Genetic variation, but no evidence of evolved tolerance, for Daphnia in road salt polluted lakes in Ontario

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Cicchetti, Lisa E.
Biology , Freshwater Ecology , Conservation Biology , Limnology , Ecotoxicology , Daphnia , Road Salt , Evolved Tolerance , Developmental Plasticity
Snowmelt and rain carry road salt from paved surfaces through the watershed via runoff and groundwater. When road salt reaches lakes, it increases the salinity, often measured as chloride concentration (Cl- mg/L). Salinization negatively impacts freshwater organisms, including Daphnia pulicaria, a common crustacean zooplankton grazer that is integral to healthy ecosystems and maintenance of ecosystem services. Daphnia are sensitive to environmental changes, but studies have shown they are capable of rapid evolution. We investigated intraspecific variation in salt sensitivity among D. pulicaria collected from 10 lakes in southeastern Ontario. Lakes were distributed along a Cl- gradient from <1 mg Cl-/L (unimpacted by road salt) to 271 mg Cl-/L, which is above the Canadian Water Quality Guidelines for chronic exposure (120 mg Cl-/L) but below the guideline for acute exposure (640 mg Cl-/L). Acclimation can influence tolerance so we tested the effect of acclimation on acute 48-hour salt tolerance for Daphnia grown at low (18 mg Cl-/L) and high (218 mg Cl-/L) Cl-. Our results indicate that acclimation to higher salt concentrations does not improve salt tolerance. Our acute toxicity tests conducted under common garden conditions show a range of salt tolerances for Daphnia originating from different lakes. Daphnia from Presqu’ile Bay (26.2 mg Cl-/L Lake Ontario, Brighton, ON, CA) and Lake Wilcox (194 mg Cl-/L, Richmond Hill, ON, CA) had the highest salt tolerances in this study, with lethal concentrations for 50% (LC50) of the test individuals of 2251 mg Cl-/L and 2328 mg Cl-/L, respectively. Verona Lake had the lowest LC50, at 1809 mg Cl-/L. However, there was no impact of lake chloride on the salt tolerance of Daphnia, providing no evidence of evolved salt tolerance in southeastern Ontario. This suggests that either the current salinities are not predominant selection forces in these lakes, or evolved tolerance is coupled with an evolutionary trade-off. Further research is needed to understand the standing variation of salt tolerance within local populations, and variation across different regions, to inform decisions regarding freshwater ecosystem conservation and roadway management.
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