Although the mining sector must meet environmental standards, its activities nevertheless have an impact on aquatic and terrestrial ecosystems. Efforts to further decrease the impacts of mining activities include the reduction of nitrate inputs to receiving waters. However, new tools are required to assess the benefits of such reductions. To this end, this project will combine in situ characterization of stream biofilms and water chemistry and biofilm exposures under controlled conditions to assess the impact of a decrease in nitrate concentrations. Biofilm response will be monitored in terms of (1) changes in diatom assemblage composition and associated IDEC values (Eastern Canadian Diatom Index), (2) changes in bacterial communities, and (3) modifications in biofilm fatty acid composition. Natural biofilms were collected in September 2021 in Rivière Blake (Abitibi). Biofilm samples were cultured in large tanks to produce enough material to inoculate 15 aquaria (10 L) for the experiment (5 treatments x 3 replicates). The experiment will be conducted for a period of 30 days over the winter of 2022. Diatom assemblages will be analysed under the microscope and bacterial and algal community composition will be obtained by rRNA gene sequencing (16S and 18S). The abundance of genes involved in nitrate reduction will be assessed by quantitative PCR. Fatty acid profiles will be analysed by GC-MS.