Rapid changes in water hardness and alkalinity: Calcite formation is lethal to Daphnia magna.

Abstract There is growing concern that freshwater ecosystems may be negatively affected by ever-increasing anthropogenic inputs of extremely hard, highly alkaline effluent containing large quantities of Ca 2 + , Mg 2 + , CO 3 2 − , and HCO 3 − ions. In this study, the toxicity of rapid and extreme shifts in water hardness (38–600 mg/L as CaCO 3 ) and alkalinity (30–420 mg/L as CaCO 3 ) to Daphnia magna was tested, both independently and in combination. Within 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 CaCO 3 water hardness and 420 mg/L as CaCO 3 alkalinity (LT50 of 60 h with a 95% CI of 54.2–66.0 h). In this treatment, a CaCO 3 (calcite) precipitate formed in the water column which was ingested by and thoroughly coated the D. magna . Calcite collected from a mining impacted stream contained embedded organisms, suggesting field streams may also experience similar conditions and possibly increased mortality as observed in the lab tests. Although further investigation is required to determine the exact fate of aquatic organisms exposed to rapid calcite precipitation in the field, we caution that negative effects may occur more quickly or at lower concentrations of water hardness and alkalinity in which we observed effects in D. magna , because some species, such as aquatic insects, are more sensitive than cladocerans to changes in ionic strength. Our results provide evidence that both calcite precipitation and the major ion balance of waters should be managed in industrially affected ecosystems and we support the development of a hardness + alkalinity guideline for the protection of aquatic life.