Effect of oil sands process-affected water on toxicity of retene to early life-stages of Japanese medaka (Oryzias latipes)

Abiotic and biotic constituents of oil sands process-affected waters

The oil sands in Northern Alberta are the largest oil sands in the world, providing an important economic resource for the Canadian energy industry. The extraction of petroleum in the oil sands begins with the addition of hot water to the bituminous sediment, generating oil sands process-affected water (OSPW), which is acutely toxic to organisms. Trillions of litres of OSPW are stored on oil sands mining leased sites in man-made reservoirs called tailings ponds. As the volume of OSPW increases, concerns arise regarding the reclamation and eventual release of this water back into the environment. OSPW is composed of a complex and heterogeneous mix of components that vary based on factors such as company extraction techniques, age of the water, location, and bitumen ore quality. Therefore, the effective remediation of OSPW requires the consideration of abiotic and biotic constituents within it to understand short and long term effects of treatments used. This review summarizes selected chemicals and organisms in these waters and their interactions to provide a holistic perspective on the physiochemical and microbial dynamics underpinning OSPW .

Effects of dietary 2-(2H-benzotriazol-2-yl)-4-methylphenol (UV-P) exposure on Japanese medaka (Oryzias latipes) in a short-term reproduction assay

Benzotriazole ultraviolet stabilizers (BZT-UVs) are added to various products to prevent damage caused by UV light and have emerged as contaminants of concern. Although BZT-UVs are detected in aquatic biota globally, few studies have assessed their potential toxic effects. The objective of the present study was to assess effects of 2-(2H-Benzotriazol-2-yl)-4-methylphenol (UV-P) on reproductive success of Japanese medaka (Oryzias latipes) in a standard 21-day reproduction assay. Japanese medaka were exposed to dietary UV-P at concentrations of 0, 36, 158, and 634 ng UV-P/g food, for a total of 28 days which included 7 days of exposure prior to the start of the 21-day reproduction assay. No significant effect on egg production or fertilization success was observed. Abundances of transcripts of erα, vtgI, cyp1a, or cyp3a4 were not significantly different in livers from male or female fish exposed to UV-P. However, abundances of transcripts of cyp11a and cyp19a were significantly lower in gonads from female fish. There was a trend of increasing concentrations of E2 and a non-significant increase of T in the 634 ng/g treatment in plasma from female fish exposed to UV-P. Concentrations of 11-KT were unchanged in plasma from males exposed to UV-P. These responses suggest weak perturbation of steroidogenesis, consistent with an antiandrogenic mode of action. However, this perturbation was insufficient to impair reproductive performance. Metabolomics analysis of female livers suggests altered concentrations of various metabolites and biological pathways, including glutathione metabolism, suggesting that UV-P might cause responses related to oxidative stress or phase II metabolism. However, metabolomics revealed no obvious mechanism of toxicity. Overall, results of this study indicate that dietary exposure to UV-P up to 634 ng/g food does not significantly impact reproductive performance of Japanese medaka but impacts on steroidogenesis could indicate a potential mechanism of toxicity which might lead to reproductive impairment in more sensitive species.

Insights into ultrasound-promoted degradation of naphthenic acid compounds in oil sands process affected water. Part I: Accelerated H-abstraction and decarboxylation of aromatic and alicyclic compounds

