Research The objective of my thesis is to test the links between (genetic and epigenetic) intraspecific diversity and ecosystems functioning. I am working on 3 species (Phoxinus phoxinus, Gammarus fossarum and Alnus glutinosa), living in fluvial ecosystems and interacting within a food web, to understand these links in a network a little more complex than what is currently done in the literature. The thesis combines molecular, observational and experimental approaches and is structured in three chapters. The first chapter aims at testing the link genetic diversity ~ ecosystem functioning in natural environments (with all the environmental complexity that it implies). For this, we will combine large-scale sampling and sequencing technologies (RAD pool-seq). In the second chapter, I will focus on a single species (Gammarus Fossarum) and the objective is to test the same link, but in controlled environments and targeting « candidate genes », which are « key » genes for the degradation of organic matter (to sum up: diversity of candidate genes ~ ecosystem functioning). The final chapter aims at decoupling the relative effects of the environment and intraspecific diversity on ecosystems functioning and stability. For this, we will use an experimental approach in a controlled environment (the Aquatron in Moulis) and genetic (RAD in pool) and epigenetic (epi-GBS) measurements will allow us to test, for example, whether the environmental conditions have led to a selection of certain variants within the basins or to a plastic response of populations. Ultimately, this thesis will generate new results that should help to further integrate intraspecific diversity within the framework of BEFs, which may ultimately improve current conservation policies (which currently aim at preserving only specific richness). Formation Master 2 Ecologie & Evolution à l’Université Paul Sabatier de Toulouse (2018/2019).