Bioaccumulation of cadmium in an experimental aquatic food chain involving phytoplankton (Chlorella vulgaris), zooplankton (Moina macrocopa), and the predatory catfish Clarias macrocephalus × C. gariepinus

The accumulation of cadmium (Cd) was studied in an experimental aquatic food chain involving the phytoplankton Chlorella vulgaris as the primary producer, the zooplankton Moina macrocopa as the primary consumer, and the catfish Clarias macrocephalus × Clarias gariepinus as the secondary consumer. C. vulgaris was first exposed to Cd solutions at 0.00, 0.35, and 3.50 mg l−1, referred to as control group and experimental groups 1 and 2, respectively. Subsequently, each group was fed to three corresponding groups of M. macrocopa. Finally, three groups of catfish were fed these corresponding groups of M. macrocopa. After C. vulgaris was exposed to 3.50 mg l−1 Cd (experimental group 2), the residual Cd in solution was only 4.01 μg l−1, lower than the maximum allowable limit of Cd in natural water sources (5 μg l−1). Cd concentrations in C. vulgaris were 0.01 ± 0.00 μg g−1 (dry wt) in the control group, 194 ± 1.80 μg g−1 (dry wt) in experimental group 1, and 1140 ± 20.06 μg g−1 (dry wt) in experimental group 2. The Cd concentrations in M. macrocopa were 0.01 ± 0.00 μg g−1 (dry wt) in the control group, 16.48 ± 2.23 μg g−1 (dry wt) in experimental group 1, and 56.6 ± 3.23 μg g−1 (dry wt) in experimental group 2. The Cd concentrations in catfish muscle increased with increasing Cd concentrations in the food. After 60 days of fish culture, the mean concentrations of Cd in fish muscle were 0.01 ± 0.00 μg g−1 (dry wt) in the control group, 0.61 ± 0.02 μg g−1 (dry wt) in experimental group 1 and 1.04 ± 0.06 μg g−1 (dry wt) in experimental group 2. Cd concentration in fish muscle of experimental group 2 was equal to the permissible limit. Cd accumulation affected fish growth: at the end of the study, the mean fresh weight (12.81 g) of catfish in the control group, was significantly higher than those experimental group 1 (10.43 g) and experimental group 2 (10.00 g). The results showed that the measurement of Cd concentration in water does not necessarily give a measure of the safety of aquatic organisms as human food. Hence, heavy metal contamination is a matter for concern when organisms are harvested, for fish and human consumption, from natural water sources.