Dietary Uptake of Highly Hydrophobic Chemicals by Rainbow Trout (Oncorhynchus Mykiss).

Rainbow trout (Oncorhynchus mykiss) was exposed through the diet to a mixture of non-ionic organic chemicals for 28 d, followed by a depuration phase, in accordance with OECD method 305. The mixture included hexachlorobenzene (HCB), 2,2',5,5'-tetrachlorobiphenyl (PCB-52), 2,2',5,5'-hexachlorobiphenyl (PCB-153), decachlorobiphenyl (PCB-209), decabromodiphenyl ether (BDE209), decabromodiphenyl ethane (DBDPE), bis-(2-ethylhexyl)-3,4,5,6-tetrabromophthalate (TBPH), perchloro-p-terphenyl (p-TCP), perchloro-m-terphenyl (m-TCP), and perchloro-p-quaterphenyl (p-QTCP), the latter six of which are considered highly hydrophobic based on n-octanol/water partition coefficients (KOW) greater than 108. All chemicals had first-order uptake and elimination kinetics except p-QTCP, whose kinetics could not be verified due to limitations of analytical detection in the elimination phase. For HCB and PCBs, the growth-corrected elimination rates (k2g), assimilation efficiencies (α), and biomagnification factors (BMFL) corrected for lipid content compared well with literature values. For the highly hydrophobic chemicals, elimination rates were faster than the rates for HCB and PCBs, and α's and BMFLs were much lower than those of HCB and PCBs, i.e., ranging from 0.019 to 2.8%, and from 0.000051 to 0.023 (g-lipid/g-lipid), respectively. As a result, the highly hydrophobic organic chemicals were found be much less bioavailable and bioaccumulative than HCB and PCBs. Based on the current laboratory dietary exposures, none of the highly hydrophobic substances would be expected to biomagnify, but Trophic Magnification Factors (TMFs) > 1 have been reported from field studies for TBPH and DBDPE. Additional research is needed to understand and reconcile the apparent inconsistencies in these two lines of evidence for bioaccumulation assessment.

Review of DDT, DDE, DDD, DDMU and DDMS Toxicity Data for Organisms Used in Estuarine and Marine Sediment Toxicity Tests.

The open literature was searched for laboratory toxicity data for marine/estuarine organisms exposed to dichlorodiphenyltrichloroethane (DDT) and its degradation products of dichlorodiphenyldichloroethylene (DDE), dichlorodiphenyldichloroethane (DDD), dichlorodiphenylchloroethylene (DDMU), and dichlorodiphenylchloroethane (DDMS). The goal of the review was to determine water-column toxicity values that could be used for porewater-based assessment of sediment toxicity. Data for individual compounds (and isomers thereof) in this group were very limited; most available data were for mixtures of multiple compounds, some defined and others undefined. Further, the majority of relevant studies involved exposure to spiked or field-contaminated sediment (rather than waterborne exposure), which requires inferring concentration in porewater from bulk sediment. Comparing data on the basis of effect concentrations for water or inferred concentration in sediment pore water, the lower reported effect concentrations were in the range of 0.05 to 0.1 µg/L, generally in studies of longer duration and/or evaluating sub-lethal effects. Because field exposures are generally to mixtures of these compounds in varied proportions, additional data on chemical-specific toxicity would aid in pore-water based toxicity assessment for marine/estuarine sediments contaminated with DDT-related chemicals.

Stochastic Framework for Addressing Chemical Partitioning and Bioavailability in Contaminated Sediment Assessment and Management.

Passive sampling to quantify net partitioning of hydrophobic organic contaminants between the porewater and solid phase has advanced risk management for contaminated sediments. Direct porewater (Cfree) measures represent the best way to predict adverse effects to biota. However, when the need arises to convert between solid-phase concentration (Ctotal) and Cfree, a wide variation in observed sediment-porewater partition coefficients (KTOC) is observed due to intractable complexities in binding phases. We propose a stochastic framework in which a given Ctotal is mapped to an estimated range of Cfree through variability in passive sampling-derived KTOC relationships. This mapping can be used to pair estimated Cfree with biological effects data or inversely to translate a measured or assumed Cfree to an estimated Ctotal. We apply the framework to both an effects threshold for polycyclic aromatic hydrocarbon (PAH) toxicity and an aggregate adverse impact on an assemblage of species. The stochastic framework is based on a "bioavailability ratio" (BR), which reflects the extent to which potency-weighted, aggregate PAH partitioning to the solid-phase is greater than that predicted by default, KOW-based KTOC values. Along a continuum of Ctotal, we use the BR to derive an estimate for the probability that Cfree will exceed a threshold. By explicitly describing the variability of KTOC and BR, estimates of risk posed by sediment-associated contaminants can be more transparent and nuanced.