Propelled by enormous increase in demand for fuel sources, Canadian oil sands are becoming increasingly important as a fuel source due to their abundance and upgrading capability. However, extraction of bitumen, a high acid crude (HAC) oil, requires 2-3 units of water per unit of oil resulting in naphthenic acid (NA)-rich oil sands process affected water (OSPW) collected in effluent ponds. This study illustrates the role of sonochemistry in the accelerated degradation through H-abstraction and subsequent decarboxylation of aromatic and alicyclic naphthenic acid model compounds. Benzoic acid and 3-methylcyclohexane carboxylic acid were selected as model NA compounds to investigate the mechanism of hydroxyl radical (OH^•) initiated carboxylic acid degradation in 378 KHz sonochemical reactor. Established FTIR methods with low resolution LCMS spectroscopy confirmation were applied to determine the extent of carboxylic acid degradation and identify the formation of products. FTIR monitoring showed a non-linear degradation of carboxylic acids with formation of many intermediates highlighting the shift from cyclic carboxylic acids to cyclic alcohols during BA degradation. Subsequent decrease in carboxylic acid groups signifies scission of cyclic structures before complete mineralization. This is confirmed with the LCMS identification of products such as: 3-hydroxybenzoic acid and phenol. This study postulated new breakdown pathways for degradation of benzoic acid with complete mineralization at a sonochemical reaction time (SRT) of 4 h. A radical quenching process was also inferred through the formation of conglomerates during sonochemical degradation of BA. Extension of the study to 3-methylcyclohexane carboxylic acid (3mCHA) shows similar non-linearity with an increase in carboxylic acid groups indicating H-abstraction followed by ring-opened compounds. However, due to the complex nature of 3mCHA's ring-opened compounds, complete mineralization is not achieved. The putative role of sonochemistry is a promising and sustainable degradation method for mitigating NAs in OSPW, but sonication periods need to be considered carefully to ensure adequate mineralization of their constituents and combinatorial methods with other advanced oxidation methods may be needed to enhance industrial application. In Part II, an in silico screening approach using first principles is reported to identify the breakdown of the organic compounds and determine molecular rates of reaction to confirm the mechanistic origins of the compounds formed.

Novel polar AhR-active chemicals detected in sediments of an industrial area using effect-directed analysis based on in vitro bioassays with full-scan high resolution mass spectrometric screening

Studies investigating aryl hydrocarbon receptor (AhR)-active compounds in the environment typically focus on non- and mid-polar substances, such as PAHs; while, information on polar AhR agonists remains limited. Here, we identified polar AhR agonists in sediments collected from the inland creeks of an industrialized area (Lake Sihwa, Korea) using effect-directed analysis combined with full-scan screening analysis (FSA; using LC-QTOFMS). Strong AhR-mediated potencies were observed for the polar and latter fractions of RP-HPLC (F3.5-F3.8) from sediment organic extracts in the H4IIE-luc in vitro bioassays. FSA was performed on the corresponding fractions. Twenty-eight tentative AhR agonists were chosen using a five-step process. Toxicological confirmation using bioassay revealed that canrenone, rutaecarpine, ciprofloxacin, mepanipyrim, genistein, protopine, hydrocortisone, and medroxyprogesterone were significantly active. The relative potencies of these AhR-active compounds compared to that of benzo[a]pyrene ranged from 0.00002 to 2.0. Potency balance analysis showed that polar AhR agonists explained, on average, ~6% of total AhR-mediated potencies in samples. Some novel polar AhR agonists also exhibited endocrine-disrupting potentials capable of binding to estrogen and glucocorticoid receptors, as identified by QSAR modeling. In conclusion, the focused studies on distributions, sources, fate, and ecotoxicological effects of novel polar AhR agonists in the environment are necessary.

Characterization of raw and ozonated oil sands process water utilizing atmospheric pressure gas chromatography time-of-flight mass spectrometry combined with solid phase microextractionun

This work describes a novel application of atmospheric pressure gas chromatography time-of-flight mass spectrometry (APGC-TOF-MS) combined with solid-phase microextraction (SPME) for the simultaneous analysis of hydrocarbons and naphthenic acids (NAs) species in raw and ozone-treated oil sands process water (OSPW). SPME method using polydimethylsiloxane (PDMS)-coated fibers was validated using gas chromatography with flame ionization detector (GC-FID) to ensure the SPME extractions were operated appropriately. The ionization pathways of the hydrocarbon species in OSPW in the APGC source were verified by analyzing a mixture of eight polyaromatic hydrocarbons which were ionized primarily via charge transfer to produce [M⁺] while NAs in OSPW were found to be ionized through protonation to generate [MH⁺] in the wet APGC source. SPME/APGC-TOF-MS analysis demonstrated a different composition profile in OSPW #1, with 74.5% of hydrocarbon species, 23.4% of O₂-NAs, and 2.1% of the oxidized NA species at extraction pH 2.0 compared with that obtained by UPLC-TOF-MS analysis (36.9% of O₂-NAs, 26.8% of O₃-NAs, 24.9% of O₄-NAs, 9.1% of O₅-NAs, 2.3% of O₆-NAs). Moreover, the peak areas of the total NAs and the total peak areas of NAs + hydrocarbons measured by SPME/APGC-TOF-MS correlated excellently with the total NA concentration determined by UPLC-TOF-MS (R² = 0.90) and the concentrations of the total acid-extractable organics determined by SPME/GC-FID (R² = 0.98), respectively. APGC-TOF-MS integrated with the SPME techniques could extend the range of target compounds and be a promising alternative to evaluate and characterize NAs and hydrocarbon in different water types.

