Widespread use of NaCl for road deicing has caused increased chloride concentrations in lakes near urban centers and areas of high road density. Chloride can be toxic, and water quality guidelines have been created to regulate it and protect aquatic life. However, these guidelines may not adequately protect organisms in low-nutrient, soft water lakes such as those underlain by the Precambrian Shield. We tested this hypothesis by conducting laboratory experiments on six Daphnia species using a soft water culture medium. We also examined temporal changes in cladoceran assemblages in the sediments of two small lakes on the Canadian Shield: one near a highway and the other >3 km from roads where salt is applied in the winter. Our results showed that Daphnia were sensitive to low chloride concentrations with decreased reproduction and increased mortality occurring between 5 and 40 mg Cl–/L. Analysis of cladoceran remains in lake sediments revealed changes in assemblage composition that coincided with the initial application of road salt in this region. In contrast, there were no changes detected in the remote lake. We found that 22.7% of recreational lakes in Ontario have chloride concentrations between 5 and 40 mg/L suggesting that cladoceran zooplankton in these lakes may already be experiencing negative effects of chloride.

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Since the 1950s, the widespread application of road salt for winter road maintenance and safety in cold regions has led to increased conductivity levels in many freshwater systems. Salting practices have adversely affected freshwater biota; however, the magnitude of ecological impacts may vary by species and ecosystem. Here, we examine diatom assemblage changes during the past ~200 years from sedimentary records from five impacted lakes (measured specific conductance values of 149 – 350 S/cm) and a reference lake (15 S/cm) located in the Muskoka River Watershed, south-central Ontario, Canada. Diatom compositional changes in the road-salt impacted sites were consistent with increasing conductivity and increased diatom-inferred conductivity was evident during the latter half of the 20th century, concurrent with known road salt application. The strongest predictor of DI-conductivity changes among the six lakes was the km equivalents of roads within the watershed (i.e. km of road x # of lanes). Similar to changes observed in a previous study focussing on cladoceran assemblages, we conclude that even modest applications of road salt have affected diatom assemblages in softwater lakes in south-central Ontario.

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