Bioaccumulation of Bis-(2-ethylhexyl)-3,4,5,6-tetrabromophthalate and Mono-(2-ethylhexyl)-3,4,5,6-tetrabromophthalate by Lumbriculus variegatus.

The brominated flame retardant bis(2-ethylhexyl)-3,4,5,6-tetrabromophthalate (TBPH) is used widely in consumer items including polyurethane foam used in furniture. Information on its bioaccumulation in aquatic species is limited. In the current study, sediment bioaccumulation tests with the oligochaete Lumbriculus variegatus were performed on a spiked natural sediment equilibrated for 14.5 months. Analysis showed the TBPH used to spike the sediment contained a small amount (0.046% by mass) of mono-(2-ethylhexyl)-3,4,5,6-tetrabromophthalate (TBMEHP), a potential biotransformation product of the parent chemical. Steady-state biota-sediment accumulation factors (BSAFs) of 0.254 and 1.50 (kg organic carbon/kg lipid) were derived for TBPH and TBMEHP, respectively. TBPH had biphasic elimination behavior where 94% of the body burden was depleted within the first 12 h of elimination (i.e., half-life of 1.2 h or less) and the remaining 6% eliminated very slowly thereafter (half-life of 15 days). There was little evidence for biotransformation of either chemical by L. variegatus. This investigation confirms the extremely hydrophobic behavior of TBPH and its impact on its bioavailability.

Bioaccumulation of Highly Hydrophobic Chemicals by Lumbriculus variegatus.

Bioaccumulation of highly hydrophobic chemicals (log KOW > 8) from contaminated sediments by Lumbriculus variegatus has been studied for relatively few chemicals, and the measured and model predicted biota-sediment accumulation factors (BSAFs) can differ by orders of magnitude. In the current study, sediment bioaccumulation tests with L. variegatus were performed on sediments dosed with chemicals having a wide range of predicted n-octanol/water partition coefficients (KOW; 106-1018), including some higher than most highly hydrophobic chemicals studied to date. The highly hydrophobic chemicals had biphasic elimination kinetics with compartments A and B having fast and slow elimination kinetics, respectively, and for compartment B, elimination followed first-order kinetics. For compartment A with fast elimination kinetics, the mechanism and its kinetic-order could not be determined. Steady-state BSAFs (kg organic carbon/kg lipid) of 0.015, 0.024, and 0.022 were derived for tetradecachloro-p-terphenyl, tetradecachloro-m-terphenyl, and octadecachloro-p-quaterphenyl, respectively. The high uncertainty in predicted KOWs for highly hydrophobic chemicals limited the comparison and evaluation of predicted BSAFs from the Arnot-Gobas food web model and BSAFs measured in this study. The results of this study point to the need to perform dietary assimilation efficiency studies with highly hydrophobic compounds to resolve uncertainties surrounding the estimation of their KOW and the need to understand mechanism and models for the biphasic elimination kinetics.

Sediment Bioaccumulation Test with Lumbriculus variegatus: Effects of Organism Loading.

At contaminated sediment sites, the bioavailability of contaminants in sediments is assessed using sediment-bioaccumulation tests with Lumbriculus variegates (Lv). The testing protocols recommend that ratio of total organic carbon (TOC) in sediment to L. variegatus (dry weight) (TOC/Lv) should be no less than 50:1. Occasionally, this recommendation is not followed, especially with sediments having low TOC, e.g., <1 %. This study evaluated the impacts and resulting biases in the testing results when the recommendation of "no less than 50:1" is not followed. In the study, seven sediments were tested with a series of TOC/Lv ratios that spanned the recommendation. With increasing loading of organisms, growth of the organisms decreased in six of the seven sediments tested. Residues of polychlorinated biphenyls (PCBs) in the L. variegatus were measured in six of the seven sediments tested, and differences in PCB residues among loading ratios across all sediments were small, i.e., ±50 %, from those measured at the minimum recommended ratio of 50:1 TOC/Lv. In all sediment, PCB residues increased with increasing loading of the organisms for the mono-, di-, and tri-chloro-PCBs. For tetra-chloro and heavier PCBs, residues increased with increasing loading of organisms for only two of the six sediments. PCB residues were not significantly different between TOC/Lv loadings of 50:1 and mid-20:1 ratios indicating that equivalent results can be obtained with TOC/Lv ratios into the mid-20:1 ratios. Overall, the testing results suggest that when testing recommendation of 50:1 TOC/Lv is not followed, potential biases in the biota-sediment accumulations factors from the sediment-bioaccumulation test will be small.

