Climate change impacts on hydrology and phosphorus loads under projected global warming levels for the Lake of the Woods watershed

Climate change and variability could directly impact inflows and nutrients from the watershed into Lake of the Woods (LoW), which has been experiencing seasonal algal blooms with a shift in community composition of bloom species mostly to toxin species over the past decades. The main contributing factor to these blooms is deemed to be external inflowing total phosphorus (TP) mostly from the Rainy River. Here, we advance our understanding of potential impacts of climate change on streamflow and non-point source (NPS) TP in the LoW watershed. To this end and for the first time, we developed LoW watershed CanSWAT (Soil and Water Assessment Tool) models forced with climate projections from seven downscaled Coupled Model Intercomparison Project Phase 6 Global Climate Models under two Shared Socioeconomic Pathways. We analyzed hydrological and water quality change at policy-relevant +1.5 to +3.0 °C Global Mean Temperature (GMT) above the pre-industrial period. Under 1.5–3.0 °C GMT increases, projected mean runoff (NPS TP) is 7.3–36.6 % (1.5–117.6 %) and 7.5–31.7 % (−1.6 to 81.4 %) higher than the 1980–2010 reference period in winter/spring in the Precambrian Shield and Agassiz zone, respectively; and lower in summer (−3.0 to −18.8 % (−0.01 to −20.1 %) for both regions). Changes in mean annual Rainy River NPS TP range from −1.8 to 3.1 %. Furthermore, there is a shift in seasonal delivery of NPS TP loads to LoW with winter/spring increases (3.5–26.8 %) and summer/autumn reductions (−0.01 to −14.8 %), which could potentially affect algal productivity and general water quality in LoW. These findings provide important insights to inform future water quality and nutrient management plans.