The extensive use of road salts as deicers during winter months is causing the salinization of freshwater systems in cold climate regions worldwide. We analyzed 20 years (2001–2020) of data on lake water chemistry, land cover changes, and road salt applications for Lake Wilcox (LW) located in southern Ontario, Canada. The lake is situated within a rapidly urbanizing watershed in which, during the period of observation, on average 785 tons of road salt were applied annually. However, only about a quarter of this salt has reached the lake so far. That is, most salt has been retained in the watershed, likely through accumulation in soils and groundwater. Despite the high watershed salt retention, time series trend analyses for LW show significant increases in the dissolved concentrations of sodium (Na+) and chloride (Cl−), as well as those of sulfate (SO42−), calcium (Ca2+), and magnesium (Mg2+). The relative changes in the major ion concentrations indicate a shift of the lake water chemistry from the mixed SO42–Cl–Ca2+-Mg2+ type to the Na + -Cl- type. Salinization of LW has further been strengthening and lengthening the lake's summer stratification, which, in turn, has been enhancing hypoxia in the hypolimnion and increasing the internal loading of the limiting nutrient phosphorus. The theoretical salinity threshold at which fall overturn would become increasingly unlikely was estimated at around 1.23 g kg−1. A simple chloride mass balance model predicts that, under the current trend of impermeable land cover expansion, LW could reach this salinity threshold by mid-century. Our results also highlight the need for additional research on the accruing salt legacies in urbanizing watersheds because they represent potential long-term threats to water quality for receiving freshwater ecosystems and regional groundwater resources.

Lake Wilcox (LW), a shallow kettle lake located in southern Ontario, has experienced multiple phases of land use change associated with human settlement and residential development in its watershed since the early 1900s. Urban growth has coincided with water quality deterioration, including the occurrence of algal blooms and depletion of dissolved oxygen (DO) in the water column. We analyzed 22 years of water chemistry, land use, and climate data (1996-2018) using principal component analysis (PCA) and multiple linear regression (MLR) to identify the contributions of climate, urbanization, and nutrient loading to the changes in water chemistry. Variations in water column stratification, phosphorus (P) speciation, and chl-a (as a proxy for algal abundance) explain 76 % of the observed temporal trends of the four main PCA components derived from water chemistry data. MLR results further imply that the intensity of stratification, quantified by the Brunt-Väisälä frequency, is a major predictor of the changes in water quality. Other important factors explaining the variations in nitrogen (N) and P speciation, and the DO concentrations, are watershed imperviousness and lake chloride concentrations that, in turn, are closely correlated. We conclude that the observed in-lake water quality trends over the past two decades are linked to urbanization via increased salinization associated with expanding impervious land cover, rather than increasing external P loading. The rising salinity promotes water column stratification, which reduces the oxygenation of the hypolimnion and enhances internal P loading to the water column. Thus, stricter controls on the application and runoff of de-icing salt should be considered as part of managing eutrophication symptoms in lakes of cold climate regions.