Sediment bioaccumulation test with Lumbriculus variegatus: effects of feeding.

Standard sediment-bioaccumulation test methods specify that Lumbriculus variegatus should not be fed during the 28-day exposure. This lack of feeding can lead to decreases in L. variegatus weight and lipid content during the 28-day exposure period. Differences in intrinsic nutritional content of sediments could lead to additional variability in organism performance and/or contaminant uptake. To evaluate the potential benefits of feeding, sediment-bioaccumulation tests were performed comparing treatments with and without supplemental feeding with tropical fish food and also comparing performance food introduced as blended slurry versus fine flakes. The ration of food provided had to be limited to 6 mg/300-mL beaker with 250 mg of L. variegatus (ww) receiving three feedings per week to maintain acceptable dissolved oxygen (DO) in the test chambers. Relative weight change during exposure varied across sediments in the absence of food from very little change to as much as a 40 % decrease from starting weight. Feeding slurry and flake foods increased the total weight of recovered organisms by 32 and 48 %, respectively, but they did not decrease variability in weight changes across sediments. Lipid contents of the organisms decreased similarly across all feeding treatments during the test. At test termination, lipid contents of L. variegatus across unfed, slurry-fed, and flake-fed treatments were not significantly different per Tukey's honest significant difference test with 95 % family-wise confidence. Feeding resulted in polychlorinated biphenyl residues in L. variegatus being generally slightly less (median 78 %) and slightly greater (median 135 %) than the unfed treatments with slurry and flake formulated foods, respectively.

Sublethal Toxicity of 17 Per- and Polyfluoroalkyl Substances with Diverse Structures to Ceriodaphnia dubia, Hyalella azteca, and Chironomus dilutus.

Seven-day sublethal toxicity tests were performed with the freshwater invertebrates Ceriodaphnia dubia, Hyalella azteca, and Chironomus dilutus to determine the effects of per- or polyfluorinated alkyl substances (PFAS) of varying chain length within four classes: perfluoroalkyl carboxylic acids (PFCAs), perfluoroalkyl sulfonic acids (PFSAs), perfluoroalkane sulfonamides, and fluorotelomer sulfonic acids. In general, toxicity increased with increasing chain length, but the slopes of these relationships varied markedly by species and chemical class. The toxicity of individual PFCAs was similar among species. The toxicity of PFSAs was similar to PFCAs for C. dubia and H. azteca, whereas PFSAs were much more toxic to C. dilutus, with median effect concentrations (EC50s) as low as 0.022 mg perfluorooctane sulfonate (PFOS)/L and 0.012 mg perfluorononane sulfonate (PFNS)/L. Despite the high sensitivity to PFOS and PFNS, C. dilutus was not very sensitive to structurally similar fluorotelomer sulfonates (6:2 and 8:2). Perfluoroalkane sulfonamides were the most toxic class tested among all species (e.g., EC50s of 0.011 and 0.017 mg perfluorooctane sulfonamide/L for C. dilutus and H. azteca, respectively). The differences in toxicity among species and chemical classes suggest that mechanisms of PFAS toxicity may differ as a function of both. Environ Toxicol Chem 2024;43:359-373. Published 2023. This article is a U.S. Government work and is in the public domain in the USA.

Gonadal Development in Smallmouth Bass (Micropterus Dolomieu) Reared in the Absence and Presence of 17-α-Ethinylestradiol.