Toxicity of Weathered Sediment-Bound Dilbit to Early Life Stages of Zebrafish (Danio rerio)

Due to high viscosity, bitumen extracted from the Alberta oil sands is diluted with natural gas condensates to form diluted bitumen (dilbit) to facilitate transport through pipelines. Dilbit that is spilled into or near a waterbody is subject to environmental weathering processes such as evaporation and interaction with sediments. This is the first study that assessed the toxicity of weathered sediment-bound dilbit (WSD) to fish early life stages. Exposure of zebrafish (Danio rerio) embryos to water-soluble fractions (WSFs) or water-accommodated fractions (WAFs) of WSD from 30 min to 120 h postfertilization resulted in pericardial edema, yolk sac edema, and incidences of uninflated swim bladder. The presence of oil-mineral aggregates (OMAs) in the WAFs greatly increased toxicity, despite all fractions having similar concentrations of dissolved polycyclic aromatic hydrocarbons (PAHs). There were greater cyp1a mRNA abundances in larvae exposed to WAFs, suggesting that there were differences in bioavailability of PAHs between fractions. However, there was little evidence that embryotoxicity was caused by oxidative stress. Results suggest that evaporation and sediment interaction do not completely attenuate toxicity of dilbit to zebrafish early life stages, and OMAs in exposures exacerbate toxicity.

Treatment of Polycyclic Aromatic Hydrocarbons in Oil Sands Process-Affected Water with a Surface Flow Treatment Wetland

This study applied a passive sampling approach using low-density polyethylene passive samplers to determine the treatment efficiency of the Kearl surface flow treatment wetland for polycyclic aromatic hydrocarbons (PAHs) in Oil Sands Process-affected Waters (OSPW). Treatment efficiency was measured as concentration-reduction and mass-removal from the OSPW. The results show that the wetland’s ability to remove individual PAHs from the influent varied substantially among the PAHs investigated. Treatment efficiencies of individual PAHs ranged between essentially 0% for certain methylated PAHs (e.g., 2,6-dimethylnaphthalene) to 95% for fluoranthene. Treatment in the Kearl wetland reduced the combined total mass of all detected PAHs by 54 to 83%. This corresponded to a reduction in the concentration of total PAHs in OSPW of 56 to 82% with inflow concentrations of total PAHs ranging from 7.5 to 19.4 ng/L. The concentration of pyrene in water fell below water quality targets in the Muskeg River Interim Management Framework as a result of wetland treatment. The application of the passive samplers for toxicity assessment showed that in this study PAHs in both the influent and effluent were not expected to cause acute toxicity. Passive sampling appeared to be a useful and cost-effective method for monitoring contaminants and for determining the treatment efficiency of contaminants in the treatment wetland.

Waste Containment Ponds Are a Major Source of Secondary Organic Aerosol Precursors from Oil Sands Operations

The surface mining and bitumen extraction of oil sands (OS) generates over one million barrels of heavy oil each day in the Alberta Oil Sands Region of Canada. Recent observations suggest that emissions from OS development contribute to secondary organic aerosol (SOA) formation, but the chemical composition, mass fluxes, and sources of those emissions are poorly delineated. Here, we simulated OS extraction and used comprehensive two-dimensional gas chromatography to quantify and characterize direct air emissions, bitumen froth, residual wastewater, and tailings components, ultimately enabling fate modeling of over 1500 chromatographic features simultaneously. During the non-ice cover season, tailings ponds emissions contributed 15 000-72 000 metric tonnes of hydrocarbon SOA precursors, translating to 3000-13 000 tonnes of SOA, whereas direct emissions during the extraction process itself were notably smaller (960 ± 500 tonnes SOA yr^-1). These results suggest that tailings pond waste management practices should be targeted to reduce environmental emissions.

