Comparing long-term changes in cladoceran and diatom assemblages from a lake impacted by road salt seepage to a nearby reference lake near Toronto (Ontario, Canada)

Urban and peri-urban lakes experience a wider array of environmental stressors, and often at a higher intensity, than their rural counterparts, including road salt runoff. A paleolimnological approach was used to determine pre-disturbance limnological conditions and to evaluate the impact of environmental stressors (nutrient inputs, climate change, and winter de-icing salt) on the long-term (~ 150 years) water quality of a small urban kettle lake in the Oak Ridges Moraine near Toronto, Ontario, Canada. Specifically, we examined Cladocera and diatom subfossils in ²¹⁰Pb-dated sediment cores from a lake with elevated measured chloride concentrations, Haynes Lake (Cl⁻ = 201 mg/l), and a nearby reference lake located in a conservation area (Swan Lake, Cl⁻ = 28 mg/l). In Haynes Lake, Cladocera compositional change is consistent with increasing Cl⁻ concentrations, showing a shift from a Bosmina spp.-dominated cladoceran assemblage to a Daphnia spp.-dominated assemblage. Concurrently, we recorded increases in the relative abundances of the diatom taxon Achnanthidium minutissimum and benthic fragilarioid taxa. These biological changes coincided closely with the onset of road salting in the region (ca. 1940s). The reference site (Swan Lake), located ~ 1 km from our salt-impacted site, displayed only minimal changes in both Cladocera and diatom assemblages, suggesting road development and salting within the Haynes Lake watershed had a larger impact than regional stressors (i.e., climate).