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.

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.

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.

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.

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^- , SO₄^2- , and HCO₃^- /CO₃^2- ), 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.

Bioaccumulation of Highly Hydrophobic Chemicals by Lumbriculus variegatus

Bioaccumulation of highly hydrophobic chemicals (log K_OW> 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 (K_OW; 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 K_OWs 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 K_OW and the need to understand mechanism and models for the biphasic elimination kinetics.

The acute toxicity of major ion salts to Ceriodaphnia dubia. III. Mathematical models for mixture toxicity

Based on previous research on the acute toxicity of major ions (Na⁺ , K⁺ , Ca²⁺ , Mg²⁺ , Cl^- , SO₄^2- , and HCO₃^- /CO₃^2- ) 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. Environ Toxicol Chem 2018;37:247-259. Published 2017 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 acute toxicity of major ion salts to Ceriodaphnia dubia. II. Empirical relationships in binary salt mixtures

Many human activities increase concentrations of major geochemical ions (Na⁺¹ , K⁺¹ , Ca⁺² , Mg⁺² , Cl^-1 , SO₄^-2 , and HCO₃^-1 /CO₃^-2 ) 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 acute toxicity of major ion salts to Ceriodaphnia dubia: I. influence of background water chemistry

The ions Na⁺ , K⁺ , Ca²⁺ , Mg²⁺ , Cl^- , SO₄^2- , and HCO₃^- /CO₃^2- (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 CaSO₄ and CaCO₃ 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:SO₄ 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.

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.

Evaluation of PCB bioaccumulation by Lumbriculus variegatus in field-collected sediments

Review of data from several contaminated sediment sites suggested that biota-sediment accumulation factors (BSAFs) declined with increasing contaminant concentrations in the sediment. To evaluate the consistency and possible causes of this behavior, polychlorinated biphenyl (PCB)-contaminated sediment samples from the Hudson, Grasse, and Fox River Superfund sites were used in sediment bioaccumulation tests with the freshwater oligochaete, Lumbriculus variegatus, with PCB concentrations in interstitial water (IW) quantified using polyoxymethylene passive samplers. Measured BSAFs tended to decrease with increasing PCB concentration in sediment, especially for the more highly chlorinated congeners. Measures of partitioning between sediment, IW, and oligochaetes showed that measured sediment-IW partition coefficients (KTOC ) tended to increase slightly with increasing sediment contamination, whereas the ratio of tissue PCB to IW PCB tended to decrease with increasing concentration in IW. Variation in accumulation among sediments was clearly influenced by bioavailability, as reflected by IW measurements, although the specific cause of varying KTOC was not clear. Calculated partitioning between IW and organism lipid (Klipid ) indicated that accumulation was generally 5 to 10-fold higher than would be predicted if Klipid was approximately equal to the n-octanol-water partition coefficient (KOW ). While affirming previous observations of decreasing BSAFs with increasing PCB contamination, the relatively shallow slope of the observed relationship in the current data may suggest that this concentration dependence is not a major uncertainty in sediment risk assessment, particularly if measurements of PCBs in IW are incorporated.

The relative importance of waterborne and dietborne arsenic exposure on survival and growth of juvenile rainbow trout

Previous work demonstrated reduced growth of rainbow trout receiving diets containing environmentally relevant concentrations of arsenic, but did not address the relative and combined potency of waterborne and dietborne exposures. In the current study, juvenile rainbow trout were exposed for 28d to a range of arsenic concentrations in water and in a live oligochaete diet, separately and in combination. In clean water, fish fed worms previously exposed to arsenate at 4 or 8mg As/L showed pronounced reductions in growth, but fish exposed to these same water concentrations and a clean diet experienced less or no effect. Increasing waterborne arsenate to 16 or 32mg As/L had substantial effects on both growth and survival, and simultaneous exposure via both routes intensified growth effects, but not mortality. Growth reduction was strongly correlated to total arsenic accumulation in the fish tissue, regardless of the route of exposure, but mortality was better correlated to waterborne arsenic concentration. The relative concentration of total arsenic in fish viscera and in the remaining carcass was not a useful indicator of exposure route. Speciation analysis showed that most arsenate was converted to arsenite within the worms, but organoarsenic species were not found. The greater toxicity of dietborne exposure when fish and prey were exposed to the same waterborne arsenate concentration emphasizes the need to address dietborne exposure when assessing the aquatic risks of arsenic contamination. This is of particular concern because risk from dietary exposure may occur at even lower water concentrations than used here when prey organisms are exposed for longer periods and via multiple routes.

