Assessing nutrient and chlorophyll drivers in Lake of the Woods: A complex system shared among Indigenous communities, Canada, and the USA

Lake of the Woods (LotW) is a large, morphologically complex water body shared between Indigenous communities, Canada, and the US with a history of eutrophication and harmful algal blooms (HABs). The highly variable geology and landscape composition (e.g., peatlands, forest, Canadian Shield) across the LotW basin complement the hydrological complexity of the lake. In an effort to ameliorate the frequency and severity of cyanobacterial harmful algal blooms (cHABs), extensive field collections and modelling efforts have been undertaken to develop adaptive management plans for LotW. However, prior modelling efforts have assumed that the Rainy River watershed is the dominant source of nutrients to LotW. In this study, we used long-term water quality observations from 2008 to 2024 collected by Environment and Climate Change Canada to develop a Bayesian structural equation model, one which allowed us to assess the drivers of in-lake nutrients and chlorophyll. While our results indicate that Rainy River load reductions will improve conditions in the southern basins, these load reductions may not affect conditions in the northerly basins. This work illustrates that future research is required to capture the drivers of phosphorus and nitrogen across LotW which result in cHAB development, to ultimately produce appropriate management strategies for the system.