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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. Environ Toxicol Chem 2022;41:2095-2106. © 2022 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA., (© 2022 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)

The results of a series of experiments on the acute toxicity of major geochemical ions (Na + , K + , Ca 2+ , Mg 2+ , Cl - , SO 4 2- , HCO 3 - /CO 3 2- ) 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), NaHCO 3 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 SO 4 and to high pH/alkalinity in CO 3 /HCO 3 -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., (© 2022 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)

An emerging Multi-Ion Toxicity (MIT) model for assessment of environmental salt pollution is based on the premise that major ion toxicity to aquatic organisms is related to a critical disturbance of the trans-epithelial potential across the gills (ΔTEP), which can be predicted by electrochemical theory. However, the model has never been evaluated physiologically. We directly tested key assumptions by examining the individual effects of eight different salts (NaCl, Na 2 SO 4 , MgCl 2 , MgSO 4 , KCl, K 2 SO 4 , CaCl 2 , and CaSO 4 ) on measured TEP in three different fish species (fathead minnow, Pimephales promelas = FHM; channel catfish, Ictalurus punctatus = CC; bluegill, Lepomis macrochirus = BG). A geometric concentration series based on previously reported 96-h LC50 values for FHM was used. All salts caused concentration-dependent increases in TEP to less negative/more positive values in a pattern well-described by the Michaelis-Menten equation. The ΔTEP responses for different salts were similar to one another within each species when concentrations were expressed as a percentage of the FHM LC50. A plateau was reached at or before 100 % of the LC50 where the ΔTEP values were remarkably consistent, with only 1.4 to 2.2-fold variation. This relative uniformity in the ΔTEP responses contrasts with 28-fold variation in salt concentration (in mmol L -1 ), 9.6-fold in total dissolved solids, and 7.9-fold in conductivity at the LC50. The Michaelis-Menten Km values (salt concentrations causing 50 % of the ΔTEP max ) were positively related to the 96-h LC50 values. ΔTEP responses were not a direct effect of osmolarity in all species and were related to specific cation rather than specific anion concentrations in FHM. These responses were stable for up to 24 h in CC. The results provide strong physiological support for the assumptions of the MIT model, are coherent with electrochemical theory, and point to areas for future research., (Copyright © 2020 Elsevier B.V. All rights reserved.)

The survival and growth of juvenile fathead minnows were investigated at various combinations of waterborne exposure to arsenate and of dietborne exposure to oligochaete worms which had been exposed to inorganic arsenic. Previous work with rainbow trout established that dietborne arsenic can reduce fish growth at environmentally relevant concentrations and could be more important than waterborne exposures. This was found to be less true for fathead minnows, which were less sensitive to dietborne exposures than rainbow trout, while being as or more sensitive to waterborne exposures. When assessed on the basis of accumulation of total As by the fish, further differences between fathead minnows and rainbow trout were evident. Fathead minnows accumulated relatively more arsenic from water versus diet than trout, and the accumulations at which growth effects occurred in minnows were different for dietborne and waterborne exposure, whereas they were the same for trout. These results suggest complex relationships involving arsenic speciation, toxicokinetics, and toxicodynamics, and underscore a need for care in relating effects information to real-world exposures. The present study also demonstrated the challenges in testing and interpreting growth effects in long-term exposures to fish, because the expression of toxicity can be confounded by the relationship of fish growth to size, the feeding regime, and wet weight versus dry weight relationships., (Published by Elsevier B.V.)

The acute and chronic toxicity of sulfate (tested as sodium sulfate) was determined in diluted well water (hardness of 100 mg/L and pH 8.2) with a cladoceran (Ceriodaphnia dubia; 2-d and 7-d exposures), a midge (Chironomus dilutus; 4-d and 41-d exposures), a unionid mussel (pink mucket, Lampsilis abrupta; 4-d and 28-d exposures), and a fish (fathead minnow, Pimephales promelas; 4-d and 34-d exposures). Among the 4 species, the cladoceran and mussel were acutely more sensitive to sulfate than the midge and fathead minnow, whereas the fathead minnow was chronically more sensitive than the other 3 species. Acute-to-chronic ratios ranged from 2.34 to 5.68 for the 3 invertebrates but were as high as 12.69 for the fish. The fathead minnow was highly sensitive to sulfate during the transitional period from embryo development to hatching in the diluted well water, and thus, additional short-term (7- to 14-d) sulfate toxicity tests were conducted starting with embryonic fathead minnow in test waters with different ionic compositions at a water hardness of 100 mg/L. Increasing chloride in test water from 10 mg Cl/L to 25 mg Cl/L did not influence sulfate toxicity to the fish, whereas increasing potassium in test water from 1 mg K/L to 3 mg K/L substantially reduced the toxicity of sulfate. The results indicate that both acute and chronic sulfate toxicity data, and the influence of potassium on sulfate toxicity to fish embryos, need to be considered when environmental guidance values for sulfate are developed or refined., (Published 2015 SETAC. This article is a US Government work and is in the public domain in the United States.)

The oligochaete, Lumbriculus variegatus, has several characteristics that make it desirable as a prey organism for conducting dietary exposure studies with fish. We conducted 21- and 30-d experiments with young fathead minnows (Pimephales promelas) and rainbow trout (Oncorhynchus mykiss), respectively, to determine whether a diet consisting solely of L. variegatus would support normal growth and to compare performance with standard diets (Artemia nauplii, frozen brine shrimp, or trout chow). All diets were readily accepted, and fish survived and grew well. Food conversion in both fathead minnows and rainbow trout was as high as or higher for the oligochaete diet compared with others, although this comparison is influenced by differences in ration, ingestion rate, or both. The oligochaete diet had gross nutritional analysis similar to the other diets, and meets fish nutrition guidelines for protein and essential amino acids. Methodologies and practical considerations for successfully using oligochaetes as an experimental diet are discussed. Considering their ready acceptance by fish, their apparent nutritional sufficiency, the ease of culturing large numbers, and the ease with which they can be loaded with exogenous chemicals, we believe that L. variegatus represents an excellent choice of exposure vector for exposing fish to toxicants via the diet.