Chronic Toxicity of Surface Water from a Canadian Oil Sands End Pit Lake to the Freshwater Invertebrates Chironomus dilutus and Ceriodaphnia dubia

Permanent reclamation of tailings generated by surface mining in the Canadian oil sands may be achieved through the creation of end pit lakes (EPLs) in which tailings are stored in mined-out pits and capped with water. However, these tailings contain high concentrations of dissolved organics, metals, and salts, and thus surface water quality of EPLs is a significant concern. This is the first study to investigate the chronic toxicity of surface water from Base Mine Lake (BML), the Canadian oil sands first large-scale EPL, to aquatic invertebrates that play a vital role in the early development of aquatic ecosystems (Chironomus dilutus and Ceriodaphnia dubia). After exposure of C. dilutus larvae for 23 days and C. dubia neonates for 8 days, no mortality was observed in any treatment with whole BML surface water. However, the emergence of C. dilutus adults was delayed by nearly 1 week, and their survival was significantly reduced (36%) compared with the controls. Reproduction (fecundity) of C. dubia was reduced by 20% after exposure to 2014 BML surface water; however, the effect was not observed after exposure to BML surface water collected a year later in 2015. Despite some adverse effects, the results of this study indicate that BML surface water quality is improving over time and is able to support certain salt-tolerant aquatic organisms. Because salinity within BML will persist for decades without manual intervention, the ecological development of the lake will likely resemble that of a brackish or estuarine ecosystem with reduced diversity.

Persistent and transgenerational effects of raw and ozonated oil sands process-affected water exposure on a model vertebrate, the zebrafish

Exposure to oil sands process-affected water (OSPW), a by-product of Canadian oil sands mining operations, can cause both acute and chronic adverse effects in aquatic life. Ozonation effectively degrades naphthenic acids in OSPW, mitigating some of the toxicological effects of exposure. In this study we examined the effect of developmental exposure to raw and ozonated OSPW had on the breeding success, prey capture, and alarm cue response in fish months/years after exposure and the transgenerational effect exposure had on gene expression, global DNA methylation, and larval basal activity. Exposure to raw and ozonated OSPW had no effect on breeding success, and global DNA methylation. Exposure altered the expression of vtg and nkx2.5 in the unexposed F1 generation. Exposure to both raw and ozonated OSPW had a transgenerational impact on larval activity levels, anxiety behaviors, and maximum swim speed compared to the control population. Prey capture success was unaffected, however, the variability in the behavioral responses to the introduction of prey was decreased. Fish developmentally exposed to either treatment were less active before exposure and did not have an anxiety response to the alarm cue hypoxanthine-3-n-oxide. Though ozonation was able to mitigate some of the effects of OSPW exposure, further studies are needed to understand the transgenerational effects and the implications of exposure on complex fish behaviors.

Risks of Mixtures of Oil Sands Contaminants to a Sensitive Mayfly Sentinel, Hexagenia

Tailings ponds in northeastern Alberta, Canada contain massive amounts of oil sands process water (OSPW) that cannot currently be released due to the toxicity of some components. Limited space and the need for reclamation of oil sands operation sites will necessitate the release of OSPW in the near future. Knowledge of the composition and toxicity of OSPW is lacking yet is crucial for both risk assessment and management planning. This study examines chronic toxicity of a mixture of OSPW components sodium naphthenate and naphthenic acid (NA) to nymphs of the mayfly Hexagenia spp. in control and polycyclic aromatic hydrocarbons (PAH)-spiked sediment treatments. The objective of this study was to determine whether the addition of the PAH-spiked sediment significantly contributed to or masked responses of these sensitive mayflies to mixtures of NA. Mean survival in nymphs exposed to NA and PAH-spiked sediment treatments was reduced by 48% compared to those exposed to the NA mixture alone. Lethal responses were observed in all of the PAH-spiked sediment treatments. However, within PAH-spiked and control sediment treatments, there was no significant difference in nymph survival due to NA concentration, indicating that changes in survivorship were predominantly a reflection of increased mortality associated with sediment PAHs and not to the NA mixture treatment. Sublethal effects on body segment ratios suggest that mayflies exposed to NA and PAH-spiked sediment, as well as those exposed to the highest NA concentration tested (1 mg/L) and control sediment, made developmental trade-offs in order to emerge faster and escape a stressful environment. These results reveal that the release of OSPW to the surrounding environment could cause a reduction in mayfly populations. Mayflies provide ecosystem services and are an important food source for higher trophic levels in both the aquatic and terrestrial communities.

