Oil spills in inland freshwater environments remain a major but under-examined environmental challenge compared to marine disasters. This study provides the first systematic, decadal review of oil spill incidents across the United States (U.S.) from 2014 to 2024, integrating records from the federal databases such as Environmental Protection Agency, U.S. Coast Guard, and the Office of Pipeline Safety. The analysis identifies national trends in spill frequency, causes, oil types, environmental settings, and response effectiveness. More than 90% of released oil volume was in rivers, while diesel represented the most frequent spill type and crude oil contributed to the greatest cumulative volume. Transportation accidents, including vessel groundings, collisions, and equipment failures, were the leading causes, often linked to aging infrastructure and high-traffic corridors. Ecological recovery was slowest in wetlands and creeks, where hydrocarbons persist in sediments and re-mobilize during flood events. Conventional mechanical containment measures such as booms and skimmers were widely used but often limited by flow conditions and site inaccessibility. In-situ burning, recently trialed in marshes and bayous, achieved up to 90% oil removal but generated combustion residues and smoke that remain poorly characterized. The findings underscore the need for freshwater-specific response frameworks, predictive spill-risk models, and the development of environmentally safe smoke-suppressant technologies. By linking hydrological, chemical, and ecological processes, this review advances a multi-sphere understanding of oil spill dynamics in inland waters and provides a foundation for improved mitigation strategies in freshwater ecosystems worldwide.

The efficacy of oil spill response depends on the speed of detecting the oil. Detecting submerged oil is more difficult than oil on the water surface, because most conventional sensors are not effective. Oil Detection Canines (ODCs) have been reliably used to detect oil during shoreline spill surveys, and preliminary laboratory studies also showed promising results for detecting oil submerged under water. To confirm their potential, a field study was conducted in a boreal freshwater lake in Northwestern Ontario, Canada to investigate the capability of an ODC to detect submerged weathered oils at depths of 1 to 5 m. Triplicate targets at each depth used weathered diluted bitumen (dilbit), Bunker C residual fuel oil, and Maya crude oil burn residue and both the ODC and handler blinded to the location of each target. Boat-based searches were conducted and the handler identified "alerts" based on ODC behaviour changes that were compared to georeferenced oil target locations. The ODC positively identified seven (7) of the eight (8) dilbit targets at 1 to 5 m, five (5) of the six (6) Bunker C targets at 1 and 3 m, and none of the burn residue targets at 1-m depth. The ability of ODCs to detect submerged or sunken oil in shallow water was clearly demonstrated, adding another technique for submerged and sunken oil surveys with the advantages of real-time data returns, the ability to detect small oil deposits, and an operational capability in shallow waters with potential for detection in deeper water.

A lake littoral enclosure study was conducted to evaluate the effects of a minimally invasive secondary remediation method (enhanced Monitored Natural Recovery eMNR) on phytoplankton and biofilm communities following a model heavy crude oil spill. Enclosures (n = 3; 27,000 L) received 1.5 kg of weathered conventional heavy crude and underwent primary remediation by freshwater shoreline flushing and oil recovery using oleophilic sorbent pads, followed by nutrient addition to stimulate microbial degradation (eMNR). Reference enclosures (n = 3) received shoreline flushing only. Phytoplankton chlorophyll-a and biofilm ash-free dry mass were monitored over 400 days, while biofilm community composition was sampled at the end of the study. No statistically significant differences were detected between eMNR and reference enclosures for any measured endpoint. Under the single nutrient-addition regime tested and the specific oligotrophic conditions of this boreal lake, eMNR did not measurably affect primary producer biomass or community structure. However, alternative formulations, nutrient doses, or environmental conditions may yield different outcomes, and caution is warranted when generalizing these results to other systems.

