Physiological and Toxicological Effects of Pb Plus Cd Mixtures on Rainbow Trout (\u3cem\u3eOncorhynchus mykiss\u3c/em\u3e) in Soft Acidic Water

The ability of a metal to cause toxicity in aquatic environments is highly dependant upon the water chemistry, which can influence metal bioavailability. Metal bioavailability is affected by the concentration of the metal, complexation of the metal by organic and inorganic ligands, and by the speciation of the metal. Predictive models of metal toxicity, such as the biotic ligand model (BLM) have mainly focused upon predicting the toxicity of individual metals rather than the toxicity of metals in mixtures, which are more commonly found in contaminated waters. Two metals that are commonly found together are Pb and Cd which can enter aquatic environments through anthropogenic sources such as mining and smelting operations, or naturally through the weathering of rock. The purpose of this research was to determine how low levels of Pb and Cd (25-2400 nmolL-1 and 6-24 nmolL-1 respectively) interacted with one another and the gills of rainbow trout (Oncorhynchus mykiss). Specific goals were to determine how acute (3-10 days) exposure influenced metal-gill binding and gill function, blood electrolytes (Na+, Ca2+, Cl-), acid-base balance and survival in these fish. These interactions were studied using repetitive blood sampling, unidirectional Na+ flux techniques, and gill Na+/K+-ATPase activity. Studies using a toxic unit approach (ITU of exposure = LC50) were also used to determine if it was possible to predict the toxicity of Pb plus Cd mixtures. The cannulation studies suggested that Pb plus Cd mixtures resulted in greater than additive reductions in plasma Ca and Na concentrations, but the decreases in plasma Na+ were corrected within 5 d. Measurements of unidirectional Na+ fluxes using the radiotracer 24Na+ revealed that the decreases in Na+ uptake caused by Pb or Cd alone at low levels were temporary and not observed during exposure to Pb plus Cd mixtures. This seemingly protective effect of Pb against Na loss was explained by competition between Pb and Cd for binding sites at the gill and accompanying increases in Na+/K+-ATPase activity. To conclude, low levels of Pb plus Cd can cause greater than additive reductions to internal Ca2+ balance, but trout are able to acclimate to these low concentrations. At higher concentrations, Pb appears to be protective against Cd toxicity. These findings underscore the need to better understand the mechanisms of Pb and Cd uptake by the gill alone and in mixtures, and when establishing water quality guidelines for these metals and for site specific risk assessments