Testicular oocytes in wild adult bass (Micropterus spp.) are considered a potential indication of exposure to estrogenic compounds in municipal, agricultural, or industrial wastewater. However, our ability to interpret links between testicular oocyte occurrence in wild fish species and environmental pollutants is limited by our understanding of normal and abnormal gonadal development. We previously reported low-to-moderate testicular oocyte prevalence (7%-38%) among adult male bass collected from Minnesota waters with no known sources of estrogenic compounds. In the present study, two experiments were conducted in which smallmouth bass (Micropterus dolomieu) fry were exposed to control water or 17-α-ethinylestradiol (EE2) during gonadal differentiation, then reared in clean water for an additional period. Histological samples were evaluated at several time points during the exposure and grow-out periods, and the sequence and timing of gonadal development in the presence of estrogen were compared with that of control fish. Testicular oocytes were not observed in any control or EE2-exposed fish. Among groups exposed to 1.2 or 5.1 ng/L EE2 in Experiment 1 or 3.0 ng/L EE2 in Experiment 2, ovaries were observed in 100% of fish up to 90 days after exposure ceased, and approximately half of those ovaries had abnormal characteristics, suggesting that they likely developed in sex-reversed males. Groups exposed to 0.1, 0.4, or 1.0 ng/L in Experiment 2 developed histologically normal ovaries and testes in proportions not significantly different from 1:1. These findings suggest that, while presumably able to cause sex reversal, juvenile exposure to EE2 may not be a unique cause of testicular oocytes in wild bass, although the long-term outcomes of exposure are unknown. Environ Toxicol Chem 2022;41:1416-1428. © 2022 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

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

The results of a series of experiments on the acute toxicity of major geochemical ions (Na+ , K+ , Ca2+ , Mg2+ , Cl- , SO4 2- , HCO3 - /CO3 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), 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.

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

Mathematical models are presented for the acute median lethal concentrations of major geochemical ions (Na+ , K+ , Ca2+ , Mg2+ , Cl- , SO4 2- , HCO3 - /CO3 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, SO4 -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.

Adverse outcome pathways: a conceptual framework to support ecotoxicology research and risk assessment.

Ecological risk assessors face increasing demands to assess more chemicals, with greater speed and accuracy, and to do so using fewer resources and experimental animals. New approaches in biological and computational sciences may be able to generate mechanistic information that could help in meeting these challenges. However, to use mechanistic data to support chemical assessments, there is a need for effective translation of this information into endpoints meaningful to ecological risk-effects on survival, development, and reproduction in individual organisms and, by extension, impacts on populations. Here we discuss a framework designed for this purpose, the adverse outcome pathway (AOP). An AOP is a conceptual construct that portrays existing knowledge concerning the linkage between a direct molecular initiating event and an adverse outcome at a biological level of organization relevant to risk assessment. The practical utility of AOPs for ecological risk assessment of chemicals is illustrated using five case examples. The examples demonstrate how the AOP concept can focus toxicity testing in terms of species and endpoint selection, enhance across-chemical extrapolation, and support prediction of mixture effects. The examples also show how AOPs facilitate use of molecular or biochemical endpoints (sometimes referred to as biomarkers) for forecasting chemical impacts on individuals and populations. In the concluding sections of the paper, we discuss how AOPs can help to guide research that supports chemical risk assessments and advocate for the incorporation of this approach into a broader systems biology framework.

The potential for salt toxicity: Can the trans-epithelial potential (TEP) across the gills serve as a metric for major ion toxicity in fish?

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, Na2SO4, MgCl2, MgSO4, KCl, K2SO4, CaCl2, and CaSO4) 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 ΔTEPmax) 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.

The relative importance of waterborne and dietborne As exposure on survival and growth of juvenile fathead minnows.

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.

Estimating n-octanol-water partition coefficients for neutral highly hydrophobic chemicals using measured n-butanol-water partition coefficients.

Direct measurement of the n-octanol partition coefficients (KOW) for highly hydrophobic organic chemicals is extremely difficult because of the extremely low concentrations present in the water phase. n-Butanol/water partition coefficients (KBW) are generally much lower than KOW due to the increased solubility of solute in the alcohol saturated aqueous phase, and therefore become easier to measure. We measured the KBW for 25 neutral organic chemicals having measured log KOWs ranging from 2 to 9 and 4 additional highly hydrophobic chemicals, with unmeasured KOWs, having estimated log KOWs ranging from 6 to 18. The measured log KBW and log KOW values were linearly related, r2 = 0.978, and using the regression developed from the data, KOWs were predicted for the 4 highly hydrophobic chemicals with unmeasured KOWs. The resulting predictions were orders of magnitude lower than those predicted by a variety of computational models and suggests the estimates of KOW in the literature for highly hydrophobic chemicals (i.e., log KOW greater than 10) are likely incorrect by several orders of magnitude.

