The study of riparian vegetation is based on the observation of natural plant communities spontaneously colonizing newly exposed areas following the erasure of the Vezins and La-Roche-Qui-Boit dams. The vegetation monitoring system includes two complementary approaches: i) a vegetation sampling method based on floristic surveys (composition & structure) carried out within a series of quadrats positioned along a transect ii) a monitoring of the vegetation units (UV) observed within the valley This dataset specifically concerns the overall characterization observed on each quadrat in terms of strata and concerns only the Sélune Observatory (2021-2026). The sampling monitoring system is based on measurements distributed at different points in space (longitudinally and laterally). It aims to identify the diversity of flora and vegetation and describe the structure of emerging formations and plant communities within the neo-valley. Implemented annually (sub-annual only for 2021, before dismantling), this monitoring aims to identify and characterize the evolution trajectories of these vegetation over time. The vegetation sampling protocol is implemented at several stations located along the course of the Sélune and at the mouth of several tributaries located in the right-of-way of the two former reservoirs. The sample of stations is supplemented by a batch of stations positioned outside the right-of-way of the two former reservoirs, downstream and upstream, which constitute so-called "reference" stations. For each station, a transect is established perpendicular to the channel, from the bank to a reference point located at the level of the former water reservoir (i.e. 60m NGF for Vezins & 28.5m NGF for LRQB). Along this transect, 25m2 (5mx5m) quadrats are positioned according to previously set distance intervals. These quadrats (named Q1 to Q1+n) constitute the elementary unit within which abiotic (slope, texture...) and biotic (composition, structure) parameters are characterized and evaluated. The number of quadrats sampled per station varies depending on the width of the exposed bank (or transect length). It is completed by a reference quadrat, located beyond this maximum value (named Q0). The inventory of vascular flora, i.e. herbaceous and woody plant species within the quadrats, aims to be exhaustive and to identify them up to the specific or even sub-specific rank. The overall characterization mentioned mainly concerns, for each quadrat, the type of soil encountered (see attribute dictionary), the slope (see attribute dictionary), the proportions of water cover, bare soil, and litter, as well as, for each of the 6 defined strata (see attribute dictionary): the considered height limits, the maximum and average height, and the proportion of overlap (relative to the 25m2 of the quadrat). This layer gives, for each monitoring quadrat of vegetation sampling from 2021 to 2026, the following information: – Quadrat information (id, code, number and distance from its centroid to the station reference point), – Campaign information (id, code, date, campaign), – Station information (code, name, protocol applied, number of quadrats, code for the associated Sélune reference point and, as a result, positioning on the catchment area, distance to sea, distance to source and watercourse, watercourse reference altitude) – Information on the transect by campaign (number and length, X, Y, LON, LAT coordinates of the reference points “NGF” and end point “bank”) – Information concerning the strata (identified or not), soil typology, water, bare and litter coverings, as well as details for each stratum (name of stratum ‘muscinal’, “herbaceous”, “young plant shrub,” “low shrub', “high shrub” and “arborescent, the height limits of the stratum, the associated overlap, the maximum and average heights of the stratum) – “Point” geometry (default) corresponding to the transect reference point

The opening of the dams should impact the functioning of Atlantic salmon populations migrating up the Sélune River and its tributaries. Restoring the river's ecological continuity will alter population flows by allowing amphihaline sea trout to migrate further upstream and native trout to move downstream and upstream of the dams. The trout population is monitored at a network of stations covering the entire Sélune basin using an abundance index, which is carried out every two years before 2021 and every year since then. Field campaigns comply with the fishing protocol developed by INRAE and OFB. The aim of this method is to estimate the abundance of juvenile Atlantic salmon (Salmo salar) at a station (or sector). This protocol targets juveniles of the year (aged 0+) whose abundance reflects the renewal of generations within the population (or recruitment) and survival after the embryonic development phases under gravel and the first months of life in the open environment. Field campaigns are carried out by INRAE and the Manche d'Ille-et-Vilaine Federation for Fishing and the Protection of the Aquatic Environment (FDAPPMA50 and FDAAPPMA35). This dataset provides individual biometric data on Atlantic salmon caught during PAS fishing sessions in the Sélune watershed, by station and by fishing session.