In the Lake Koocanusa-Kootenai River system (Montana, USA and British Columbia, Canada), selenium (Se) contamination has become an international concern and is suspected to contribute to the observed burbot (Lota lota) population collapse. Due to our limited ability to sample burbot in Lake Koocanusa for monitoring studies, we used a reference population to develop tools to model tissue Se disposition for a focal species in systems with elevated Se. Total Se concentrations in otoliths, biofluids (blood and endolymph), and tissues (muscle, liver, and ovary) from burbot in reference lakes in northwestern Ontario, Canada, were measured to document tissue-to-tissue Se relationships and evaluate the potential for otoliths to retrace Se exposure in fish. Among burbot tissue, Se concentrations were the highest in the ovary (mean +/- SD = 4.55 +/- 2.23 mu gg(-1) dry mass [dm]), followed by the liver (2.69 +/- 1.96 mu gg(-1)dm) and muscle (1.87 +/- 1.14 mu gg(-1)dm), and decreased with body size (p<0.05). In otoliths, Se was detected at low levels (<1 mu gg(-1)). Selenium concentrations in burbot samples were positively correlated among muscle, ovary, liver, and endolymph tissues, but not for the most recent annually averaged or lifetime-averaged Se concentrations in otoliths. We hypothesize that Se concentrations were too low in this study to establish links between otoliths and other fish tissues and to detect significant lifetime variation in individuals, and that further validation using archived otoliths from burbot exposed to elevated Se levels in Lake Koocanusa-Kootenai River is needed to reconstruct exposure histories. However, intercompartmental models proved valuable for estimating Se concentrations in burbot tissues only available by means of lethal sampling (i.e., ovary), although additional work should confirm whether the established models are reliable to predict concentrations in Se-impaired systems as tissue distributions are likely to differ with increasing Se levels.

Selenium (Se) is a naturally occurring metalloid in soils and rocks that is released by weathering processes; it is also enriched by some anthropogenic activities, including mining and agriculture. The mechanism of Se aquatic toxicity has been understood for several decades; at elevated concentrations, dietary Se can accumulate in maternal tissues of fish and birds, become deposited into their eggs, and can potentially result in impaired embryological development. North American environmental regulations have acknowledged differences in species sensitivity and variation among aquatic environments (i.e., lentic and lotic) that influence Se toxicity. The above subjects were thoroughly reviewed in a state-of-science SETAC Pellston workshop and book, entitled Ecological assessment of selenium in the aquatic environment, published in 2010. Since the publication of that book, regulatory guidance in North America has evolved further to enhance the protection of aquatic life exposed to Se. This IEAM special series entitled "Regulatory issues surrounding the management of selenium" is a compilation of recent research and reviews from North American experts addressing critical environmental, physiological, and operational factors warranting consideration in support of Se regulatory frameworks.

A pangenome analysis offers a unique exploration of the metabolic and genetic diversity, range of ecological niches, and evolution of a particular genus or species. However, such pangenomic analyses are uncommon among environmentally relevant genera. Here, we present freshwater pangenomes of three environmentally relevant genera, Limnohabitans, Aquabacterium, and Novosphingobium. These genera had been detected in hydrocarbon degrading cultures in previous research by our group. Using pangenomic tools we attempted to characterize the extent of hydrocarbon degradation potential within each pangenome and determine what ecological niche each genus occupies within hydrocarbon degradation. In total 46 Limnohabitans, 10 Aquabacterium, and 32 Novosphingobium freshwater genomes were collected from various databases and compiled into pangenomes. We found that each pangenome harbours downstream hydrocarbon degrading potential and unexpected genetic diversity within its core and accessory pangenomes possibly stemming from geographic and metagenomic data processing influences. This work was the first to explore pangenomes of these environmentally relevant genera.

