Deposition, acute toxicity, and bioaccumulation of nickel in some freshwater organisms with best-fit functions modeling

Although nickel (Ni) frequently enters into water, documents on speciation of the metal and its toxicity to freshwater organisms are scanty. Laboratory experiments, made in this study, with 1.0–5.0 mg/L of Ni revealed that Ni was not quickly removed from water. Application of cubic regression followed by classical optimization technique showed that maximum reduction time (T) of Ni in water ranged between 60 to 65 h. Ninety-six hours of LC₅₀ value of Ni to crustacean Diaptomus forbesi, fish Cyprinus carpio, and worm Branchiura sowerbyi was respectively 5.43, 14.70, and 19.73 mg/L. Normalizing the lethal values and plotting them against time, it was observed that C. carpio was more sensitive than D. forbesi, which was not reflected in the 96-h LC₅₀ values. However, sensitivity of these organisms to Ni was better explained by power regression equation (M = aNi^b), which exhibited that mortality (M) of D. forbesi and B. sowerbyi increased slowly between 24 to 72 h, increasing sharply at 96 h, while mortality of C. carpio increased steadily every 24 till 96 h. Experiment with 1.0 mg/L of Ni in outdoor vats showed that crustacean zooplankton and worms accumulated high concentration of Ni, while only gut and liver tissues of fish C. carpio accumulated trace and moderately high concentration of Ni, respectively. It is assumed that toxicity of Ni to fish is mediated primarily through gill.