Toxicity of manganese to Ceriodaphnia dubia and Hyalella azteca

Manganese is a toxic element frequently overlooked when assessing toxicity of effluents, sediments, and pore waters. Manganese can be present at toxic levels in anoxic solutions due to increased solubility under chemically re ducing conditions, and it can remain at those levels for days in aerated te st waters due to slow precipitation kinetics. Ceriodaphnia dubin and Hyalel la azteca are freshwater organisms often used for toxicity testing and reco mmended for assessments of effluents and pore waters. Lethal and reproducti ve-inhibition concentrations of Mn were determined for C. dubia in acute 48 -h tests and chronic three-brood tests using animals < 24 h old and between 24 and -48 h old. Sensitivity of H. azteca to Mn was determined with 7-day -old animals in acute 96-h tests. Tests were run at three levels of water h ardness to assess the amelioratory effect, which was often significant. Man ganese concentrations were measured analytically at test initiation and aft er 96 h for calculation of toxicity and determination of Mn precipitation d uring the tests. Minimal amounts of Mn (less than or equal to 3%) precipita ted within 96 h. LC(50)s determined for H. azteca progressively increased f rom 3.0 to 8.6 to 13.7 mg Mn/L in soft, moderately hard, and hard waters, r espectively. The tolerance of C. dubia to Mn was not significantly differen t between moderately hard and hard waters, but was significantly lower in s oft water. Manganese sensitivity of C. dubia was not significantly differen t between the ages tested. Acute LC50 values for C. dubia averaged 6.2, 14. 5 and 15.2 mg Mn/L and chronic IC50 values averaged 3.9, 8.5 and 11.5 mg Mn /L for soft, moderately-hard and hard waters. respectively. Manganese toxic ity should be considered when assessing solutions with concentrations appro aching these levels.