Small scale oil spills and secondary clean up methods were implemented in shoreline enclosures within the littoral zone of a boreal lake to examine impacts on lower trophic organism (periphyton, phytoplankton and zooplankton) communities. Treatment enclosures received 1.5 L (1300 g) of weathered diluted bitumen on June 21st, 2019, followed four days later by primary recovery (i.e. freshwater flushing and absorbent recovery of floating oil) and then either the shoreline washing agent (SWA) Corexit EC9580A or enhanced monitored natural recovery (eMNR) with a slow-release nutrient fertilizer. Indicators of water quality and biota were tracked for 95 days, until September 24th, 2019. Maximum polycyclic aromatic compound (PAC) concentrations in both SWA and eMNR treatments reached 7.3 - 9.4 mg m-3. Phytoplankton appeared resilient to direct toxic effects of oil and these treatments and no significant effects on photosystem II (Fv/Fm) or community structure were detected. Phytoplankton biomass increased in both the SWA (c. +130 %) and eMNR (c. +60 %) treatments during the first 33 days after oil additions, relative to the reference treatment, which did not receive oil, SWA, or eMNR. DNA metabarcoding and microscopy revealed that the phytoplankton community structure was dominated by chrysophytes and dinoflagellates with a greater similarity between eMNR and reference treatments relative to the SWA treatment. While no difference in periphyton biomass was detected between the eMNR and reference treatments, within the SWA treatment periphyton biomass was c. -77 % lower than within the reference for the first 48 days of the experiment. No statistically significant differences in zooplankton biomass were detected between the reference and either the eMNR or SWA treatments, but values within the SWA treatment dropped to < 4 % of values within the reference treatment on day 11 of the experiment, before rebounding by Day 33 of the experiment, suggesting reduced grazing pressure on phytoplankton. While our results revealed no acute toxic effects to the phytoplankton, the experimental oil spills and the use of shoreline cleaners within freshwaters resulted in complex changes in food web structure including increases in phytoplankton biomass and declines in both periphyton and zooplankton biomass.

The type 2 diabetes drug metformin is among the most frequently prescribed, dispensed, and consumed pharmaceuticals worldwide; and its heavy use and poor breakdown means it is consequently detected in various wastewater treatment plant (WWTP) effluents and surface waters. The resulting environmental concentrations of metformin can have adverse impacts on aquatic ecosystems. An 8-week in-lake mesocosm experiment was conducted in a freshwater boreal lake at the IISD-Experimental Lakes Area (Ontario, Canada) to determine the environmental fate and effects of metformin. Mesocosms were assigned to nominal concentrations of 0, 5, or 50 μg L−1 metformin, in replicates of four. Biotic communities (i.e. microbial, phytoplankton, zooplankton, benthic macroinvertebrate) were assessed and complementary Lemna gibba and Daphnia magna bioassays were conducted in the laboratory. Metformin was extremely stable during the 8-week experiment, with mean measured concentrations of 0.00, 5.42, and 42.79 μg L−1 in mesocosm surface waters for the 0, 5, and 50 μg L−1 treatments, respectively. While trace amounts of metformin were detected in mesocosm sediments, the compound was primarily found in the water column. After over 1 year following metformin additions, the mean loss of metformin from surface water was 94.0 % and 91.0 % for the 5 and 50 μg L−1 treatment groups, respectively. No adverse effects of metformin treatment on the diversity, abundance, or biomass of microbial, phytoplankton, zooplankton, or benthic macroinvertebrate communities were found. Additionally, no survival or reproductive effects were observed in the 21-d Daphnia magna bioassays and no significant effects were observed in the 7-d Lemna gibba growth assays. No substantial metformin transformation to guanylurea was observed in mesocosm surface waters or sediments, likely due to a lack of bacterial degradation occurring within mesocosms relative to WWTPs.