The effects of arsenic speciation on accumulation and toxicity of dietborne arsenic exposures to rainbow trout.

The effects on juvenile rainbow trout survival, growth, food consumption, and food conversion efficiency from dietborne exposures to inorganic arsenic (arsenite, arsenate) and to the organoarsenicals monomethylarsonate (MMA), dimethylarsinate (DMA), and arsenobetaine (AsB) were investigated in two experiments: (1) a 28-d exposure using live diets of oligochaete worms separately exposed via water to these five arsenic compounds and (2) a 56-d exposure using pellet diets prepared from commercial fish food to which arsenite, MMA, or DMA were added. In the live diet experiment, the degree to which worms could be contaminated with the organoarsenicals was limited by toxicity to the worms and other experimental constraints, so that their toxicity relative to inorganic arsenic could not be fully established, but AsB was concluded to have low toxicity, consistent with published results for mammals. For the pellet diet experiment, MMA and DMA were found to be at least an order of magnitude less toxic than inorganic As on the basis of concentration in the diet, as well as much less toxic on the basis of accumulation in the fish. The need to consider speciation in aquatic risk assessments for arsenic was further demonstrated by tissue analyses of three macroinvertebrate species from a mining-impacted stream, which showed large variations in both total arsenic and the relative amounts of inorganic and organic arsenic. Additionally, although effects of arsenic on trout appear to be well correlated with inorganic arsenic, worms were found to be more sensitive to waterborne DMA than to inorganic arsenic, showing that low toxicity of organoarsenicals cannot be assumed for all aquatic organisms. Various difficulties in evaluating and applying studies on dietborne exposures and fish growth are also discussed.

Chronic toxicity of major ion salts and their mixtures to Ceriodaphnia dubia.

In previous work we intensively studied the acute responses of the cladoceran Ceriodaphnia dubia to major geochemical ions (Na+ , K+ , Ca2+ , Mg2+ , Cl- , SO42- , and HCO3- /CO32- ), culminating in the development of models to predict acute toxicity from ionic composition. To quantitatively evaluate whether the toxicological behavior of major ions observed for C. dubia extends to chronic toxicity, we conducted 58 chronic toxicity tests with individual major salts and binary mixtures thereof. Chronic responses paralleled those demonstrated previously for acute exposure, specifically 1) similar relative toxicity of individual salts; 2) different Na salts showing similar potency when exposure is expressed as osmolarity; 3) toxicity of Mg, Ca, and K salts related to cation activity; 4) decreased toxicity of Na and Mg salts when Ca activity is increased at less than toxic concentrations; 5) additive behavior for salt mixtures sharing a common cation; and 6) independent behavior for salt mixtures with dissimilar cations, except Mg/Ca mixtures which appeared additive. Acute-to-chronic ratios were fairly consistent among salts, with values of approximately 1.8 for acute 50% lethal concentration (LC50) to chronic 50% effect concentration (EC50) and 2.8 for LC50/EC20 when expressed on an activity basis. Adjusting the previous acute toxicity model for acute-to-chronic ratios yielded chronic models that predict chronic toxicity within the range of intertest variability. Because these models are informed by a wide range of ion mixtures, they should provide robust assessment tools for natural waters enriched with major ions. Environ Toxicol Chem 2019;38:769-783. © Published 2018 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.

Effects of low dissolved oxygen on organisms used in freshwater sediment toxicity tests.

