Insect pests are the most widespread disturbance in Canadian forests, but resulting impacts of forest defoliation on stream ecosystem functions are poorly understood. This study investigated the effects of a spruce budworm outbreak on water quality and the structure and function of microbial communities in streams of 12 catchments across a gradient of cumulative defoliation severity in the Gaspésie Peninsula, Québec, Canada. Bi-weekly stream habitat sampling was conducted spring to fall 2019-2021, with stream flow rates measured and water samples collected and analyzed for water chemistry parameters, nutrients, and dissolved organic matter (DOM) structure and quality. Algal communities were assessed at the same time by measuring in-situ biomass. Bacteria and fungi communities on leaf packs were assessed by incubating six leaf packs for five weeks (mid-August- late September) in one stream reach per watershed. Microbial community composition of leaf packs was determined using metabarcoding of 16S and ITS rRNA genes, and functions were examined using extracellular enzyme assays, leaf litter decomposition rates, and taxonomic functional assignments. This study determined that cumulative defoliation increased stream temperatures, flow rates, and SUVA (DOM aromaticity), but not nutrients. It increased algal biomass and altered microbial community composition, with a stronger influence on bacteria than fungi. The observed increases in SUVA and algal biomass corresponded with changes to bacteria carbon cycling functions, which indicated that microbes were preferentially selecting carbohydrates produced by algae rather than the aromatic compounds from increased terrestrial inputs. There were no changes to other bacteria or fungi functions and no changes to taxonomic or functional diversity. Overall, results indicate that forest pest outbreaks alter carbon inputs to streams and the structure and function of stream microbial communities associated with carbon cycling. / Thesis / Master of Science (MSc) / Terrestrial and aquatic landscapes are tightly linked, and forest disturbances can influence stream ecosystems. Insect pests defoliate millions of hectares of forests each year, but the resulting impacts on stream ecosystems are poorly understood. This study investigated the effects of a spruce budworm outbreak on water quality and microbial communities in streams in Gaspésie, QC, Canada. Microbial communities are critical to the functioning of stream ecosystems as they convert energy (e.g., carbon) into useable forms for other organisms. Results indicate that defoliation altered stream flow rates, temperatures, and carbon composition, as well as the microbial communities involved in carbon cycling processes. Carbon is essential to aquatic food webs and this improved understanding of how carbon flow is altered by a widespread forest disturbance can inform pest management decisions for spruce budworm outbreaks.