Geodetic markers were installed along the main course of the Sélune. Physically marked out in the field, these markers have been located with a high-precision GPS (planimetric and z-precisions are given in the table). Please note that these markers may move slightly with the wetting/drying cycles of mainly clay soils, and their coordinates may be updated if necessary. Geodetic markers can be used as reference points to define a device or a new study area.

As part of the Biomonitoring project and the Sélune observatory , pilot stations have been set up along the main course of the Sélune, distributed from upstream to downstream of the hydroelectric dams. Three stations are located in the former reservoirs of the dams, while the other two, known as the reference stations, are outside the area of influence of the former dams (one downstream and the other upstream). At these stations, aquatic biocenoses (including benthic macroinvertebrates, biofilms, macrophytes, etc.) are monitored. 2 sampling campaigns (spring and autumn) for benthic macroinvertebrates (bmi) have been carried out since September 2014. During each campaign, 2 sampling protocols are implemented: - in accordance with standard NF T90-333, which enables the calculation of the IBG-DCE and I2M2 (‘SURBER’) - via artificial substrates immersed for 1 month (‘SUBART’), in order to focus on measuring the ‘effect of dams’. The protocol is described in: Piscart C., Moreteau, J.C., Beisel J.N. (2006). Monitoring changes in freshwater macroinvertebrate communities along a salinity gradient using artificial substrates. Environmental Monitoring and Assessment 116: 529-542. This layer details the benthic macroinvertebrate taxa and their abundance obtained after analysis (according to standard NF T90-388). These data are distributed by campaign and by sector. Taxa presenting all the morphological criteria clearly visible and characteristic allowing the exactitude of their determination and an irreproachable state of conservation are integrated into the reference collection of the Sélune Observatory. This collection can be consulted on request (see contact point). Taxon determination is based on version v17 of the TAXREF reference. The dataset is currently under embargo.

The opening of the dams should impact the functioning of Atlantic salmon populations migrating up the Sélune and its tributaries. Restoring the river's ecological continuity will alter population flows by allowing Atlantic salmon to migrate further upstream and downstream of the dams. The Atlantic salmon population is monitored at a network of stations covering the entire Sélune basin using an abundance index, which was carried out every two years before 2021 and annually since then. Field campaigns comply with the fishing protocol developed by INRAE and OFB. The aim of this method is to estimate the abundance of juvenile Atlantic salmon (Salmo salar) at a station (or sector). This protocol targets juveniles of the year (aged 0+) whose abundance reflects the renewal of generations within the population (or recruitment) and survival after the embryonic development phases under gravel and the first months of life in the open environment. Field campaigns are carried out by INRAE and the Manche d'Ille-et-Vilaine Federation for Fishing and the Protection of the Aquatic Environment (FDAPPMA50 and FDAPPMA35). This dataset provides the locations of these study sectors for IAS fishing sessions in the Sélune watershed by fishing session.

The study of riparian vegetation is based on the observation of natural plant communities spontaneously colonizing newly exposed areas following the erasure of the Vezins and La-Roche-Qui-Boit dams. The vegetation monitoring system includes two complementary approaches: i) a vegetation sampling method based on floristic surveys (composition & structure) carried out within a series of quadrats positioned along a transect ii) a monitoring of the vegetation units (UV) observed within the valley This sheet specifically concerns the monitoring of vegetation units (UV) observed within the valley, only for data from the Sélune Observatory (2021-2026). The monitoring system for Vegetation Units is based on the identification of vegetation distributed along the transect at each station in the main stream. It aims to obtain a representative view of diversity and the organization of plant formations and communities. This monitoring consists of identifying the discontinuities in terms of composition and structure of the various vegetation types that develop within the neo-valley along the transect that descends to the bank, and to report their influence. (distance in metres from the reference point at the edge of the old dam). These data can be used to establish a pro rata of the different plant formations encountered along the transect. Although this value is thus really valid only at the transect of each station, this information can potentially constitute an approximation of the heterogeneity and distribution of vegetation at the valley scale. This layer gives, for each riparian vegetation monitoring station and each campaign from 2021 to 2026, the following information: – Campaign information (id, code, date, campaign), – Station information (code, name, protocol applied, number of quadrats, code for the associated Sélune reference point and, as a result: location on the watershed, distance to sea, distance to source and water course) – Information on the transect by campaign (number and length, X, Y, LON coordinates, LAT of reference points "NGF" and end point "rive") – Information regarding the Vegetation Units (VU) identified by campaign (number, description: code, start position, end position, EUNIS3 characteristics, 4) – Origin of the data ("Observatoire Sélune," "Hors Observatoire Sélune," "Projet Séripage," or "Projet Restaure") – “Point” geometry (default) corresponding to the transect reference point – Line (alternative) geometry of the associated transect for each campaign. The study of riparian vegetation in exposed areas is based on the observation of natural plant communities colonizing newly exposed mudflats, at different time scales (intra- and interannual variations) and at nested spatial scales. The botanical inventory is carried out according to the Braun-Blanquet method. The sampling protocol is stratified in order to maximize the recorded diversity. For each study area, a transect perpendicular to the riverbed is established, where strips parallel to the watercourse are marked on the ground according to the exposure rate. This layer indicates the location of these study areas, the layout of transects and information regarding vegetation units recorded between 2021 and 2026.