Minimum dissolved oxygen requirements are part of standard guidelines for toxicity testing of freshwater sediments with several benthic invertebrates, but the data underlying these requirements are somewhat sparse. We exposed three common test organisms to ranges of dissolved oxygen concentrations to determine their responses in 10-d exposures, relative to published guidelines for sediment toxicity tests. The oligochaete, Lumbriculus variegatus, showed 100% survival in all exposures down to the lowest concentration tested, 0.7 mg O(2)l(-1). Midge (Chironomus dilutus) larva showed a more pronounced response; while survival was less than 90% only below 1.0mg O(2)l(-1), the biomass endpoint showed EC(50), EC(20), and EC(10) values of 1.00 (0.91-1.10), 1.41 (1.16-1.71), and 1.67 (1.25-2.24) mg O(2)l(-1). The amphipod, Hyalella azteca, showed no adverse effects at concentrations as low as 2.12 mg O(2)l(-1). The combination of these data with other literature data suggest that DO minima in current North American 10-d sediment test guidelines are reasonable.

Influence of dilution water ionic composition on acute major ion toxicity to the mayfly Neocloeon triangulifer.

Field and laboratory studies have shown that mayflies (Ephemeroptera) tend to be relatively sensitive to elevated major ion concentrations, but little is known about how ionic composition influences these responses. The present study evaluated the acute toxicity of major ion salts to the mayfly Neocloeon triangulifer over a range of background water quality conditions. The mayfly was particularly sensitive to Na2 SO4 , with the median lethal concentration (LC50) of 1338 mg SO4 /L being lower than LC50s reported for 7 other species at that hardness. Increasing hardness of the dilution water from 30 to 150 mg/L (as CaCO3 ) resulted in doubling of LC50s for sodium salts, and an approximately 1.5-fold increase in LC50 for MgSO4 . Potassium salt toxicity was not strongly influenced by hardness, consistent with findings for other species. When hardness was held constant but the Ca to Mg ratio was manipulated, the ameliorative effect on Na2 SO4 and NaCl did not appear as strong as when hardness was varied; but for MgSO4 the amelioration relative to Ca activity was similar between the 2 experiments. The toxicity of K salts to N. triangulifer was similar to Na salts on a millimolar basis, which contrasts with several other species for which K salts have been much more toxic. In addition, the toxicity of KCl to N. triangulifer was not notably affected by Na concentration, as has been shown for Ceriodaphnia dubia. Finally, plotting LC50s in terms of ion activity (Cl, SO4 , Na, Mg, or K) over the range of Ca activities in dilution water resulted in significant positive relationships, with comparable slopes to those previously observed for C. dubia over the same range of Ca activities. Environ Toxicol Chem 2018;37:1330-1339. © 2018 SETAC.

Acute toxicity of sodium chloride and potassium chloride to a unionid mussel (Lampsilis siliquoidea) in water exposures.

Freshwater mussels (order Unionoida) are one of the most imperiled groups of animals in the world. However, many ambient water quality criteria and other environmental guideline values do not include data for freshwater mussels, in part because mussel toxicity test methods are comparatively new and data may not have been available when criteria and guidelines were derived. The objectives of the present study were to evaluate the acute toxicity of sodium chloride (NaCl) and potassium chloride (KCl) to larvae (glochidia) and/or juveniles of a unionid mussel (fatmucket, Lampsilis siliquoidea) and to determine the potential influences of water hardness (50, 100, 200, and 300 mg/L as CaCO3 ) and other major ions (Ca, K, SO4 , or HCO3 ) on the acute toxicity of NaCl to the mussels. From the KCl test, the 50% effect concentration (EC50) for fatmucket glochidia was 30 mg K/L, similar to or slightly lower than the EC50s for juvenile fatmucket (37-46 mg K/L) tested previously in our laboratory. From the NaCl tests, the EC50s for glochidia increased from 441 to 1597 mg Cl/L and the EC50s for juvenile mussels increased from 911 to 3092 mg Cl/L with increasing water hardness from 50 to 300 mg/L. Increasing K from 0.4 to 1.9 mg/L, SO4 from 13 to 40 mg/L, or HCO3 from 44 to 200 mg/L in the 50 mg/L hardness water did not substantially change the NaCl EC50s for juvenile mussels, whereas increasing Ca from 9.9 to 42 mg/L increased the EC50s by a factor of 2. The overall results indicate that glochidia were equally or more sensitive to NaCl and KCl compared with juvenile mussels and that the increased water hardness ameliorated the acute toxicity of NaCl to glochidia and juveniles. These responses rank fatmucket among the most acutely sensitive freshwater organisms to NaCl and KCl. Environ Toxicol Chem 2018;37:3041-3049. © 2018 SETAC. This article is a US government work and, as such, is in thepublic domain in the United States of America.