The scientific program on the Sélune River, which started in 2012, aims at providing a complete feedback on dam removal. This restoration operation is unique in Europe and requires a long-term environmental monitoring effort before, during and after dam removal. The Selune observatory is in charge of this long-term monitoring effort through the measurement of various environmental parameters and by making the collected and processed data publicly available.
The construction of a dam obstructs the circulation of water, sediments and aquatic and riparian organisms. It thus greatly modifies ecosystems by fragmenting them. Conversely, the removal of a dam should restore natural ecosystem functioning by re-establishing the ecological continuity between the source, the estuary and the ocean.
In order to follow this re-establishment of continuity on the Selune River, scientists have set up an observatory to monitor environmental data. It will make it possible to measure the recovery dynamics of fluxes, whether they are related to water, sediments, chemical or biological elements.
The objectives of the Sélune Observatory are to:
- Monitor environmental parameters,
- Analyse/Process these parameters,
- Make data available to all the scientists involved in the Sélune program and to the general public.
Environmental parameters monitored by the Selune observatory
A hydroelectric dam has multiple and interconnected impacts on:
- Abiotic conditions: hydrological regimes, water chemistry and temperature, transport of solid matter (including fine sediments)
- Aquatic and riparian biocenosis, which are impacted by changes in habitat (abiotic conditions) and by river fragmentation (i.e. interruption of ecological continuity). This affects the movement of animals and the dispersal of aquatic and terrestrial plants, around the river.
More precisely, the Selune Observatory monitors the dynamics of the following parameters:
- Hydrological, sedimentary and chemical fluxes
- The hydro geomorphology of the river
- Migratory fish
- Invasive crayfish
- Benthic macroinvertebrates
- Photosynthetic biofilms
- Riparian vegetation
Hydrological, sedimentary and chemical fluxes: Towards a more welcoming habitat?
The presence of dams has a strong impact on hydrological, fine and coarse sediment, and chemical fluxes. These parameters play a major role in determining the quality of habitats for aquatic communities. Moreover, dam removal on the Selune river, is supplemented by a specific sediment management plan to prevent massive sediment transfers downstream.
The Selune observatory monitors hydrological, sedimentary and chemical fluxes using probes and through the regular collection of water samples from upstream to downstream, in order to measure the concentrations of suspended matter and dissolved elements.
Benthic macroinvertebrates: a bioindicator
Benthic macroinvertebrates are one of the most sensitive freshwater community when looking at changes in environmental conditions. This sensitivity make them good bioindicators to understand the effect of dam removal.
The Selune observatory monitors these macroinvertebrates communities to observe their dynamics by looking at:
- The evolution of these communities following the changes in water and sediment fluxes downstream of the dams,
- Their potential settlement in the former lake areas, which may provide new available lotic habitats for them.
Migratory fish: the (re)conquest of the upstream river ?
Removal of the Selune dams will profoundly change the distribution of fish species across the watershed. In particular, migratory amphihaline fish will once again be able to circulate on either side of the old dams.
Electric fishing inventories are carried out each year on the main course and in the main tributaries of the Selune river. These inventories report on the reestablishment of biological fluxes.
Amphihaline fish species with a strong stake in conservation ecology are monitored, particularly:
- European eels,
- Sea and river lampreys,
- Atlantic salmon,
- Sea trout.
More parameters are monitored to follow the direct consequences of dam removal and the restoration of fluxes. Several communities whose assemblages are highly dependent on hydrodynamic conditions and water quality (such as macrophytes, benthic macroinvertebrates, and photosynthetic biofilm) are monitored several times a year.
Riparian vegetation: what recolonisation ?
The disappearance of the retention lakes behind the dams exposes the old riverbanks and therefore leads to their re-colonization by vegetation. The recolonizing species and the rate of revegetation of the banks are also characterized within the Selune observatory.
Signal crayfish: a risk of invasion ?
The signal crayfish (Pacifastacus leniusculus) is present in the Selune watershed, but still absent in some tributaries of the Selune river. The dissemination of this invasive crayfish and carrier of the “crayfish plague” currently seems to be hampered by the presence of retention lakes behind the dams.
The Selune observatory monitors the distribution of signal crayfish every year throughout the watershed. It will continue after the complete removal of the dams.
Organisation of the Selune observatory
The Selune observatory is based on a collective work in which scientists from INRAE and CNRS collaborate:
- UMR ESE (INRAE/Agrocampus-Ouest)
- UMR SAS (INRAE/Agrocampus-Ouest)
- UMR Géosciences (CNRS/University Rennes 1)
- UMR LETG (CNRS/University Rennes 2 and Caen)
- Experimental Unit U3E (INRAE)
The monitoring of environmental parameters is coordinated by:
- The experimental unit U3E, for aquatic and riparian biocenosis,
- The mixed research unit SAS, for abiotic parameters (physico-chemistry and geomorphology)
The Sélune observatory is financially supported by the Seine Normandie Water Agency (l’Agence de l’eau Seine Normandie).
An information system (IS) will be created to make all monitored and processed data from the Selune observatory available. Supported by the program coordination unit, the IS aims to help the scientists associated with the Selune program and make data available to everyone.