Early chemical and toxicological risk characterization of inorganic constituents in surface water from the Canadian oil sands first large-scale end pit lake

End pit lakes (EPLs) have been proposed as a method for the long-term reclamation of process water and fluid fine tailings (FFT) produced from surface mining within the Canadian oil sands. These waste products contain elevated concentrations of dissolved organics, metals, and salts which reduce surface water quality and are toxic to aquatic organisms. This study measured the concentrations of inorganic constituents in surface water from the industry's first large-scale EPL over the course of a three-year period (2014-2016). The toxicological risk was subsequently assessed to identify constituents of concern that may impair surface water quality necessary for the development of a functional aquatic ecosystem or for release to the surrounding environment. Changes in surface water concentrations over the three-year period were strongly correlated with hydrological processes occurring within the lake: advective-diffusive chemical influx from FFT pore water to the overlying surface water was offset by efflux via continuous manual pumping (freshwater in, process water out). These processes resulted in a net dilution effect of approximately 5-10% per year, however, a significant chemical mass is expected to persist within the underlying FFT. Elevated salinity (as Na⁺, Cl^-, HCO₃^-) and concentrations of boron and nickel were predicted to pose very high toxicological risk to aquatic organisms. Despite these risks, the discovery of wild Daphnia pulex in the August 2016 sample suggested that surface water quality was sufficient to support populations of certain salt-tolerant zooplankton and primary producers. However, the time required for development into a robust aquatic ecosystem remains unknown.

Assessment of raw and ozonated oil sands process-affected water exposure in developing zebrafish: Associating morphological changes with gene expression

With the ever-increasing amounts of oil sands process-affected water (OSPW) accumulating from Canada's oil sands operations, its eventual release must be considered. As OSPW has been found to be both acutely and chronically toxic to aquatic organisms, remediation processes must be developed to lower its toxicity. Ozone treatment is currently being studied as a tool to facilitate the removal of organic constituents associated with toxicity. Biomarkers (e.g. gene expression) are commonly used when studying the effects of environmental contaminants, however, they are not always indicative of adverse effects at the whole organism level. In this study, we assessed the effects of OSPW exposure on developing zebrafish by linking gene expression to relevant cellular and whole organism level endpoints. We also investigated whether or not ozone treatment decreased biomarkers and any associated toxicity observed from OSPW exposure. The concentrations of classical naphthenic acids in the raw and ozonated OSPW used in this study were 16.9 mg/L and 0.6 mg/L, respectively. Ozone treatment reduced the total amount of naphthenic acids (NAs) in the OSPW sample by 92%. We found that exposure to both raw and ozonated OSPW had no effect on the survival of zebrafish embryos. The expression levels of biotransformation genes CYP1A and CYP1B were induced by raw OSPW exposure, with CYP1B being more highly expressed than CYP1A. In contrast, ozonated OSPW exposure did not increase the expression of CYP1A and only slightly induced CYP1B. A decrease in cardiac development and function genes (NKX2.5 and APT2a2a) was not associates with large changes in heart rate, arrhythmia or heart size. We did not find any indications of craniofacial abnormalities or of increased occurrence of apoptotic cells. Overall, our study found that OSPW was not overtly toxic to zebrafish embryos.

Growth and recovery of zebrafish embryos after developmental exposure to raw and ozonated oil sands process-affected water

Due to the increasing volume of oil sands process-affect water (OSPW) and its toxicity to aquatic organisms, it is important to fully understand its effects and study remediation processes that will enable its release to the environment. Ozone treatment is currently being considered as a tool to expedite remediation, as it is known to degrade toxic organic compounds present in OSPW. In this study, we aimed to measure the effects of OSPW exposure on the growth, development and recovery of zebrafish (Danio rerio) embryos. We also used ozone-treated OSPW to determine whether ozonation negated any effects of raw OSPW exposure. As biomarkers of exposure, we assessed the expression of genes involved in neurodevelopment (ngn1, neuroD), estrogenicity (vtg), oxidative stress (sod1), and biotransformation (cyp1a, cyp1b). Our study found that exposure to both raw and ozonated OSPW did not impair growth of zebrafish embryos, however, otoliths of exposed embryos were smaller than those of control embryos. The expression levels of both cyp1a and cyp1b were induced by raw OSPW exposure. However, after the exposure period, expression levels of these genes returned to control levels within two days of residence in clean water. We found no changes in the expression levels of ngn1, neuroD and vtg genes with exposure to treated or untreated OSPW. Overall, our study found that raw OSPW exposure did not have many negative effects on zebrafish embryos and embryos appeared to recover relatively quickly after exposure ended. Furthermore, ozone treatment decreased the induction of cyp1a and cyp1b.

