Food quantity affects the sensitivity of Daphnia to road salt

Road deicing operations have raised chloride (Cl) levels in many temperate lakes in Europe and North America. These lakes vary widely in trophic status, but to date, no one has quantified the interaction between food quantity and road salt toxicity. We examined the effects of food quantity (particulate algal C concentration (C)) on the chronic toxicity of Cl to Daphnia in soft-water bioassays. There was a strong positive linear relationship (r(2) = 0.92 for NaCl and r(2) = 0.96 for CaCl2) between food quantity and Cl LC50. As food quantity increased from 0.2 to 1.0 mg C/L (levels characteristic of oligotrophic to eutrophic lakes, respectively), the chronic Cl LC50 increased from 55.7 to 284.8 mg Cl/L. Salt type (NaCl or CaCl2) did not affect the Cl LC50, Daphnia life history parameters, or the intrinsic rate of population increase (r). The life history parameter most sensitive to Cl was neonate production. Cl did not inhibit egg production, nor was the maternal lipid investment in eggs changed, but egg viability and the subsequent release of live neonates decreased as Cl levels increased and food decreased. Our results suggest the trophic status of lakes should be considered when assessing ecological threat from Cl.