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Effects of road-salt application on Cladocera assemblages in shallow Precambrian Shield lakes in south-central Ontario, Canada

Road-salt application is an integral part of winter road maintenance and safety in many cold regions. However, chloride (Cl) runoff from salting activities may have adverse ecological effects. Lakes affected by road salt, particularly in urban areas, may have Cl concentrations up to 300% higher than natural conditions. Zooplankton are sensitive to changes in water chemistry; however, little research has been conducted thus far into the biological effects of elevated Cl concentrations from road-salt runoff. Using paleolimnological analyses, we studied the long-term effects of Cl from road salt runoff on shallow, soft-water lakes within the southern Canadian Shield. Specifically, we examined Cladocera subfossils in dated sediment cores from 6 lakes within the Muskoka River Watershed of south-central Ontario, Canada. We compared long-term changes in cladoceran assemblages in the 5 lakes with the highest regional Cl concentrations (32.8–90.9 mg/L) with a reference lake (0.91 mg/L). Prior to salting (ca. 1950), all of the lakes were dominated by Bosmina spp., and cladoceran assemblages showed little variation over time. After salting began, species assemblages in the 5 impacted lakes showed distinct taxonomic shifts, including increases in relative abundances of Chydorus brevilabris (Frey, 1980), Eurycercus spp., and the Daphnia pulex complex, and decreases in the relative abundance of Bosmina spp. and Alona spp. In 2 of the lakes, these changes were distinct and coincided with the onset of salting activities. In contrast to the species changes observed in the affected lakes, we recorded minimal directional changes in cladoceran assemblages in the nearby reference site. Overall, biological changes in the impacted lakes were best explained by changing Cl concentrations associated with road-salt application. Among lakes, the severity of impact may have been indirectly mediated by differences in food quantity through its effect on Cl toxicity.