Acute Toxicity of an Acid Mine Drainage Mixing Zone to Juvenile Bluegill and Largemouth Bass

—The toxicity of an acid mixing zone produced at the confluence of a stream that was contaminated by acid mine drainage (AMD) and a pH-neutral stream was investigated in toxicity tests with juvenile bluegill Lepomis macrochirus and largemouth bass Micropterus salmoides. Fish mortalities in instream cages located in the mixing zone, below the mixing zone, and upstream in both tributaries were compared to determine relative toxicity at each site. In all tests and for both species, significantly higher mortality was observed in the mixing zone than at any other location, including the acid stream, which had lower pH (2.9–4.3). The mixing zone was defined chemically by rapid precipitation of dissolved aluminum and iron, which arrived from the low-pH stream, and by the presence of white precipitates, which were attached to the substratum and which extended below the confluence. Possible seasonal changes in mixing zone toxicity were investigated by conducting field tests with bluegill in June, July, and August 1996 and in January 1997 and by conducting field tests with largemouth bass in April and May 1997. Toxicity was not significantly different at the extremes of temperature, pH, and metal concentration that occurred in June and July, as compared with January. Toxicity was significantly lower in August; however, elevated stream discharge during the August test may have disturbed mixing zone characteristics. High toxicity in AMD mixing zones may lower the survival of fishes in streams, reduce available habitat, and impede movements of migratory fish. Acid mine drainage (AMD) results from the oxidation of sulfide minerals that are exposed to the atmosphere through erosion of coal-mine spoils (Van Breeman 1973; Herrman and Baumgartner 1992). Receiving streams typically have low pH, * Corresponding author: [email protected] 1 Present address: Department of Fisheries and Allied Aquacultures, 203 Swingle Hall, Auburn University, Auburn, Alabama 36849, USA. 2 Cooperators are: U.S. Geological Survey, Biological Resources Division; Alabama Department of Conservation and Natural Resources, Game and Fish Division; and Alabama Agricultural Experiment Station, Auburn University. Received June 8, 1998; accepted December 17, 1998 high conductivity, and high metal and sulfate concentrations (Herlihy et al. 1990). In streams that are severely polluted by AMD (pH ,3.5), fish may be absent; in less severe conditions (pH 5 4.5– 6.0), low abundance and diversity may occur (Harris et al. 1983; Short et al. 1990; Rutherford and Mellow 1994; Saiki et al. 1995). Negative effects on stream biota can result from low pH, dissolved metals (aluminum [Al], iron [Fe], zinc [Zn], manganese [Mn], copper [Cu], or cadmium [Cd]), or from sedimentation that is increased by mining activity (for review see Sorensen 1991; Grippo and Dunson 1996). In addition, AMD-contaminated streams affect neutral streams through the production of acid mixing zones at their confluence (McKnight and Feder 1984).