Assessing the Effects of Road Salt Application (NaCl) on Cladocera and Diatoms Across a Gradient of Chloride Concentrations

Lakes across the Northern Hemisphere are being affected by multiple environmental stressors and of particular concern in north temperate regions is the application of rock salt (NaCl) as a road de-icer. Despite its near ubiquitous use in north temperate regions, little is known about how road salt contamination of surface waters effects freshwater ecosystem structure. This thesis combines paleolimnological approaches and laboratory bioassays to assess the overall impact of Cl− additions on freshwater biota (Cladocera and diatoms). Paleolimnological techniques were used to assess baseline conditions, to track biotic changes through time, and to investigate the long-term effects of Cl− additions on freshwater biota in both softwater, oligotrophic systems as well as hardwater, mesotrophic lakes. A multiproxy study of shallow, softwater lakes receiving direct road salt runoff assessed how multiple trophic levels have changed over the past ~200 years. Prior to salting (ca. 1950), all of the lakes were dominated by Bosmina spp. After salting began, distinct taxonomic shifts were recorded, including increases in relative abundances of Chydorus brevilabris and the Daphnia pulex complex, and decreases in the relative abundance of Bosmina spp. Diatom compositional change in the road-salt impacted sites was also consistent with increasing Cl− concentrations (i.e., increases in Achnanthidium minutissimum and benthic fragilarioid taxa). Increases in diatom-inferred conductivity were also evident, occurring concurrently with known road salt application. Building on the paleolimnological findings, laboratory bioassay experiments were used to determine the specific tolerances of dominant Cladocera taxa to Cl−. To evaluate the specific tolerances of species identified in the paleolimnological studies as Cl− sensitive (Bosmina longirostris) and tolerant (C. brevilabris), a series of 14-day bioassays were conducted. Both B. longirostris and C. brevilabris showed sensitivity below the current Canadian water quality guidelines for Cl− (LC50 = 24.3 mg/L and 60.3 mg Cl−/L, respectively). Finally, an investigation of a highly impacted, hardwater, mesotrophic lake recorded shifts similar to those recorded in moderately impacted softwater lakes. This urban lake shifted from a Bosmina dominated assemblage to a Daphnia dominated assemblage with concurrent increases in A. minutissimum and benthic fragilarioid taxa, coinciding closely with the onset of road salting. Collectively, this research shows that the biological impacts of Cl− additions are observed at concentrations below many water quality guidelines and that future changes may exacerbate the harmful impacts of road salt additions (e.g., changes in water hardness, nutrients, and climate warming).

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