Despite decades of fate and effects studies, environmental selenium (Se) contamination and management remain an issue for many freshwater systems in North America. Several regulatory bodies have promulgated updated targets or management levels for Se; however, additional guidance on best practices for monitoring Se to protect freshwater aquatic life is warranted. In this article, we describe current approaches to assessing the ecological risks of Se in impaired freshwater systems and outline recommended methods for collecting and analyzing biological and abiotic samples and interpreting data. Because reproductive impairment of fish populations is most commonly used to determine the potential impacts of Se, several biological factors that could affect Se toxicity are explored, including diet, trophic positions, reproductive biology, body size and maturity, migratory movements, and use of seasonal habitats. Measuring Se concentrations in mature eggs is the most reliable metric for estimating potential reproductive impairment in fish populations because the range of toxicity thresholds is relatively narrow for all but a few tolerant fish species. In situations where collecting mature eggs is not feasible, we review the use of alternative fish tissue for estimating potential effects. Factors affecting Se uptake from freshwater are also considered with guidance on collecting abiotic (e.g., water and sediment) and biotic components of aquatic food webs (e.g., macroinvertebrates, biofilm). Integr Environ Assess Manag 2024;00:1-16. (c) 2024 SETAC Despite decades of fate and effects studies, environmental selenium (Se) contamination and management remain an issue for many freshwater systems in North America. Measuring Se concentrations in mature eggs is the most reliable metric for estimating potential reproductive impairment in fish populations, because the range of toxicity thresholds is relatively narrow for all but a few tolerant fish species. Several regulatory bodies have promulgated updated targets or management levels for Se; however, additional guidance on best practices for monitoring Se to protect freshwater aquatic life is warranted. In situations where collecting mature eggs is not feasible, alternative fish tissue can be used as a screening tool to estimate potential effects.

Due to its widespread use for the treatment of Type-2 diabetes, metformin is routinely detected in surface waters globally. Laboratory studies have shown that environmentally relevant concentrations of metformin can adversely affect the health of adult fish, with effects observed more frequently in males. However, the potential risk to wild fish populations has yet to be fully elucidated and remains a topic of debate. To explore whether environmentally relevant metformin exposure poses a risk to wild fish populations, the present study exposed wild fathead minnows (Pimephales promelas) to 5 or 50 mu g/L metformin via 2 m diameter in-lake mesocosms deployed in a natural boreal lake in Northern Ontario at the International Institute for Sustainable Development Experimental Lakes Area (IISD-ELA). Environmental monitoring was performed at regular intervals for 8-weeks, with fish length, weight (body, liver and gonad), condition factor, gonadosomatic index, liver-somatic index, body composition (water and biomolecules) and hematocrit levels evaluated at test termination. Metabolic endpoints were also evaluated using liver, brain and muscle tissue, and gonads were evaluated histologically. Results indicate that current environmental exposure scenarios may be sufficient to adversely impact the health of wild fish populations. Adult male fish exposed to metformin had significantly reduced whole body weight and condition factor and several male fish from the high -dose metformin had oocytes in their testes. Metformin- exposed fish had altered moisture and lipid (decrease) content in their tissues. Further, brain (increase) and liver (decrease) glycogen were altered in fish exposed to high -dose metformin. To our knowledge, this study constitutes the first effort to understand metformin's effects on a wild small -bodied fish population under environmentally relevant field exposure conditions.

The aquaculture industry is among the fastest growing food production sectors in the world. Land-based aquaculture systems continue to increase in popularity as they offer the benefits of controlling diseases, managing water quality, and minimizing threats to wild populations of fish. However, these systems discharge wastewater high in N and P. The ability of cattail (Typha spp.) constructed wetlands (CWs) to remove N and P from aquaculture wastewater (AWW) was examined here. Cattail CWs were established in mesocosms and had a gradient of AWW applied weekly for a total of 5 weeks. Total N and P loadings ranged from 13.7 to 209.2 mg m-2 and 3.01 to 45.97 mg m-2 over 28 days, respectively. Additions of AWW did not cause elevations in total dissolved N, total ammonia N, or nitrite N in CW water; however, concentrations of nitrate N and P in CW water were related to nutrient loading conditions. Elevations in P persisted for 3-4 weeks among high nutrient loading treatments, providing an opportunity for eutrophic conditions to develop in CW systems. However, after 33 days of treatment, >95% total P concentration reduction was achieved in all mesocosms with final concentrations <0.05 mg L-1, equivalent to reference conditions. High-loading treatments achieved greater P load reduction (856.8-955.0 mg m-2 year-1) than low-loading and reference treatments (591.7-792.7 mg m-2 year-1). This study demonstrates the effectiveness of cattail CWs to remove nutrients during AWW treatment and highlights the potential for end-of-season use in northern climates, providing insights regarding the operational timeline of such systems.

