentration de phosphore total) et la profondeur du lac etaient egalement des variables explicatives de la structure de tailles pour quatre des six especes. En outre, nous avons constate une forte densite dependance de la structure de tailles de toutes les especes qui se traduit par des tailles du corps moyennes et une diversite de tailles plus faibles, ainsi que des pentes plus grandes des distributions de tailles avec l'augmentation de la densite. Ces resultats donnent a penser que la densite dependance est un facteur cle de la structure de tailles des poissons., Introduction Size structure has traditionally been used for elucidating trophic interactions and patterns of energy transfer through trophic levels (Jennings et al. 2002; Woodward et al. 2005; Trebilco et al. [...]

Planktivorous and benthivorous fish have been documented to influence the density and size structure of their prey communities in lakes. We hypothesized that piscivorous fish modify their prey fish communities in the same way and sought to find evidence for such predation effects from a comparison across 356 lakes located in nine European ecoregions. We categorized individual fish as being piscivore, nonpiscivore, or prey of piscivores, depending on species and individual size. We calculated piscivore, nonpiscivore, and piscivore prey densities, respectively, and fit linear abundance size spectra (SS) on lake-specific piscivore, nonpiscivore, and piscivore-prey size distributions. Multiple linear regressions were calculated to quantify the effect of piscivore density and SS slopes on nonpiscivore and piscivore-prey densities and SS slopes by accounting for potentially confounding factors arising from lake morphometry, productivity, and local air temperature. Piscivore density correlated positively with piscivore-prey density but was uncorrelated with density of nonpiscivores. Across a subset of 76 lakes for which SS slopes of piscivores were statistically significant, SS slopes of piscivores were uncorrelated with SS slopes of either nonpiscivores or piscivore prey. However, densities of piscivores, nonpiscivores, or piscivore prey were a significant negative predictor of SS slopes of the respective groups. Our analyses suggest that direct predation effects by piscivorous fish on density and size structure of prey fish communities are weak in European lakes, likely caused by low predator-prey size ratios and the resulting size refuges for prey fish. In contrast, competition may substantially contribute to between-lake variability in fish density and size. Il est documente que les poissons planctonivores et benthivores influencent la densite et la structure de taille des communautes de leurs proies dans les lacs. Nous avons postule que les poissons piscivores modifient leurs communautes de poissons-proies de la meme maniere et tente de trouver des indices de tels effets de la predation en comparant 356 lacs situes dans neuf ecoregions europeennes. Nous avons classe chaque poisson en tant que piscivore, non piscivore ou proie de piscivores, selon son espece et sa taille. Nous avons calcule les densites des poissons piscivores, non piscivores et proies de piscivores, respectivement, et cale les spectres lineaires d'abondance selon la taille (SS) sur les distributions de tailles des poissons piscivores, non piscivores et proies de piscivores propres a chaque lac. Des regressions lineaires multiples ont ete calculees pour quantifier l'effet de la densite de poissons piscivores et des pentes des SS sur les densites des poissons non piscivores et des proies de piscivores et de leurs pentes des SS, en tenant compte de facteurs de confusion potentiels decoulant de la morphometrie et de la productivite du lac et de la temperature locale de l'air. La densite des piscivores etait positivement correlee a la densite des proies de piscivores, mais non correlee a la densite des non-piscivores. Pour un sous-ensemble de 76 lacs dont les pentes de SS des piscivores etaient statistiquement significatives, ces dernieres n'etaient pas correlees aux pentes de SS des non-piscivores, ni des proies de piscivores. Les densites des piscivores, des non-piscivores et des proies de piscivores constituaient toutefois une variable explicative negative significative des pentes de SS de leurs groupes respectifs. Nos analyses donnent a penser que les effets directs de la predation par les poissons piscivores sur la densite et la structure de taille des communautes de poissons-proies sont faibles dans les lacs europeens, probablement en raison de faibles rapports des tailles de predateurs et proies et des tailles-refuges qui en resultent pour les poissons-proies. En revanche, la concurrence pourrait contribuer de maniere sensible aux variations de la densite et de la taille des poissons entre les lacs. [Traduit par la Redaction], Introduction Following two seminal papers on the effect of predation by planktivorous fish on zooplankton communities (Hrbacek et al. 1961; Brooks and Dodson 1965), pelagic food webs have long been [...]