Effects of water hardness on the toxicity of manganese to developing brown trout (Salmo trutta)

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Abstract
Manganese is a common constituent of point and nonpoint discharges from mining and smelting activities. Available data indicate that Mn is acutely toxic at relatively high aqueous concentrations, when compared with trace metals, and its toxicity is affected by water hardness. Little information is available regarding the chronic toxicity of manganese. Early‐life‐stage (ELS) tests were conducted to determine the toxicity of manganese to brown trout (Salmo trutta) and to evaluate the extent to which water hardness (ranging from 30 to 450 mg/L as CaCO3) affects the chronic toxicity of Mn. Water hardness significantly affected Mn chronic toxicity, with toxicity decreasing with increasing hardness. Decreased survival was the predominant effect noted in the 30‐mg/L hardness experiment, while significant effects on growth (as measured by changes in body weight) were observed in both the 150‐ and 450‐mg/L hardness experiments. Twenty‐five percent inhibition concentration (IC25) values, based on the combined endpoints (i.e., survival and body weight), were 4.67, 5.59, and 8.68 mg Mn/L (based on measured Mn concentrations) at hardness levels of approximately 30, 150, and 450 mg/L as CaCO3, respectively.