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.

Photoactivated polycyclic aromatic hydrocarbon toxicity in medaka (Oryzias latipes) embryos: relevance to environmental risk in contaminated sites

The hazard for photoactivated toxicity of polycyclic aromatic hydrocarbons (PAHs) has been clearly demonstrated; however, to our knowledge, the risk in contaminated systems has not been characterized. To address this question, a median lethal dose (LD50) for fluoranthene photoactivated toxicity in medaka (Orvzias latipes) embryos was determined experimentally and then compared with ultraviolet-A (UV-A; 320-400 nm) radiation exposures in a PAH-contaminated field site. The dose metric, J/cm2/ microg fluoranthene/g egg wet weight, provided the means to estimate risk as the depth where the LD50 level would be exceeded at realistic field PAH concentrations, based on estimates of UV-A exposure. The estimates were made using 30 years of solar radiation data for Duluth (MN, USA) and measurements of water-column UV-A transmittance in a PAH-contaminated field site. Medaka embryo failure was strongly related to tissue PAH concentration and UV-A exposure. The LD50 was estimated to be 12.64 J/cm2/ microg fluoranthene/g egg wet weight; the 95% confidence interval was 8.46 to 19.7 J/cm2/microg fluoranthene/g egg wet weight. Embryo failures were characterized by undifferentiated cell proliferation that occurred very early in development. No partial effects or embryo/larval malformations were observed. Estimates of the depth at which the LD50 would be exceeded in the contaminated field site ranged from 10.7 cm (clear-sky conditions and lowest attenuation) to 0.0 cm (cloudy conditions and highest attenuation). Similar calculations were done using water-column attenuation estimates from 12 sites across the Great Lakes (USA). For these, the depths at which the LD50 would be exceeded ranged from 0.00 to 271.6 cm under the conditions described above. These results suggest that PAH phototoxicity may be a risk factor in specific contaminated sites, and they provide a framework for assessing that risk.

Use of the oligochaete, Lumbricuilus variegatus, as a prey organism for toxicant exposure of fish through the diet

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.

Method for testing the aquatic toxicity of sediment extracts for use in identifying organic toxicants in sediments

Biologically directed fractionation techniques are a fundamental tool for identifying the cause of toxicity in environmental samples, but few are available for studying mixtures of organic chemicals in aquatic sediments. This paper describes a method for extracting organic chemicals from sediments and then re-introducing them into water column toxicity tests in a way that mimics, at least in part, the partitioning processes that govern bioavailability in sediment. This involves transferring solvent extracts of sediment into triolein and then placing the mixture inside low-density polyethylene dialysis tubing in a configuration similar to semipermeable membrane devices (SPMDs) used for environmental monitoring. For four model compounds, SPMDs were shown to effectively maintain water column exposure in static systems for 10-14 d, with partition coefficients similar to K(OW). Toxicity tests indicated that the SPMDs were compatible with four of five freshwater organisms tested and could be used to measure both lethal and sublethal end points. An example application showed good correspondence between organism responses in intactsediment and extracts in SPMDsfor both field-collected and spiked sediments. The SPMD-based method offers a simple, flexible test design, amenable to several different test organisms, and the ability to work with complex mixtures of contaminants while maintaining partitioning behavior similar to that within intact sediments.