LAUSR

The results of a series of experiments on the acute toxicity of major geochemical ions (Na+, K+, Ca2+, Mg2+, Cl−, SO42−, HCO3−/CO32−) to fathead minnows (Pimephales promelas) are reported. Tests of individual major ion salts in various dilution waters demonstrated that the toxicities of Na, Mg, and K salts decrease as the overall ion content of the dilution water increases. For Na and Mg salts, this is attributable to Ca content as previously reported for Ceriodaphnia dubia. For K salts, the cause is unclear, but it is not due to Na as reported for C. dubia. In an unregulated test at high pH (9.3), NaHCO3 was also found to be twice as toxic compared to when the pH was reduced to 8.4. Experiments with binary salt mixtures indicated the existence of multiple independent mechanisms of action. These include K‐specific toxicity and Ca/Mg‐specific toxicity previously reported for C. dubia, but also apparent toxicities related to SO4 and to high pH/alkalinity in CO3/HCO3‐dominated exposures. Previous work with C. dubia also suggested a general ion toxicity involving all ions that was correlated with osmolarity. For fathead minnow, similar correlations were observed, but multiple mechanisms were indicated. At higher Ca, this general toxicity could be attributable to osmotic effects, but at lower Ca, osmolarity may be more a covariate than a cause, with this toxicity being related to a combined effect of ions other than via osmolarity. Environ Toxicol Chem 2022;41:2078–2094. © 2022 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.