Fish bioturbation of cadmium‐contaminated sediments: Factors affecting Cd availability to Daphnia magna

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Abstract
Benthic fish bioturbation of contaminated sediments is thought to enhance exposure and, potentially, bioaccumulation into planktonic organisms. Exposures were conducted with cadmium‐spiked sediment, 1.0 mg/kg nominal concentrations, and koi carp (Cyprinus carpio). Daphnia magna were placed in aquaria with and without fish for 6 d and Cd bioaccumulation was measured every 48 h. Koi carp bioturbation increased mean total suspended solids (TSS) in two trials from 0.001 mg/L to 44.4 mg/L and 19.2 mg/L to 762.4 mg/L. Mean aqueous Cd concentrations increased from 1.4 μg/L to 2.8 μg/L, and from 1.6 μg/L to 13.2 μg/L. Cadmium binding capacity increased from 28.9 μg/L to 169.8 μg/L in with‐fish treatments when compared to controls. However, Daphnia magna body burdens did not increase. Mean Cd residues of daphnids exposed with fish, 9.2 μg/g, were not statistically different from without‐fish exposures, 8.0 μg/g. Body burdens slightly decreased in the first trial after the with‐fish treatment, 9.4 μg/g to 8.3 μg/g. Fish size was partially correlated with TSS and aqueous Cd concentrations and TSS positively correlated with binding capacity. Because increased TSS in the with‐fish treatment resulted in increased binding capacity, it is probable that cadmium bioavailability decreased. Although koi carp were capable of remobilizing Cd from sediment, Cd bioaccumulation into Daphnia magna was not significant.