Note d'expertise et d'appui à la Décision Publique "Evaluation de l'Etat Ecologique des Très grands Cours d'Eau à partir du compartiment diatomique et des EQR-IBD2007 régionalisés" Version 1-2 (21-02-2018, modifiée le 07-03-2018)

Fiche techniqueDans le cadre de la mise en application de la DCE en France métropolitaine, du fait de certaines difficultés spécifiques dont l'absence quasi-totale de situations de référence vraie sur tout notre territoire, la mise au point d'un système d'évaluation diatomique dédié aux très grands cours d'eau (TGCE) a commencé d'être envisagé seulement à partir de 2015-2016, une fois que les systèmes d'évaluation dédiés aux cours d'eau plus petits, plus simples à concevoir, étaient en place et avaient subi une intercalibration européenne. Dans les faits, cette action nationale a pu être prise en charge et soutenue par le biais d'une Fiche-Action AFB-Irstea N° 23 (réalisation 2016-2018). Dans les mêmes temps s'est organisé un 2ème exercice d'intercalibration Européenne sur l'évaluation des Large Rivers de plus de 10 000 km2 de bassin versant intégré, auquel ont participé 8 Pays-Membres dont la France. Participant à cet exercice au titre d'experts diatomiques représentant la France auprès de l'UE, nous avons pu dans le même temps mettre au point des scénarios d'évaluation basés sur la biotypologie des grands cours d'eau français par ensemble naturel et tester au fil de l'eau leur DCE-compatibilité dans le cadre de l'exercice en cours. Au final, cela a permis d'adopter directement un nouveau système d'évaluation Français de ces types de cours d'eau conforme aux résultats de l'intercalibration réalisée. Cependant, quelques marges de manoeuvre peuvent exister pour la mise en application concrète sur le plan national, portant notamment sur le périmètre d'assise des types et en particulier, sur le seuil de surface adopté dans notre système national. Avent de formuler le nouvel Arrêté d'Evaluation 2018 incorporant ce nouveau dispositif TGCE, le Ministère en charge de l'Environnement et l'AFB ont sollicité la réalisation d'une étude d'impact comparé de 4 scénarios envisageables, afin de les aider à décider sur le dispositif final le plus adéquat à retenir. Nous avons donc comparé 4 scénarios dont un représentant l'état initial (application exacte de Arrêté évaluation du 27 Juillet 2015 en cours de validité) servant de base de départ. Les 3 nouveaux scénarios font intervenir le même dispositif de grilles d'évaluation TGCE récemment intercalibré et visé OK par l'UE, mais sur la base de 3 variantes de typification des TGCE, les cours d'eau non-TGCE restant évalués selon le contenu de l'Arrêté antérieur. Un scénario utilise le type donné par la typologie nationale abiotique existante, basé sur l'Ordre de Strahler du cours d'eau au site à évaluer, les 2 autres scénarios utilisent chacun un seuillage des TGCE basé sur une surface minimale de bassin versant intégré au site de surveillance (8 000 km2, qui serait une variante nationale plutôt en phase avec le seuil de surface caractéristique de la taille atteinte par les principaux affluents de nos fleuves juste avant confluence avec le drain principal, ou 10 000 km2 qui est le seuil retenu pour la définition du type Large Rivers utilisé pour l'intercalibration. Les résultats statistiques procurés par ces 3 scénarios d'évaluation y sont comparés et commentés. En fin de note figurent quelques suggestions et recommandations opérationnelles, du point de vue des experts-maillon, visant à aider à la réalisation des choix finaux relatifs à la mise en application du nouveau dispositif. Au final, la décision prise n'a pas été celle recommandée dans la présente note (utilisation d'un seuil national de 8 000 km2, a priori plus adapté aux caractéristiques de notre réseau hydrographique), mais celle découlant de l'utilisation du seuil européen de 10 000 km2 tel que défini pour le cadrage et l'homogénéisation des différents jeux de données nationaux dans le cadre de ce type d'exercice d'intercalibration

Global climate change in large European rivers: long-term effects on macroinvertebrate communities and potential local confounding factors

International audienceAquatic species living in running waters are widely acknowledged to be vulnerable to climate-induced, thermal and hydrological fluctuations. Climate changes can interact with other environmental changes to determine structural and functional attributes of communities. Although such complex interactions are most likely to occur in a multiple-stressor context as frequently encountered in large rivers, they have received little attention in such ecosystems. In this study, we aimed at specifically addressing the issue of relative long-term effects of global and local changes on benthic macroinvertebrate communities in multistressed large rivers. We assessed effects of hydroclimatic vs. water quality factors on invertebrate community structure and composition over 30years (19792008) in the Middle Loire River, France. As observed in other large European rivers, water warming over the three decades (+0.9 degrees C between 19791988 and 19992008) and to a lesser extent discharge reduction (80m3s1) were significantly involved in the disappearance or decrease in taxa typical from fast running, cold waters (e.g. Chloroperlidae and Potamanthidae). They explained also a major part of the appearance and increase of taxa typical from slow flowing or standing waters and warmer temperatures, including invasive species (e.g. Corbicula sp. and Atyaephyra desmarestii). However, this shift towards a generalist and pollution tolerant assemblage was partially confounded by local improvement in water quality (i.e. phosphate input reduction by about two thirds and eutrophication limitation by almost one half), explaining a significant part of the settlement of new pollution-sensitive taxa (e.g. the caddisfly Brachycentridae and Philopotamidae families) during the last years of the study period. The regain in such taxa allowed maintaining a certain level of specialization in the invertebrate community despite climate change effects

