Early research with typical laboratory-cultured aquatic species showed that major ion toxicity is dependent on the ionic composition of the water, and chloride and sulfate water quality standards reflecting the influence of background water chemistry were developed and adopted by several States. However, as more field-based studies were published on the influence of elevated major ions on benthic macroinvertebrate communities, particularly in Central Appalachian low-order streams, a distinct gap was observed between field based effect levels, and ion concentrations considered safe according to the laboratory-developed standards. Field studies observed that mayflies (Ephemeroptera) in particular tended to be more sensitive to elevated major ions than other benthic macroinvertebrates and most other species tested in the laboratory. Potential explanations for the perceived disparity between field and lab effect levels included: differences in specific ionic compositions, differences in specific responses of taxa, differences in duration or timing of exposure, or presence of correlated but confounding factors in field studies like subtle habitat differences or presence of other trace contaminants. With the goal of both confirming the environmentally relevant field responses observed by others and investigating potential causal mechanisms, our research group has conducted field community assessments and laboratory studies with relevant taxa.
Results/Conclusions
In our field studies, maintaining specific site selection criteria allowed us to approach elimination of confounding factors while verifying the observations of others that benthic macroinvertebrate community structure changes at relatively minor elevations of major ion concentrations. We verified the sensitivity of mayflies in particular, and identified seasonal patterns in both major ion concentrations and community responses. In laboratory studies, we used a parthenogenetic mayfly cultured in the laboratory to conduct full life-cycle toxicity tests with various salts. These tests not only confirmed the exceptional sensitivity of the mayfly, but also identified two particularly sensitive life-stages. Further testing showed wide variation in responses of this mayfly based not only on the salt tested, but also on the background chemistry of the dilution water. Distinct differences in responses to various cations and anions were observed when compared to other studies conducted with more commonly tested crustaceans. In combination, our laboratory and field studies confirm the exceptional sensitivity of mayflies to elevated major ions, and demonstrate the importance of careful study design for field studies and the use of appropriate ionic compositions and relevant laboratory species and life stages to represent the responses of taxa of interest in community studies.