POSTER PRESENTER CO-AUTHORS: Matthew Hudson, Sarah Johnson, Erik Olson (Burke Center for Ecosystem Research) TITLE: Eurasian watermilfoil invasion and macrophyte community change in the Turtle-Flambeau Flowage ABSTRACT: Impoundment lakes and flowages created by dams alter natural water-level regimes, reshaping the composition and dynamics of aquatic biotic communities. Due to elevated susceptibility to invasion, capacity to support multiple aquatic invasive species, and potential to act as sources of aquatic invasive species to nearby natural lakes, impoundments warrant enhanced monitoring. The Turtle-Flambeau flowage in Northern Wisconsin is a species-rich impoundment system with long-term macrophyte data and a recent invasion of Eurasian watermilfoil (Myriophyllum spicatum; EWM). Our study quantified the distribution, rate, and magnitude of EWM spread in the flowage and evaluates impact on native macrophytes. In summer 2025, we conducted a system-wide survey using standard point-intercept methods at 1,445 points surveyed once prior in 2014-2019 and within additional finer-scale grids of 698 points established in 2024. We recorded water depth, substrate type, and the presence of all macrophyte species using rake fullness metrics. Surveyed points supporting at least one macrophyte species increased by 13% compared to baseline data from 2014-2019 and occurred at deeper water depths, indicating the flowage became more vegetated. Across the flowage, native community composition remains diverse (68 species) and of high floristic quality (Mean C = 7.1, FQI = 52.1). This community is at risk, as EWM has spread rapidly, affecting 20% more locations between 2024 and 2025. We will present additional metrics of community composition and dynamics. The Turtle-Flambeau flowage provides a valuable opportunity to explore environmental drivers and rates of change in aquatic macrophyte communities within a large, annually managed impoundment system. BIO: Abby Stasko is a first-year graduate student in the biology program at Grand Valley State University in Allendale, MI. Her primary area of study is plant community ecology, and she is currently working on a project focused on the aquatic macrophyte community in the Turtle-Flambeau Flowage near Mercer, WI. Abby graduated from Northland College in 2023 with bachelor’s degrees in biology and natural resources.