Acute Toxicity of Major Geochemical Ions to Fathead Minnows (Pimephales promelas): Part B—Modeling Ion Toxicity

Abstract

Mathematical models are presented for the acute median lethal concentrations of major geochemical ions (Na + , K + , Ca 2+ , Mg 2+ , Cl − , SO 4 2− , HCO 3 − /CO 3 2− ) to fathead minnows (Pimephales promelas), based on an extensive series of experiments presented in a companion article. Toxicity relationships across different dilution waters, individual salts, and salt mixtures suggest six independent mechanisms of toxicity to consider in modeling efforts, including Mg/Ca-specific toxicity, osmolarity-related toxicity, SO 4 -specific toxicity, K-specific toxicity, effects of high pH/alkalinity, and a multiple ion-related toxicity at low Ca distinct from the other mechanisms. Models are evaluated using chemical activity-based exposure metrics pertinent to each mechanism, but concentration-based alternative models that are simpler to apply are also addressed. These models are compared to those previously provided for Ceriodaphnia dubia, and various issues regarding their application to risk assessments are discussed.

Acute Toxicity of Major Geochemical Ions to Fathead Minnows (Pimephales Promelas): Part A—Observed Relationships for Individual Salts and Salt Mixtures

“…Testing the toxicity of mannitol to fathead minnow in dilution waters with different [Ca] was important to the evaluation of the relationship of salt toxicity to osmolarity and to model development in the companion article (Erickson et al, 2022). For C. dubia the effects of Ca on mannitol were not tested by Mount et al (2016), so an ancillary experiment was conducted to fill this data gap and is described in Supporting Information.…”

“…Equation 1 with {Mg} as the exposure metric is thus useful for mixtures containing up to at least half MgCO 3 , but the metric for MgCO 3 -dominated exposures is uncertain. Addressing toxicity in high carbonate exposures is further discussed in the companion modeling article (Erickson et al, 2022).…”

“…As for C. dubia (Erickson et al, 2017(Erickson et al, , 2018, this additivity indicates a common mechanism with {Mg} and {Ca} as the exposure metrics. A prediction for this isobole is not provided in the present study because Equation 1 does not address the complete Ca range; rather, this relationship is further addressed in the companion modeling article (Erickson et al, 2022).…”

“…The Ca-dependent toxicity at lower Ca is not addressed by the predictions in Figure 6 because it involves only two data points and these have different mechanisms of toxicity. More complete consideration of this part of the relationship is provided in the companion modeling article (Erickson et al, 2022).…”

“…Based on the information in the present study, a suitable mechanistic metric based on the activities of specific ions is not clear. The implication of this to the development of models for ion toxicity is further addressed in the companion modeling article (Erickson et al, 2022).…”