Measuring intraspecific variation in tolerance to road salt for Daphnia populations sampled along spatial gradients in lake chloride levels using standardized toxicity (LC50) tests

Road salt levels are increasing in many northern temperate lakes, and many species of zooplankton are sensitive to increasing chloride [Cl] with up to 50% abundance declines at existing chronic concentration guidelines. Yet, much is still unknown about spatial variability in salt loadings, background water chemistry, and species responses to variable chloride levels in space and time. I tested if intraspecific variation in Daphnia galeata’s chloride tolerance was positively related to previous [Cl] exposure. Live D. galeata and grab water samples were collected at six sites that differed in background [Cl]. Genetically identical D. galeata of a standard age (<24 hours) were placed into 10 mL test tubes with 10 replicated [Cl] treatments (18-2700 mg/L) to determine 48-hour survival. Survival declined with increasing chloride concentrations, and the lethal concentration-50 (LC50) ranged between 820.45-1220.63 [Cl] mg/L across sites. Intraspecific variation found did not correlate with the measured lake chloride levels.