Canadians are the stewards of more inland water bodies than any other country in the world, a key resource that faces increasing pressures from agricultural intensification. To track these impacts and to assess the ecological integrity of our inland waters, monitoring programs rely on organisms that serve as indicators of water quality, for example bottom-dwelling algae and invertebrates. These biomonitoring activities are however limited in their spatial and temporal scope, mainly because the microscopic enumeration of organisms is a time-consuming process that requires expert taxonomic knowledge. An emergent technology that could solve this problem is the characterization of environmental DNA (eDNA), namely DNA traces left by organisms in the environment. For example, traces of DNA in a single water sample collected at the mouth of a river can reveal the thousands of plants and animals that occur in that river's watershed. Thus, by sequencing eDNA, one could in principle determine the species composition of a site without having to identify organisms with traditional, labour-intensive methods. Moreover, as many genetic markers have recently been optimized to detect various taxa, it is now possible to reconstruct the entire food web of an aquatic ecosystem (including microbes, invertebrates, and fish) by sequencing multiple markers from the same DNA sample, which could provide a richer understanding of biodiversity trends than traditional monitoring focused on a single group of organisms. We propose to develop a multimarker eDNA monitoring approach for the Lac Saint-Pierre UNESCO Biosphere Reserve, a large fluvial lake along the St-Lawrence River. In partnership with three branches of Quebec's government, 4 watershed management organizations, a federal cross-agency genomics project, and an Abenaki first nation community, we will: 1) calibrate a comprehensive eDNA biomonitoring tool that can be used by various stakeholders in the region; 2) assess impacts of land use intensification on streams and wetlands in a provincial biodiversity hotspot; and 3) provide one of the first studies quantifying the response of 'multitrophic' freshwater biodiversity to watershed land use. Les eaux intérieures du Canada représentent une ressource unique qui fait face aux pressions croissantes de l'agriculture intensive. Pour suivre ces impacts et évaluer l'intégrité écologique de nos plans d'eau, les programmes de surveillance utilisent des