pH, Hardness, and Humic Acid Influence Aluminum Toxicity to Rainbow Trout (Oncorhynchus mykiss) in Weakly Alkaline Waters

Abstract

Juvenile rainbow trout (Oncorhynchus mykiss) were exposed for either 96 h (acute) or 16 d (subacute) to various combinations of aluminum and hardness concentrations or aluminum and humic acid concentrations between pH 7.14 and 8.58. Aluminum-induced mortality was higher at weakly alkaline pH (7.95–8.58) (attributed to up to 10-fold higher filterable aluminum concentrations) than near-neutral pH (7.14–7.64). Growth rates were higher in 16-d hardness tests at weakly alkaline pH (0.188–1.600% of initial weight∙d⁻¹) than for fish exposed to similar aluminum concentrations at near-neutral pH (−0.535–0.756%∙d⁻¹). This suggested that polymeric and colloidal forms of aluminum were more potent than soluble forms in restricting growth. Hardness and humic acid appeared most protective to trout against subacute aluminum toxicity. Cumulative mortality of trout exposed to 1.50 mg aluminum∙L⁻¹at 103 or 20 mg∙L⁻¹hardness as CaCO₃was 10 and 45%, respectively. Hardness did not significantly protect against aluminum-induced growth inhibition. Trout exposed for 16 d to aluminum (0.53–2.56 mg∙L⁻¹) and humic acid 4.31–5.23 mg∙L⁻¹) had higher specific growth rates and decreased mortality than those exposed to aluminum and ho humic acid at any pH.