THEODORE B. HENRY AND ELISE R. IRWIN, Alabama Cooperative Fish and Wildlife Research Unit, Auburn University, Alabama 36849, USA

Acid mixing zones occur at the confluence of waters of different pH and are often visible by plumes of metal precipitates in the water column and attached to the substratum. Metals (e.g., Al, Fe, Mn) soluble in the low pH stream become insoluble as rapid pH elevation occurs in the mixing zone. Mixing zone toxicity to juvenile bluegill Lepomis macrochirus was evaluated by measuring mortality rates in instream cages during six hour tests. Mortality rates in the mixing zone were much higher than those placed in the acid or neutral stream or downstream below the effects of mixing. A mixing channel was assembled in the field allowing the toxicity of mixed water to be compared after various mixing times (0-2.5 min). Water immediately after mixing was significantly more toxic than after 2.5 minutes when it reached caged fish at the terminus of the channel. The suspected mechanism of mortality, Al precipitation and polymerization on fish gills, was supported by water quality analysis and Al speciation. Reactive Al species were present at higher levels early in the mixing process and had precipitated into unreactive Al species before reaching bottom cages.