THE EFFECT OF SULFATE ON THE BIOCONCENTRATION OF SELENATE BY CHIRONOMUS-DECORUS AND DAPHNIA-MAGNA

Agricultural drainage containing high concentrations of selenium (Se) poses a continuing threat to wildlife in California's San Joaquin Vall ey. Drainage water from this area frequently contains high concentrati ons of sulfate, which are known to have mediating effects on the bioac cumulation and toxicity of Se in some organisms. It has been proposed that sulfate concentration should be a consideration in determining wa ter quality criteria for Se. As a step toward analyzing the viability of such a plan, this study evaluated the effect of varying sulfate con centration 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. T he 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 su lfate concentrations significantly reduced the accumulation of Se by b oth organisms. However, D. magna and C. decorus bioconcentrate Se diff erently at low sulfate concentrations. This difference can be explaine d by a two permease model for selenate/sulfate absorption. Although th is experiment showed that sulfate may reduce selenate bioavailability to aquatic invertebrates, there is no indication that sulfate may comp letely eliminate selenate absorption. Thus, further research should be performed before sulfate concentration becomes a factor in the determ ination of water quality standards for selenium.