Effects of major ions on natural benthic communities: an experimental assessment of the US Environmental Protection Agency aquatic life benchmark for conductivity

Elevated concentrations of HCO3-, Cl-, SO42-, Mg2+, Na+, and Ca2+ in freshwater ecosystems are often associated with anthropogenic disturbances. The US Environmental Protection Agency developed a field-based specific conductance (SC) benchmark of 300 mu S/cm for streams affected by mountain-top mining operations. The benchmark has been criticized because of the potential influence of confounding variables and difficulty in demonstrating a causal relationship between elevated SC and macroinvertebrate responses. We conducted 4 stream mesocosm experiments to quantify the effects of major ions on aquatic insect assemblages. We exposed insects from streams with low (60-72 S/cm) and moderate (200-250 mu S/cm) SC to major ions at values bracketing 300 mu S/cm. We measured community metabolism, macroinvertebrate drift, community composition, and survival. Sixty-six taxa were exposed to NaHCO3, MgSO4, and NaCl in 4 mesocosm experiments, and 8 dominant families/subfamilies occurred in sufficient densities to develop SC-response relationships. Significant SC-response relationships occurred for each major ion tested. Drift increased and community metabolism decreased with increasing SC. Ephemeroptera were highly sensitive, whereas Trichoptera and Diptera were relatively tolerant. EC20 values (the SC that resulted in a 20% difference from controls) ranged from 151 to 3615 mu S/cm and were >300 mu S/cm for most endpoints. Mayfly drift, abundance of baetid and heptageniid mayflies, total mayfly abundance, and community metabolism were affected at SC levels near or <300 mu S/cm. EC20 values were lower for NaHO3 and MgSO4 than for NaCl, indicating greater toxicity of these 2 salts. Effects were greater on communities from the low- than the high-SC stream. Thus, accounting for context-dependent responses may be important when establishing contaminant benchmarks or thresholds. The 300-mu S/cm benchmark is protective of aquatic insect communities in naturally low-conductivity streams.