Human activity causes various negative impacts on freshwater ecosystems. The Rideau Canal, and its watersheds, in Ontario, Canada is a freshwater ecosystem altered by humans which faces pressure from nutrient loading, land-use change, and invasive species. This thesis examined spatial and temporal drivers of algal abundance, biomass, and community composition in the Rideau Canal. First, the chemical and biological changes that have occurred since 1995 were quantified by comparing water samples and periphyton biomass collected in 1995 and 2019. This study found that periphyton biomass, despite experiencing changes in its environment, demonstrated little temporal variation. Second, environmental, spatial, and temporal drivers of phytoplankton abundance, biomass, and community composition in the Rideau Canal in 2019 were quantified using a FlowCam to enumerate and identify the phytoplankton. This study showed that phytoplankton community composition in the waterway is largely driven by environmental factors, with spatial and temporal controls having lesser effects.