Lake water chemistry and local adaptation shape NaCl toxicity in Daphnia ambigua

The increasing application of road deicing agents (e.g., NaCl) has caused widespread salinization of freshwater environments. Chronic exposure to toxic NaCl levels can impact freshwater biota at genome to ecosystem scales, yet the degree of harm caused by road salt pollution is likely to vary among habitats and populations. The background water chemistry may strongly impact NaCl toxicity, with greater harm occurring in ion-poor freshwater environments. In addition, populations exposed to salinization may evolve increased NaCl tolerance. We examined the potential for these two factors, genetics and environmental context, to interact in shaping NaCl toxicity in natural populations of the water flea Daphnia ambigua. We performed a reciprocal transplant toxicity trial using Daphnia clones from three lakes varying in ion availability. NaCl toxicity greatly increased in calcium-poor (1.7 mg/L) lake water compared with a high-calcium (7.2 mg/L) environment, especially for Daphnia from the high-calcium lake. Meanwhile, Daphnia from an ion-rich lake showed evidence of adaptation to NaCl exposure, especially in their natal lake water. Our findings that the lake water environment, adaptation to that environment, and adaptation to a contaminant of interest may interact to shape toxicity demonstrate the importance of considering environmental and genetic complexity in mitigating pollution impacts.