The ions Na , K , Ca , Mg , Cl , SO , and HCO /CO (referred to in the present study as "major ions") are present in all freshwaters and physiologically required by aquatic organisms but can increase to harmful levels from a variety of anthropogenic activities. It is also known that the toxicities of major ion salts can vary depending on the concentrations of other ions, and understanding these relationships is key to establishing appropriate environmental limits. The authors present a series of experiments with Ceriodaphnia dubia to evaluate the acute toxicity of 12 major ion salts and to determine how toxicity of these salts varies as a function of background water chemistry. All salts except CaSO and CaCO were acutely toxic below saturation, with the lowest median lethal concentrations found for K salts. All 10 salts that showed toxicity also showed some degree of reduced toxicity as the ionic content of the background water increased. Experiments that independently varied Ca:Mg ratio, Na:K ratio, Cl:SO ratio, and alkalinity/pH demonstrated that Ca concentration was the primary factor influencing the toxicities of Na and Mg salts, whereas the toxicities of K salts were primarily influenced by the concentration of Na. These experiments also indicated multiple mechanisms of toxicity and suggested important aspects of dosimetry; the toxicities of K, Mg, and Ca salts were best related to the chemical activity of the cation, whereas the toxicities of Na salts also reflected an influence of the anions and were well correlated with osmolarity. Understanding these relationships between major ion toxicity and background water chemistry should aid in the development of sensible risk-assessments and regulatory standards. Environ Toxicol Chem 2016;35:3039-3057. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Many human activities increase concentrations of major geochemical ions (Na , K , Ca , Mg , Cl , SO , and HCO /CO ) in freshwater systems, and can thereby adversely affect aquatic life. Such effects involve several toxicants, multiple toxicity mechanisms, various ion interactions, and widely varying ion compositions across different water bodies. Previous studies of individual salt toxicities have defined some useful relationships; however, adding single salts to waters results in atypical compositions and does not fully address mixture toxicity. To better understand mechanisms and interactions for major ion toxicity, 29 binary mixture experiments, each consisting of 7 to 8 toxicity tests, were conducted on the acute toxicity of major ion salts and mannitol to Ceriodaphnia dubia. These tests showed multiple mechanisms of toxicity, including: 1) nonspecific ion toxicity, correlated with osmolarity and to which all ions contribute; and 2) cation-dependent toxicities for potassium (K), magnesium (Mg), and calcium (Ca) best related to their chemical activities. These mechanisms primarily operate independently, except for additive toxicity of Mg-dependent and Ca-dependent toxicities. These mixture studies confirmed ameliorative effects of Ca on sodium (Na) and Mg salt toxicities and of Na on K salt toxicity, and further indicated lesser ameliorative effects of Ca on K salt toxicity and Mg on Na salt toxicity. These results provide a stronger basis for assessing risks from the complex mixtures of ions found in surface waters. Environ Toxicol Chem 2017;36:1525-1537. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

The effects of diet-borne copper, cadmium, lead, and arsenic on juvenile fish were evaluated using a live diet consisting of the oligochaete Lumbriculus variegatus . In 30 d exposures, no effects were observed on the growth and survival of rainbow trout ( Oncorhynchus mykiss ), fathead minnow ( Pimephales promelas ), and channel catfish ( Ictalurus punctatus ) fed diets contaminated with copper [130–310 µg Cu·(g dm)−1], cadmium [90–540 µg Cd·(g dm)−1], and lead [850–1000 µg Pb·(g dm)−1]. However, rainbow trout growth was reduced in a dose-dependent manner for diets contaminated with arsenic [26–77 µg As·(g dm)−1]. These effects of arsenic on fish growth were accompanied by slower feeding rate, reduced food conversion efficiency, liver cell abnormalities, and fecal matter changes suggestive of digestive effects, and occurred to a similar extent whether the diet was exposed to arsenate or arsenite. Effects from these dietary levels of arsenic, and the absence of effects from these dietary levels of metals, were generally consistent with literature reports using laboratory diets amended with toxicant salts. These results also indicated that reported growth effects on rainbow trout fed diets of invertebrates collected from mining-contaminated areas of the Clark Fork River (Montana, USA) or exposed in the laboratory to Clark Fork River sediments are likely more attributable to the arsenic than the metals in those diets.

Based on previous research on the acute toxicity of major ions (Na þ , K þ , Ca 2þ , Mg 2þ , Cl -, SO 4 2-, and HCO 3 -/CO 3 2-) to Ceriodaphnia dubia, a mathematical model was developed for predicting the median lethal concentration (LC50) for any ion mixture, excepting those dominated by K-specific toxicity. One component of the model describes a mechanism of general ion toxicity to which all ions contribute and predicts LC50s as a function of osmolarity and Ca activity. The other component describes Mg/Ca-specific toxicity to apply when such toxicity exceeds the general ion toxicity and predicts LC50s as a function of Mg and Ca activities. This model not only tracks well the observed LC50s from past research used for model development but also successfully predicts LC50s from new toxicity tests on synthetic mixtures of ions emulating chemistries of various ion-enriched effluents and receiving waters. It also performs better than a previously published model for major ion toxicity. Because of the complexities of estimating chemical activities and osmolarity, a simplified model based directly on ion concentrations was also developed and found to provide useful predictions.

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