The overall impact of a crude oil spill into a pristine freshwater environment in Canada is largely unknown. To evaluate the impact on the native microbial community, a large-scale in situ model experimental spill was conducted to assess the potential role of the natural community to attenuate hydrocarbons. A small volume of conventional heavy crude oil (CHV) was introduced within contained mesocosm enclosures deployed on the shoreline of a freshwater lake. The oil was left to interact with the shoreline for 72 h and then free-floating oil was recovered using common oil spill response methods (i.e. freshwater flushing and capture on oleophilic absorptive media). Residual polycyclic aromatic hydrocarbon (PAH) concentrations returned to near preoiling concentrations within 2 months, while the microbial community composition across the water, soil, and sediment matrices of the enclosed oligotrophic freshwater ecosystems did not shift significantly over this period. Metagenomic analysis revealed key polycyclic aromatic and alkane degradation mechanisms also did not change in their relative abundance over the monitoring period. These trends suggest that for small spills (<2 l of oil per 15 m2 of surface freshwater), physical oil recovery reduces polycyclic aromatic hydrocarbon concentrations to levels tolerated by the native microbial community. Additionally, the native microbial community present in the monitored pristine freshwater ecosystem possesses the appropriate hydrocarbon degradation mechanisms without prior challenge by hydrocarbon substrates. This study corroborated trends found previously (Kharey et al. 2024) toward freshwater hydrocarbon degradation in an environmentally relevant scale and conditions on the tolerance of residual hydrocarbons in situ.

With the increase in crude oil transport throughout Canada, the potential for spills into freshwater ecosystems has increased and additional research is needed in these sensitive environments. Large enclosures erected in a lake were used as mesocosms for this controlled experimental dilbit (diluted bitumen) spill under ambient environmental conditions. The microbial response to dilbit, the efficacy of standard remediation protocols on different shoreline types commonly found in Canadian freshwater lakes, including a testing of a shoreline washing agent were all evaluated. We found that the native microbial community did not undergo any significant shifts in composition after exposure to dilbit or the ensuing remediation treatments. Regardless of the treatment, sample type (soil, sediment, or water), or type of associated shoreline, the community remained relatively consistent over a 3-month monitoring period. Following this, metagenomic analysis of polycyclic aromatic and alkane hydrocarbon degradation mechanisms also showed that while many key genes identified in PAH and alkane biodegradation were present, their abundance did not change significantly over the course of the experiment. These results showed that the native microbial community present in a pristine freshwater lake has the prerequisite mechanisms for hydrocarbon degradation in place, and combined with standard remediation practices in use in Canada, has the genetic potential and resilience to potentially undertake bioremediation.

Due to the sudden nature of oil spills, few controlled studies have documented how oil weathers immediately following accidental release into a natural lake environment. Here, we evaluated the weathering patterns of Cold Lake Winter Blend, a diluted bitumen (dilbit) product, by performing a series of controlled spills into limnocorrals installed in a freshwater lake in Northern Ontario, Canada. Using a regression-based design, we added seven different dilbit volumes, ranging from 1.5 to 180 L, resulting in oil-to-water ratios between 1:71,000 (v/v) and 1:500 (v/v). We monitored changes in the composition of various petroleum hydrocarbons (PHCs), including n-alkanes, polycyclic aromatic hydrocarbons (PAHs), and oil biomarkers in dilbit over time, as it naturally weathered for 70 days. Depletion rate constants (kD) of n-alkanes and PAHs ranged from 0.0009 to 0.41 d-1 and 0.0008 to 0.38 d-1, respectively. There was no significant relationship between kD and spill volume, suggesting that spill size did not influence the depletion of petroleum hydrocarbons from the slick. Diagnostic ratios calculated from concentrations of n-alkanes, isoprenoids, and PAHs indicated that evaporation and photooxidation were major processes contributing to dilbit weathering, whereas dissolution and biodegradation were less important. These results demonstrate the usefulness of large scale field studies carried out under realistic environmental conditions to elucidate the role of different weathering processes following a dilbit spill.

