Amelioration of the toxicity of H⁺ to larval stoneflies by metals found in coal mine effluent

Stonefly nymphs (Acroneuria carolinensis) were used as a bioindicator of the toxicity of coal mine drainage, and of the interaction between low pH and dissolved metals at severe pollutant levels. Laboratory experiments were done to determine the mean life expectancy of stoneflies subjected to simulated mine drainage with various levels of metals and pH. Both iron and aluminum had significant protective effects against low pH toxicity (at levels found in mine drainage streams of pH 2.8-3.3). The ameliorative effect of Al and Fe was additive. At pH 2.8, the protective effect of aluminum increased with concentration, approaching a maximum near 12 mM. Increased water hardness due to Ca and Mg had a lesser protective effect. A laboratory simulation of a field site at pH 2.8 (modelling field levels of Na, K, Mg, Ca, Al, and Fe) was more toxic than the field site it was patterned after, indicating that there is an ameliorating factor that is not among the ions considered. The other metals tested (Mn, Zn, Cu, and Cd at concentrations prevalent in central Pennsylvania mine effluent) did not reduce the toxicity of H+ ions at pH 3.3. Stonefly nymphs placed in an acid mine drainage stream with a pH of 4.0, lost approximately 50% of their body sodium. However, when these field water chemistry conditions were mimicked in the laboratory there was no significant body Na loss. This indicates that some factor(s) other than the cations considered is affecting toxicity, and that field conditions were more toxic than the laboratory simulation at pH 4.0. Larval stoneflies are more tolerant of acidic mine effluent than are trout. Hence, they can be a useful bioindicator of the toxicity of severe acid and metal mixtures, especially when enough detail to understand the mechanisms is sought.