Manganese toxicity to tropical freshwater species in low hardness water

Andrew J Harford, Thomas J Mooney, Melanie A Trenfield, Rick A van Dam

Elevated Manganese (Mn) is a common contaminant issue for mine water discharges and previous studies have reported that its toxicity is ameliorated by H + , Ca2+ and Mg2+ ions. The toxicity of manganese (Mn) was assessed in a high risk scenario, i.e. the slightly acidic, soft waters of Magela Creek, Kakadu National Park, Northern Territory, Australia. Toxicity estimates were derived for six tropical freshwater species (Chlorella sp., Lemna aequinoctialis, Amerianna cumingi, Moinodaphnia macleayi, Hydra viridissima, and Mogurnda mogurnda). Low effect chronic inhibition concentration (IC10) and acute lethal concentration (LC05) values ranged between 140-80000 µg L-1, with three of the species tested (M. macleayi, A. cumingi and H. viridissima) being more sensitive to Mn than all but one species in the international literature (Hyalella azteca). A loss of Mn was observed on the final day for two of the H. viridissima toxicity tests, which may be a result of the complex speciation of Mn and biological oxidation. International data from toxicity tests conducted in natural water with a similar physico-chemistry to Magela Creek water, were combined with this current data to increase the sample size to produce a more reliable Species Sensitivity Distribution (SSD). A 99% protection Guideline Value (GV) of 73 (33- 466) µg L-1 was derived, the low value of this GV reflects the higher toxicity of Mn in slightly acidic soft waters. This article is protected by copyright. All rights reserved.