Aquatic ecosystems offer a continuum of water flow from headwater streams to inland lakes and coastal marine systems. This spatial connectivity influences the structure, function and dynamics of aquatic communities, which are among the most threatened and degraded on the Earth. Here, we determine the spatial resolution of environmental DNA (eDNA) in dendritic freshwater networks, which we use as a model for connected metacommunities. Our intensive sampling campaign comprised over 420 eDNA samples across 21 connected lakes, allowing us to analyse detections at a variety of scales, from different habitats within a lake to entire lake networks. We found strong signals of within-lake variation in eDNA distribution reflective of typical habitat use by both fish and zooplankton. Most importantly, we also found that connecting channels between lakes resulted in an accumulation of downstream eDNA detections in lakes with a higher number of inflows, and as networks increased in length. Environmental DNA achieves biodiversity surveys in these habitats in a high-throughput, spatially integrated way. These findings have profound implications for the interpretation of eDNA detections in aquatic ecosystems in global-scale biodiversity monitoring observations.

Following an oil spill into water, bacteria can biodegrade petroleum hydrocarbons which could lead to petrogenic carbon assimilation by aquatic biota. We used changes in the isotope ratios of radio- (Δ14C) and stable (δ13C) carbon to examine the potential for assimilation of petrogenic carbon into a freshwater food web following experimental spills of diluted bitumen (dilbit) into a boreal lake in northwestern Ontario, Canada. Different volumes (1.5, 2.9, 5.5, 18, 42, 82, and 180 L) of Cold Lake Winter Blend (a heavy crude blend of bitumen and condensate) dilbit were applied to seven 10-m diameter littoral limnocorrals (approximate volume of 100 m3), and two additional limnocorrals had no added dilbit to serve as controls. Particulate organic matter (POM) and periphyton from oil-treated limnocorrals had lower δ13C (up to 3.2‰ and 2.1‰ for POM and periphyton, respectively) than the control at every sampled interval (3, 6 and 10 weeks for POM and 6, 8 and 10 weeks for periphyton). Dissolved organic and inorganic carbon (DOC and DIC, respectively) had lower Δ14C in the oil-treated limnocorrals relative to the control (up to 122‰ and 440‰ lower, respectively). Giant floater mussel (Pyganodon grandis) housed for 25 days in aquaria containing oil-contaminated water from the limnocorrals did not show significant changes in δ13C values of muscle tissue compared to mussels housed in control water. Overall, the changes in δ13C and Δ14C observed indicated small amounts (up to 11% in DIC) of oil carbon incorporation into the food web. The combined δ13C and Δ14C data provide evidence for minimal incorporation of dilbit into the food web of this oligotrophic lake, suggesting that microbial degradation and subsequent incorporation of oil C into the food web may play a relatively small role in the ultimate fate of oil in this type of ecosystem.

