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Influence of dilution water composition on acute major ion toxicity to the mayfly Neocloeon triangulifer.
Citation:
Soucek, D., A. Dickinson, Dave Mount, R. Erickson, AND R. Hockett. Influence of dilution water composition on acute major ion toxicity to the mayfly Neocloeon triangulifer. SETAC North America, Salt Lake City, UT, November 01 - 05, 2015.
Impact/Purpose:
not applicable
Description:
A number of field studies have shown that mayflies (Ephemeroptera) tend to be more sensitive than other benthic macroinvertebrates to elevated levels of total dissolved solids in streams. Until relatively recently, difficulties with culturing have limited the use of mayflies as laboratory toxicity testing organisms, but recent efforts to develop cultured diets and chronic toxicity testing methods for the mayfly (Neocloeon triangulifer) have proven successful and will help to move toward standardization of methods for this species. Work with other species has shown that major ion toxicity is dependent on the ionic composition of the water; however, little is known about how ionic composition influences responses of mayflies to elevated major ions. Because N. triangulifer appears to be more sensitive to some major ions than more commonly tested organisms, studies with this organism that could contribute to the improvement of ion toxicity prediction models are needed. We conducted acute toxicity tests with N. triangulifer designed to determine how changes in ionic composition impact responses to elevated major ions. First we tested toxicity of salts in dilution waters that had varying hardness levels, but that also had increased concentrations of other ions, e.g. bicarbonate, as would be expected for natural waters. In these tests, the toxicity of NaCl, Na2SO4, and MgSO4 decreased with increasing hardness. However, when we held hardness constant but manipulated Ca:Mg ratios in dilution waters, Ca alone could not explain all of the response observed in the initial hardness manipulation tests with NaCl and Na2SO4. This may suggest that other ions besides Ca, for example bicarbonate, influence toxicity of sodium salts. We also tested the influence of sodium on potassium salt toxicity and vice versa, observing that these cations do not modify the toxicity of each other. Comparing toxicity expressed as total concentration or as chemical activity showed that in solutions dominated by sulfate salts, a narrow range of sulfate activities resulted in a toxic effect, but other ions in solution have a strong effect on sulfate activity. In addition, tests with D-mannitol in which mayflies were exposed to an osmolarity approximately double that of the highest observed LC50 in terms of osmolarity in the salt tests, resulted in no effect on survival. This suggests that specific ions, rather than overall osmolarity, were responsible for observed toxicity.