and alkalinity (30–420 mg/L as CaCO3) to Daphnia magna was tested, both independently and in combination.With in these ranges, where no precipitation event occurred, shifts in water hardness and/or alkalinity were not toxic to D. magna. In contrast, 98–100% of D. magna died within 96 h after exposure to 600 mg/L as CaCO3 water hardness and 420 mg/L as CaCO3 alkalinity (LT50 of 60 h with a 95% CI of 54.2–66.0 h). In this treatment,