Aluminum-dominated acute toxicity to the cladoceran Ceriodaphnia dubia in neutral waters downstream of an acid mine drainage discharge

Acid mine drainage (AMD) is traditionally considered to impact aquatic ecosystems by acidification, metal precipitation smothering stream substrates, and sediment toxicity in association with trace metals. Using Ceriodaphnia dubia as a test organism, we conducted whole effluent toxicity (WET) tests with both field-collected and laboratory-synthesized AMD samples to investigate possible mechanisms of reduced benthic macroinvertebrate community diversity in neutral (pH > 7.0) waters downstream of an acidified tributary. Our results indicate that Al and Fe in transition from acidic waters to neutralizing receiving streams can cause acute toxicity to standard invertebrate test organisms at neutral pH. Aluminum, with an average LC₅₀ (50% lethal concentration) of 2880 μg·L^-1, was determined to be the cause of acute toxicity in the field for up to a mile downstream of the AMD-influenced tributary and was the likely cause of reduced community diversity at those sites. Although Fe singly may cause acute toxicity in this type of system, it appears to reduce the toxicity of combinations of other metals such as Al, Cu, and Zn.