Effects of bicarbonate alkalinity and calcium on the acute toxicity of copper to juvenile channel catfish ( Ictalurus punctatus)

Three experiments were conducted to evaluate the relative importance of calcium hardness and bicarbonate alkalinity to the acute response of juvenile channel catfish ( Ictalurus punctatus) exposed to a toxic concentration of copper sulfate. A preliminary bioassay revealed 28 mg·l −1 copper sulfate caused 50% mortality within 48 h (48-hLC 50) in juvenile channel catfish placed in water with calcium hardness and bicarbonate alkalinity, set at 75 mg·l −1 CaCO 3. Catfish were then exposed to 28 mg·l −1 copper sulfate concentrations in environments where hardness or alkalinity concentrations were varied. Bicarbonate alkalinities above 75 mg·l −1 CaCO 3, with calcium hardness held at 20 mg·l −1 CaCO 3, significantly reduced catfish mortalities from 97–100% to 63–70%. Copper-induced mortalities were 100% for all fish placed in calcium hardness treatments (20–250 mg·l −1 CaCO 3) in which bicarbonate alkalinity was held at 20 mg·l −1 CaCO 3. When bicarbonate alkalinity was held constant at 75 mg·l −1 CaCO 3 and calcium hardness was varied from 20 to 250 mg·l −1 CaCO 3, copper related catfish mortalities displayed high variability and means ranged from 6.7 to 60%. Mortalities decreased as calcium concentrations increased. Although differences in mortalities were not statistically significant, the latter hardness findings appear to suggest a biologically significant calcium effect on copper toxicity in the presence of sufficient alkalinity concentrations.