The re-establishment of the river continuum on the Sélune after the dams have been levelled will enable colonization of the upper reaches of the basin, which were previously inaccessible to eels. Before the dams were levelled, two main tributaries were used by amphihaline fish for reproduction, in addition to the main river: the Beuvron and the Oir. The eel population of the Sélune was monitored in the pre-flushing phase by abundance index campaigns carried out in 2013, 2015, 2019 on a network of stations covering the entire Sélune hydrographic network. Since 2021, this network of stations has evolved to monitor changes in the part of the watershed accessible following the dismantling of the 2 dams. These data enable us to estimate changes in eel abundance along the Sélune and its tributaries, as well as their size structure before and after the removal of the dams. The inventory is carried out in early September, every other year between 2013 and 2019, then every year since, using an electric fishing device and dip nets. Thirty thirty-second fishing stations are set up. A fishing station comprises approximately 100 meters of river. All eels caught are anaesthetized and biometric measurements are taken (weight, length, horizontal and vertical eye diameter, determination of yellow or silver stage). All eels are released directly at the fishing site. Two different protocols were implemented: the Eel Abundance Index (Pottier and Chapon, 2022) at 23 stations and exhaustion fishing at eight other stations (IAA01 to IAA08) on the Oir River and some of its tributaries, according to an internal protocol. This layer provides station abundances by fishing session.

Microbial communities play a major role in the functioning of ecosystems: they are at the base of the food chain (primary production) and participate in the degradation of organic matter. These communities are also known to respond rapidly to environmental changes. Like macro-invertebrates, they can be used as ecological indicators. Benthic diatoms are the main photosynthetic organisms in this biological community. The relative abundance of the different species is used to calculate an environmental quality index (EQI). Major differences in communities between the upstream and downstream zones were observed when the dams were in place. These differences will change once the dams are removed. As part of the Sélune observatory, pilot stations have been set up along the main course of the Sélune, distributed from upstream to downstream of the hydroelectric dams. Three stations are located in the former reservoirs of the dams, while the other two, known as the reference stations, are outside the area of influence of the former dams (one downstream and the other upstream). At these stations, aquatic biocenoses (including benthic macroinvertebrates, biofilms, macrophytes, etc.) are monitored. Since September 2014, the stations are ideally sampled every month from April to October (7 annual surveys) using artificial substrates (glass slides placed in the water). In addition to the diatom survey (floristic list, IBD calculation), the chlorophyll-a concentration is measured. During each campaign, an INRAe experimental sampling protocol is implemented. This protocol involves the immersion (1 month) of glass slides, an in situ measurement of the chlorophyll-a concentration via BentoTorch and then the collection of the biofilm on these glass slides. The biofilm is conditioned in (1) 99.9% ethanol to determine IBDs and floristic lists (outsourced service, based on standard NF T90-354) and (2) mineral water to measure chlorophyll-a concentration using a spectrometer. NB: the year 2023 is not covered in terms of measuring chlorophyll-a concentration via spectrometry due to a problem with the low-temperature storage of samples. This dataset provides measurements of the average daily concentration of chlorophyll-a and pheopigment in µg/cm2/day.