Assessing the Relative Toxicity of Different Road Salts and Effect of Temperature on Salinity Toxicity: LC x Values versus No-Effect Concentration (NEC) Values.

Freshwater biota are at risk globally from increasing salinity, including increases from deicing salts in cold regions. A variety of metrics of toxicity are used when estimating the toxicity of substances and comparing the toxicity between substances. However, the implications of using different metrics are not widely appreciated. Using the mayfly Colobruscoides giganteus (Ephemeroptera: Colobruscoidea), we compare the toxicity of seven different salts where toxicity was estimated using two metrics: (1) the no-effect concentrations (NEC) and (2) the lethal concentrations for 10, 25 and 50% of the test populations (LC x ). The LC x values were estimated using two different models, the classic log-logistic model and the newer toxicokinetic-toxicodynamic (TKTD) model. The NEC and both types of LC x values were estimated using Bayesian statistics. We also compared the toxicity of two salts (NaCl and CaCl 2 ) for C. giganteus at water temperatures of 4 °C, 7 °C and 15 °C using the same metrics of toxicity. Our motivation for using a mayfly to assess salinity toxicity was because mayflies are generally salt sensitive, are ecologically important and are common in Australian (sub-)alpine streams. The temperature ranges were chosen to mimic winter, spring and summer water temperatures for Australian (sub-)alpine streams. Considering 144-h classical LC x values, we found toxicity differed between various salts, i.e., the lowest 144-h LC 50 (8 mS/cm) for a salt used by a ski resort was half that of the highest 144-h LC 50 from artificial marine salts and CaCl 2 applied to roads (16 mS/cm). The analytical grade NaCl (as shown by 144-h LC 50 value at 7 °C) was substantially more toxic (7.3 mS/cm) compared to analytical grade CaCl 2 (12.5 mS/cm). Yet for NEC values, there were comparably fewer differences in toxicity between salts and none between the same salts at different temperatures. We conclude that LC x values are better suited to compare the difference in toxicity between substances or between the same substance at different test temperatures, while NEC values are better suited to estimating concentrations of substances that have no effect to the test species and endpoint measured under laboratory conditions.