Functioning of the Selune estuary

Functioning of the Selune estuary

This project is drawing up an inventory of fish stocks and the functioning of aquatic food webs in the Selune estuary before the dams are removed.

Effects of dam removal on the benthic biocenoses and trophic functioning of the Selune estuary: initial state


Hervé Le Bris et Nicolas Desroy

Scientific context

The Selune estuarine area - credit: L.Goyot © Ludovic Goyot

The emptying of the two reservoirs on the Selune will cause a change in sediment flows downstream of the dams, which will have an effect on the biological functioning of the downstream areas, particularly in the estuarine zone. The benthic communities, living close to the bottom, in the estuarine zone are likely to be affected by these changes, with changes in their structure and diversity. The effect of these modifications can also be integrated by the fish in the estuarine zone, particularly juveniles, which find in these sheltered and biologically productive zones favourable conditions for their development before returning to deeper marine areas. These two types of community are closely linked from a dietary point of view, with the former containing a large majority of the prey exploited by the latter.

Trophic relationships, in terms of food supply, are themselves likely to evolve. Food webs such as the supply and demand for energy between prey and predators could be transformed, benefiting from a greater supply of energy from upstream.

Subject(s) of study

This project focuses on the benthic communities of invertebrates and fish living in the Selune estuary.


The aim of the project is to establish the initial state of the communities present in the Selune estuarine zone, the little bay of Mont Saint-Michel, and their trophic interactions. This initial assessment will help answering the following questions during and after dam removal operations:

  • How are the invertebrate and fish populations in the Selune and the Little Bay changing in response to the environmental modifications associated with the emptying operations?
  • How does the ecological quality of the area evolve through the characteristics of these populations?
  • How are the trophic relationships between these two components of the benthic biocenosis changing?
  • What are the repercussions on the profitability of this area, particularly for juvenile fish?


The study is based on a sampling strategy incorporating the Little Bay of Mont Saint-Michel (shared Sée and Selune estuary), the Sée estuary (control) and the Selune estuary for two seasons. Several sampling methods are used:

  • Sampling of macrobenthos, with sorting and analysis of samples in the laboratory,
  • Trawl tows for benthic and demersal fish, with sorting and analysis of samples in the laboratory,
  • Water and particulate organic matter sampling,
  • Analysis of fish digestive contents,
  • Stable isotope analyses for trophic interactions.
Location of sampling sites - map taken from Le Bris et al. 2018

Laboratories involved

  • ESE – UMR Ecology and Ecosystem Health, INRA/Agrocampus Ouest, Rennes
  • IFREMER – Biological station of Dinard

See also

Monitoring of the functioning of the Little Bay of Mont Saint-Michel were integrated to the Sélune Observatory in 2022 in order to understand the effect of dam removal on the ecosystem of the estuary zone.

This project has been the subject of several publications and reports which you can consult and download.

Modification date : 21 August 2023 | Publication date : 31 July 2023 | Redactor : Selune Team