Des indicateurs multi-substances basés sur la bioaccumulation lors d'encagement de gammares révèlent l'influence de la contamination chimique sur l'abondance des macroinvertébrés des cours d'eau en France

International audienceMost anthropogenic stressors affecting freshwater systems are qualitatively known. However, the quantitative assessment of contaminant exposure and effects to aquatic communities is still difficult, limiting the understanding of consequences on aquatic ecosystem functioning and the implementation of effective management plans. Here, multisubstance indicators based on caged gammarid bioaccumulated contamination data are proposed (for metals and persistent organic pollutants, POPs) to map the bioavailable contamination level of freshwater ecosystems at a large spatial scale. We assessed the ability of these indicators to highlight the relationships between chemical exposure gradients and alteration in the abundance of macroinvertebrate populations on a data set of 218 watercourses distributed throughout France. We identified spatial regional heterogeneities in the levels of bioavailable contamination of metals (18 compounds) and POPs (43 compounds). Besides this, a degradation of Gammaridae, Ephemeridae, and Hydrobiidae densities with increasing levels of metal contamination are identified relative to Baetidae, Chironomidae, and Hydropsychidae. We show here that active biomonitoring allows the establishment of multisubstance indicators of bioavailable contamination, which reliably quantify chemical exposure gradients in freshwater ecosystems. Our ability to identify species-specific responses to chemical exposure gradients demonstrates the promising possibility to further decipher the effects of chemical contamination on macroinvertebrate assemblages through this type of indicator

Stream solutes and particulates export regimes: A new framework to optimize their monitoring

International audienceThe quantification of solute and sediment export from drainage basins is challenging. A large proportion of annual or decadal loads of most constituents is exported during relatively short periods of time, a "hot moment", which vary between constituents and catchments. We developed a new framework based on concentration-discharge (C-Q) relationship to characterize the export regime of stream particulates and solutes during high water periods when the majority of annual and inter-annual load is transported. We evaluated the load flashiness index (percentage of cumulative load that occurs during the highest 2% of daily load, M2), a function of flow flashiness (percentage of cumulative Q during the highest 2% of daily Q, W2) and export pattern (slope of the logC-logQ relationship for Q higher than the daily median Q, b50high). We established this relationship based on long-term water quality and discharge datasets of 580 streams sites of France and USA, corresponding to 2507 concentration time series of total suspended sediments (TSS), total dissolved solutes (TDS), total phosphorus (TP), nitrate (NO3) and dissolved organic carbon (DOC), generating 1.5 million data points in highly diverse geologic, climatic and anthropogenic contexts. Load flashiness (M2) increased with b50high and/or W2. Also, M2 varied as a function of the constituent transported. M2 had the highest values for TSS and decreased for the other constituents in the following order: TP, DOC, NO3, TDS. Based on these results, we constructed a load-flashiness diagram to determine optimal monitoring frequency of dissolved or particulate constituents as a function of b50high and W2. Based on M2, optimal temporal monitoring frequency of the studied constituents decreases in the following order: TSS, TP, DOC, NO3, and TDS. Finally, we analyzed relationships between these metrics and catchments characteristics. Depending on the constituent, we explained between 30 to 40% of their M2 variance with simple catchment characteristics, such as stream network density or percentage of intensive agriculture. Therefore, catchment characteristics can be used as a first approach to set up water quality monitoring design where no hydrological and/or water quality monitoring exist. Abbreviations: W2: percentage of cumulative discharge that occurs during the highest 2% of daily discharge values, termed as flow flashiness M2: percentage of cumulative load that occurs during the highest 2% of daily load values, termed as load flashiness b50high: slope of the logC-logQ relationship for discharge higher than daily median discharge Q50, termed export pattern C-Q: concentration-discharge Q: discharge Cdf: cumulative distribution functio

The recovery of European freshwater biodiversity has come to a halt

Owing to a long history of anthropogenic pressures, freshwater ecosystems are among the most vulnerable to biodiversity loss(1). Mitigation measures, including wastewater treatment and hydromorphological restoration, have aimed to improve environmental quality and foster the recovery of freshwater biodiversity(2). Here, using 1,816 time series of freshwater invertebrate communities collected across 22 European countries between 1968 and 2020, we quantified temporal trends in taxonomic and functional diversity and their responses to environmental pressures and gradients. We observed overall increases in taxon richness (0.73% per year), functional richness (2.4% per year) and abundance (1.17% per year). However, these increases primarily occurred before the 2010s, and have since plateaued. Freshwater communities downstream of dams, urban areas and cropland were less likely to experience recovery. Communities at sites with faster rates of warming had fewer gains in taxon richness, functional richness and abundance. Although biodiversity gains in the 1990s and 2000s probably reflect the effectiveness of water-quality improvements and restoration projects, the decelerating trajectory in the 2010s suggests that the current measures offer diminishing returns. Given new and persistent pressures on freshwater ecosystems, including emerging pollutants, climate change and the spread of invasive species, we call for additional mitigation to revive the recovery of freshwater biodiversity