Food webs

Effects of the Selune dams on the functioning of food webs: pre-removal assessment

Coordinator(s)

Jean-Marc Roussel

Scientific context

By restoring natural river dynamics, the removal of the two dams on the Séelune will permanently modify the exchanges between marine (Mont Saint-Michel Bay), lotic (Selune and tributaries) and terrestrial ecosystems. These reconnections will have an impact on the trophic interaction networks - food interaction - between species upstream and in the estuary zone. New balances between these species will be established.

The study of trophic interaction networks is the study of all the compartments in the food chain, from organic matter and micro-organisms to fish. The removal of dams will modify the sources of energy (organic matter and micro-organisms) in the food chain, as well as their transport, which was blocked by the presence of the dams. The disappearance of reservoirs will also lead to the creation of new lotic (flowing water) ecosystems. All this will have large-scale effects on the functioning of animal and plant communities. The case of the Selune offers a unique opportunity to study the interactions between different species through food webs.

Subject(s) of study

This project is looking at trophic interactions between animal and plant communities in the Selune in the presence of dams.

Objectives

This project provides an overview of the ecological situation before the disappearance of the two large lakes on its main course, the removal of the dams and the restoration of source-ocean continuity. The aim is to study the animal and plant communities from several angles:

• Community structure: through inventories in each group.
• Ecological functions: monitoring photosynthetic activity and the degradation of organic matter.
• Functioning of trophic networks: stable isotope studies.

Methods

16 reference stations are set up, spread over the main course of the river (6 stations) and some of its tributaries (10 stations). Different methods are deployed:

• Measurements of the physico-chemical quality of the water, in particular temperature, dissolved oxygen, pH, conductivity and nutrient concentration.
• Community inventories using dedicated protocols depending on the taxonomic groups targeted. In each case, the communities are described from a trophic point of view.
• Estimation of two essential ecosystem functions, the degradation of coarse organic matter (tree leaf litter) by the detrital pathway, and the photosynthetic activity of microorganisms and biofilms in the algal pathway.
• Analysis of trophic interactions using the stable isotope method.

Additional investigations are carried out beyond the 16 reference stations:

• Study of stable isotope variations along the source-estuary continuum to highlight possible functional discontinuities in the aquatic ecosystem.
• Updating the spatial distribution of invasive crayfish in the Selune basin, by analysing existing historical data, using in situ trapping and developing a detection method based on DNA testing of water samples.

Laboratories involved

• ESE- UMR Ecology and Ecosystem Health, INRA/Agrocampus Ouest, Rennes
• Pôle MIAME – Centre for the management of amphihaline migrants in the environment, AFB/INRA/Agrocampus Ouest/Université Pau Pays de l’Adour, Rennes
• ECOBIO – UMR Ecosystems, Biodiversity, Evolution, CNRS/Université Rennes 1, Rennes