Influence of some abiotic environmental factors on acute toxicity of inorganic lead to Cyprinus carpio var communis (Linn.) and Catla catla (Ham.) in simulated toxic aquatic environment

With the increase of water hardness from 60 to 720 mg/L CaCO₃, total alkalinity from 32 to 376 mg/L CaCO_3, pH from 7.6 to 7.9 and chloride from 28 to 350 mg/L, 96 h LC₅₀ on the basis of total lead increased from 8.2 to 1291 mg/L for Cyprinus carpio and 5.3 to 865 mg/L for Catla catla, when soil sediments were included these values were further raised to 1356 and 874 mg/L, respectively. The dissolved lead LC₅₀ values in all the treatments of soil and water was consistent with fixed amount of dissolved lead (1.04-1.78 mg/L) being needed for median lethal toxicity. Total lead toxicity also decreased with increase in pH from 6.3 to 11.3. 96 h LC₅₀ values increased for common carp 15 to 631 mg/L and for catla, 8 to 355 mg/L. But dissolved lead toxicity was found to increase with the increase of pH from 6.3 to 11.3 for both common carp (LC₅₀, 3.53 to 0.24 mg/L) and catla (LC₅₀, 2.21 to 0.09 mg/L). Removal of dissolved Pb with increasing carbonate content, particulate matter and pH due to adsorption, precipitation or coprecipitation reaction, reduced the dissolved lead concentration and thus the total lead toxicity. Increase in toxicity of lead with increase of exposure time was the biological response of longer contact time and decrease in dissolved lead toxicity with decrease in pH was due to increase H⁺ ion competition.