Impacts of water hardness and road deicing salt on zooplankton survival and reproduction

Rising salinity from road deicing salts threatens the survival and reproduction of freshwater organisms. We conducted two experiments to address how Daphnia pulex survival and reproduction were affected by road salt concentration (control, 120, 640 and 1200 mg Cl(−)/L) crossed with three concentrations of water hardness (20, 97, 185 mg CaCO(3) /L). D. pulex survival was poor in our hard water treatment in both experiments (185 mg CaCO(3) /L), potentially indicating a low tolerance to hard water for the strain used in our experiments. With the remaining two hardness treatments (20 and 97 mg CaCO(3) /L), we found no evidence of an interactive effect between salt concentration and water hardness on D. pulex survival. In our population-level experiment, D. pulex survival was reduced by > 60% at 120 mg Cl(−)/L compared to the control. In the individual experiment, survival was similar between the control and 120 mg Cl(−)/L, but ≤ 40% of individuals survived in 640 and 1200 mg Cl(−)/L. For the surviving individuals across all treatments, the number of offspring produced per individual declined with increasing Cl(−) concentration and in hard water. Our results indicate that current Cl(−) thresholds may not protect some zooplankton and reduced food availability per capita may enhance the negative impacts of road salt.