Towards an improvement in the ecological status of the watercourse following the removal of the Selune dams?

What is a "good status" watercourse?

A watercourse in "good condition" has sufficient water quantity and quality to support a rich and varied animal and plant life. According to the water agencies, in France in 2015: 62% of rivers were considered to be in good chemical status, 44.8% of rivers were considered to be in good ecological status and 69.1% of groundwater bodies were considered to be in good chemical status.
The Water Framework Directive, signed in 2000, defines and sets the objectives and methods for achieving good water status. In particular, the WFD calls for all rivers and groundwater bodies to achieve good ecological status by 2027.

• The chemical status of a watercourse is assessed according to environmental quality standards (EQS) and compliance with threshold values (status: 'good' or 'not good'). Several substances, such as heavy metals, pesticides, PCBs, hydrocarbons, etc., are monitored to establish this chemical status.
• The ecological status of a watercourse is assessed by monitoring the structure and functioning of the aquatic ecosystems associated with the watercourse. Quality indices are determined according to biological (bio-indicator species), hydro-morphological and physico-chemical criteria. Ecological status is determined by the difference between the quality index and the reference conditions for the type of watercourse (status: 'very good', 'good', 'average', 'poor' and 'bad').

This article and the results presented here relate solely to the assessment of the ecological status of the Selune.

What is the current ecological state of the Selune?

In summer, with the dams, it was common to see blooms of toxic algae and cyanobacteria, which led to the suspension (by order of the prefect) of swimming and other water sports activities in the lakes. These blooms were the result of nutrient-rich water that stagnated and warmed up in summer. In addition, catfish (Silurus glanis) had flourished in the lakes, where they had become the dominant species to the detriment of other species. The aquatic ecosystem in the reservoirs was therefore unstable and not very resilient, i.e. not conducive to withstanding increasingly hot summers, as is happening with climate change. Removing the dams immediately stopped the problem of summer blooms of cyanobacteria.

The warming of the water mass in the lakes was reflected in an increase in water temperatures downstream compared with the upstream parts of the Selune. Thus, after the dams were removed and the lakes disappeared, the water temperature recorded at the Pont de Signy - downstream - was around 2°C cooler during the hottest period in the summer of 2022, compared with previous years. Oxygen levels in water are inversely related to temperature. Cooler water therefore contains more oxygen, which is good for aquatic fauna.

This change is important because these are essential conditions for improving the quality of the environment, which will encourage the establishment and return of sensitive species that are key to the stability of the new Selune ecosystem.

This is particularly the case for aquatic invertebrates, which are the larvae of many insects. These aquatic insects, which live on the river bed, are bio-indicators of the ecological quality of the environment. On the Selune, these aquatic insects have been monitored for several years at various stations located in the former reservoirs, upstream and downstream of the dams, in order to understand their evolution and calculate a quality index: the I2M2. The I2M2, which stands for Multi-Metric Invertebrate Index, incorporates the difference between the reference conditions of the watercourse and several measurements of the structure and functioning of aquatic invertebrates. On the Selune, the I2M2 shows a gradual improvement in the stations at the former Vezins reservoir after it was emptied. The quality of the environment went from poor (the year following the emptying and work on the sediments) to good in just 3 to 4 years, reaching a level identical to that upstream of the old reservoirs.

This is evidence of the rapid colonisation of the new river by aquatic invertebrates, particularly insect species sensitive to pollutants and oxygen. The same pattern is occurring in the river at the site of the former La-Roche-Qui-Boit reservoir, following the emptying of the reservoir in 2022. Other bio-indicators, such as photosynthetic biofilms, are also showing positive trends, or have a longer response time. It is therefore important to continue monitoring these indicators in the coming years.