Toxicity of Cold Lake Blend and Western Canadian Select dilbits to standard aquatic test species

Dilbits are blends of bitumen and natural gas condensates or crude oils with only limited toxicity data. Two dilbits, Cold Lake Blend and Western Canadian Select, were tested as either unweathered or weathered oils for acute and chronic toxicity to standard freshwater and estuarine organisms. Water accommodated fractions of the dilbits were characterized for total petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAHs), and monoaromatics (BTEX). Acute toxicity of unweathered and weathered dilbits ranged from 4 to 16 mg/L TPH, 8 to 40 μg/L total PAHs, and 0.7 to 16 mg/L BTEX in Ceriodaphnia dubia, Pimephales promelas, Americamysis bahia, and Menidia beryllina. Concentrations of weathered dilbits causing impaired growth (A. bahia) and reproduction (C. dubia) ranged from 0.8 to 3.5 mg/L TPH and 6 to 16 μg/L PAHs. The two dilbits had generally similar acute and short term chronic toxicity expressed as TPH or total PAHs as other crude oils and other petroleum products.

The toxicity of oil sands process-affected water (OSPW): A critical review

Large volumes of oil sands process-affected water (OSPW) are produced by the surface-mining oil sands industry in Alberta. Both laboratory and field studies have demonstrated that the exposure to OSPW leads to many physiological changes in a variety of organisms. Adverse effects include compromised immunological function, developmental delays, impaired reproduction, disrupted endocrine system, and higher prevalence of tissue-specific pathological manifestations. The composition of OSPW varies with several factors such as ore sources, mining process, and tailings management practices. Differences in water characteristics have confounded interpretation or comparison of OSPW toxicity across studies. Research on individual fractions extracted from OSPW has helped identify some target pollutants. Naphthenic acids (NAs) are considered as the major toxic components in OSPW, exhibiting toxic effects through multiple modes of action including narcosis and endocrine disruption. Other pollutants, like polycyclic aromatic hydrocarbons (PAHs), metals, and ions may also contribute to the overall OSPW toxicity. Studies have been conducted on OSPW as a whole complex effluent mixture, with consideration of the presence of unidentified components, and the interactions (potential synergistic or antagonistic reactions) among chemicals. This review summarizes the toxicological data derived from in vitro and in vivo exposure studies using different OSPW types, and different taxa of organisms. In general, toxicity of OSPW was found to be dependent on the OSPW type and concentration, duration of exposures (acute versus sub chronic), and organism studied.

Molecular responses of Walleye (Sander vitreus) embryos to naphthenic acid fraction components extracted from fresh oil sands process-affected water

Naphthenic acid fraction components (NAFCs) are constituents of oil sands process-affected water (OSPW), which is generated as a result of unconventional oil production via surface mining in the Athabasca oil sands region. NAFCs are often considered to be major drivers of OSPW toxicity to various taxa, including fishes. However, the molecular targets of these complex mixtures are not fully elucidated. Here we examined the effects in walleye (Sander vitreus) embryos after exposure to NAFCs extracted from fresh OSPW. Eleutheroembryos (exposed to 0, 4.2 or 8.3mg/L NAFCs from 1day post-fertilization to hatch) were subsampled, measured for growth and deformities, and molecular responses were assessed via real-time polymerase chain reaction (PCR). Fourteen genes were evaluated, with a focus on the aryl-hydrocarbon receptor (AhR) - cytochrome P450 pathway (arnt, cyp1a1), the oxidative stress axis (cat, gst, sod, gpx1b), apoptosis (e.g. casp3, bax and p53), growth factor signaling (e.g. insulin-like growth factors igf1, igf1b, and igf1bp), and tissue differentiation (vim). NAFC exposure was associated with an increase in the expression of cyp1a1, and a decrease in gpx1b and ribosomal protein rps40. These results indicate that NAFC effects on walleye early-life stages may be mediated through oxidative stress via pathways that include AhR.