The effect of sulfate on the bioconcentration of selenate by Chironomus decorus and Daphnia magna
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Agricultural drainage containing high concentrations of selenium (Se) poses a continuing threat to wildlife in California's San Joaquin Valley. Drainage water from this area frequently contains high concentrations of sulfate, which are known to have mediating effects on the bioaccumulation and toxicity of Se in some organisms. It has been proposed that sulfate concentration should be a consideration in determining water quality criteria for Se. As a step toward analyzing the viability of such a plan, this study evaluated the effect of varying sulfate concentration on Se bioconcentration by two aquatic invertebrates. Fourth instar Chironomus decorus and neonate Daphnia magna were exposed, for a 48 h period, to 5.92 and 0.71 mg Se/L, as selenate, respectively. The selenium:sulfur (Se:S) ratio in the dilution waters ranged from 1:0 to 1:480 for C. decorus and 1:3 to 1:240 for D. magna. Increasing sulfate concentrations significantly reduced the accumulation of Se by both organisms. However, D. magna and C. decorus bioconcentrate Se differently at low sulfate concentrations. This difference can be explained by a two permease model for selenate/sulfate absorption. Although this experiment showed that sulfate may reduce selenate bioavailability to aquatic invertebrates, there is no indication that sulfate may completely eliminate selenate absorption. Thus, further research should be performed before sulfate concentration becomes a factor in the determination of water quality standards for selenium.
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