The effect of road salt (NaCl) treatment on the hatching success of Daphnia magna and Thamnocephalus platyurus

Salinization is an emerging problem for freshwater biodiversity. Road salt treatment results in salinization of local freshwater, influencing its entire ecosystem throughout the year. The present study examines the impact of road salt on the hatching success of two zooplankton species, Daphnia magna and Thamnocephalus platyurus, using pure laboratory lines from MicroBioTest. Hatching success was determined without incubation (T0) and after 3, 7 and 28 days of incubation at 5°C (T3, T7 and T28, respectively), at different NaCl concentrations. The low-temperature incubation was intended to simulate winter conditions - 5°C corresponds to the water temperature at the bottom of the ponds during winter in a temperate climate. The number of hatchings decreased with increasing NaCl concentration for T. platyurus, and decreased rapidly after a crucial value of 7 600 mg NaCl/L for D. magna. At the second hatching (eggs washed after first incubation and put into standard freshwater), D. magna demonstrated a low hatching rate regardless of NaCl concentration, while most T. platyurus hatchings were observed at the highest NaCl concentration; hence, the highest (15 200 mg/L) and lowest (1 900 mg/L) NaCl concentrations resulted in similar total hatching for the two species. The incubation time for three and seven days in NaCl at 5°C had no significant impact on hatching success; however, T28 resulted in significantly lower hatching success at all NaCl concentrations and controls. No significant differences in hatching success were noted if the eggs were first washed after incubation in NaCl and then put into the standard freshwater for hatching.