Lake Morphometry Controls Chloride Retention and Modulates Impacts of Road Salt on Stratification in Urban Lakes

Abstract Road salt inputs have caused widespread salinization of urban lakes in northern temperate regions. Watershed characteristics related to road salt application rates are known to be important drivers of lake chloride concentrations, but there has been less focus on how lake morphometry and climate influence seasonal and interannual dynamics in lake chloride, and how these chloride levels may alter mixing in the water column. We analyzed chloride retention for two urban lakes in Saint Paul, Minnesota with adjacent watersheds and similar surface areas that contrast in depth and water residence time. Summer chloride concentrations were negatively related to total summer precipitation for the shallower lake (Como Lake; maximum depth 2.2 m), but the relationship was substantially weaker for the deeper lake (Lake McCarrons; maximum depth 7.6 m). We used a zero‐dimensional model to simulate chloride dynamics in both lakes and tracked the fate of chloride over time. In Como Lake, the mass of chloride in the lake turns over within three years, whereas chloride inputs are retained for >10 years in Lake McCarrons. We then used a one‐dimensional hydrodynamic lake model (GLM‐AED) to examine how chloride loading rates alter lake mixing across a wide range of lake morphometry. Salt inputs significantly extended the duration of summer stratification for simulated lakes with depths >8 m, which may exacerbate anoxic conditions. These results underscore the importance of considering lake morphometry in understanding and managing for the effects of salt inputs on lake ecosystems.