Aquatic ecological communities
The project’s fourth thematic area is focused on the river’s biotic community; the goal is to describe, understand, and model ecosystem restoration in the post-dam period. Research will be carried out at different scales—from the individual scale to the community scale—and will examine biotic and abiotic interactions. In particular, researchers will be looking at organisms of conservation interest that serve as bioindicators: photosynthetic organisms (phytoplankton, biofilm-forming microorganisms, and macrophytes), macroinvertebrates , diadromous fish, and certain mammals. The functional importance of these different groups within trophic webs will also be examined.
The project’s biomonitoring program will explore how the dams’ removal will affect invertebrate (Fig. 1), phytoplankton, biofilm, and macrophyte communities. These organisms are often bioindicators for habitat quality.
However, phytoplankton can also cause problems for humans, when they occur at high levels. For example, certain cyanobacteria produce toxins and can therefore present health risks. These communities, and especially those found in the reservoirs, are expected to change significantly after the dams are removed. At present, they are being sampled at regular intervals. Changes in the macroinvertebrate community are also expected, as river habitats start to be recolonized by particular species in the post-dam period. The decreased water level will facilitate the establishment of macrophytes, which play key ecological roles. For example, they create microhabitats that are exploited by macroinvertebrates.
Dispersal and recolonization
The re-establishment of ecological continuity along the upper stretch of the Sélune will benefit anadromous fish like the Atlantic salmon (Salmo salar) (Fig. 2a), the sea lamprey (Petromyzon marinus), the European river lamprey (Lampetra fluviatilis) as well as catadromous fish like the European eel (Anguilla anguilla). It should also allow the sympatry of fresh water and salt water trout (Salmo trutta ecotypes) and different lamprey species (Lampetra fluviatilis and L. planeri), which will have a significant impact on life-history strategies and population dynamics. Trout and lamprey genetics will be studied, as will the recolonization dynamics of the Atlantic salmon. One concern is that dam removal will allow the spread of invasive species (particularly the signal crayfish, Pacifastacus leniusculus) (Fig. 2b) that currently occur upstream from the dams. The populations of all these organisms (fish and crayfish) are being characterized using different methods (electrofishing, trapping, environmental DNA sampling, and acoustic cameras). The Sélune’s colonization/recolonization by these organisms will be tracked once the dams have been removed.
Researchers will also look at the different aquatic communities found in the Sélune, from its headwaters to its estuary. Instead of concentrating on a specific taxonomic group or community of species, the goal is to answer the question “who is eating whom?” by examining the interactions between the members of the trophic web, from microorganisms to fish. Equilibria within ecosystems largely rely on networks of trophic interactions. Consequently, the project will help answer the following broad questions: (1) What are the effects of restoring ecological continuity (e.g., organismal, material, nutritional) on current aquatic communities and ecosystem functioning, from the river’s headwaters to its estuary? (2) What is occurring as the river ecosystem re-establishes itself at the former location of the reservoir(s)? (3) What are the roles played by certain key species with strong dispersal abilities (notably migratory fish and invasive species) in new ecological equilibria as the river reconnects to the ocean ?