Uranium toxicity and speciation during chronic exposure to the tropical freshwater fish, Mogurnda mogurnda

The effects of chronic uranium (U) exposure on larval Northern trout gudgeon, Mogurnda mogurnda, were assessed in two experiments using a newly-developed 28 d survival and growth toxicity test. Significant effects were observed in both tests, but toxicity was markedly higher in Test 2 than Test 1. The LC50s for Tests 1 and 2 were 2090 μg L−1 and 1070 μg L−1, respectively. Larval growth IC10s for Tests 1 and 2 were 860 μg L−1 and 660 μg L−1 (dry weight), and 1160 μg L−1 and 850 μg L−1 (length), respectively. Uranium speciation modelling showed that a lower pH in Test 2 (mean of 6.0) compared to Test 1 (mean of 6.7) resulted in a greater proportion of free uranyl ion (UO22+), the predominant bioavailable form of U. A higher dissolved organic carbon concentration (DOC) in Test 2 (4.2 mg L−1) compared to Test 1 (2.1 mg L−1) resulted in a higher proportion of U-DOC in Test 2, but this was insufficient to counter the effect of pH on the proportion of UO22+. The difference in U toxicity between the two tests could be explained by normalising for UO22+; the concentrations of UO22+ at the LC50s for Tests 1 and 2 were calculated to be 13.3 and 13.7 μg L−1, respectively. Finally, the results of this study, and comparisons with other studies suggest that U toxicity to M. mogurnda appears to be as much, if not more, a function of exposure water quality and feeding regime, as exposure duration.