Pipelines carrying diluted bitumen (dilbit) traverse North America and may result in dilbit release into sensitive freshwater ecosystems. To better understand the potential effects of a freshwater oil release, the Boreal-lake Oil Release Experiment by Additions to Limnocorrals project at the International Institute for Sustainable Development-Experimental Lakes Area (Ontario, Canada) modeled seven dilbit spills contained within a 10-m diameter of littoral limnocorrals in a boreal lake. Wild finescale dace (Phoxinus neogaeus) were released in the limnocorrals 21 days after oil addition and remained there for 70 days. Dilbit volumes covered a large range representing a regression of real spill sizes and total polycyclic aromatic compounds (TPAC) between 167 ng L-1  day-1 and 1989 ng L-1  day-1 . We report the effects of chronic exposure on reproductive potential as well as physiological responses in the gallbladder and liver. In exposures >1000 ng L-1  day-1 , there was a significant decrease in fish retrieval, culminating in zero recapture from the three highest treatments. Among the fish from the limnocorrals with lower levels of TPAC (<500 ng L-1  day-1 ), effects were inconsistent. Gallbladder bile fluorescence for a naphthalene metabolite was significantly different in fish from the oil-exposed limnocorrals when compared to the lake and reference corral, indicating that fish in these lower exposures were interacting with dilbit-derived polycyclic aromatic compounds. There were no significant differences in condition factor, somatic indices, or hepatocyte volume indices. There were also no significant changes in the development of testes or ovaries of exposed dace. The results from the present study may serve to orient policymakers and emergency responders to the range of TPAC exposures that may not significantly affect wild fish. Environ Toxicol Chem 2022;41:2745-2757. © 2022 SETAC.

Selenium (Se) is an environmental contaminant of global concern that can cause adverse effects in fish at elevated levels. Fish gut microbiome play essential roles in gastrointestinal function and host health and can be perturbed by environmental contaminants, including metals and metalloids. Here, an in-situ Se exposure of female finescale dace (Phoxinus neogaeus) using mesocosms was conducted to determine the impacts of Se accumulation on the gut microbiome and morphometric endpoints. Prior to this study, the gut microbiome of finescale dace, a widespread Cyprinid throughout North America, had not been characterized. Exposure to Se caused a hormetic response of alpha diversity of the gut microbiome, with greater diversity at the lesser concentration of 1.6 μg Se/L, relative to that of fish exposed to the greater concentration of 5.6 μg Se/L. Select gut microbiome taxa of fish were differentially abundant between aqueous exposure concentrations and significantly correlated with liver-somatic index (LSI). The potential effects of gut microbiome dysbiosis on condition of wild fish might be a consideration when assessing adverse effects of Se in aquatic environments. More research regarding effects of Se on field-collected fish gut microbiome and the potential adverse effects or benefits on the host is warranted.

Transport of diluted bitumen (dilbit) from Canada’s oil sands region poses risk for leaks and spills of petroleum-derived contaminants into the environment. Exposure of fish to dilbit is known to cause cardiotoxicity, developmental deformities, and impairment in swim performance. However, previous studies have examined the toxicity of dilbit in laboratory settings which does not account for environmental and biological food-web variables that may alter exposure and/or toxicity of dilbit. Moreover, most methods of assessing organism health following oil exposure require lethal sampling. This work is a part of a larger set of experiments where dilbit spills were simulated within enclosures on a lake; the present study assesses the impacts of residual levels of dilbit that may have entered the surrounding lake environment from the enclosures following model spill cleanup. In order to understand the impacts of residual dilbit in an ecosystem setting without use of lethal sampling, epidermal mucus was collected and sequenced from lake charr (Salvelinus namaycush) exposed to residual dilbit in a boreal lake. While concentrations reached a maximum of 2.29 μg/L total polycyclic aromatic compounds (ΣPAC) within surface waters, surface water ΣPAC concentrations generally remained below 1 μg/L. Results of RNA sequencing were compared to sequencing data from mucus collected prior to dilbit additions. Differential gene expression and pathway analyses indicated dysregulation of genes associated with intermediary and energy metabolism as well as a trend in upregulation of cyp1a3 in epidermal mucus following dilbit exposure. Thus, results of the present study suggest that lake charr undergo consistent biological responses after exposure to residual levels of dilbit following a model spill, and that mRNA-based analysis of mucus may be a viable method for non-lethal oil exposure assessment. Overall, the results provide insight on the response of wild fish to very dilute dilbit exposures after a model spill cleanup.

In our research topic, we received 5 papers -one review, two method article, and two 12 original research articles from a globally diverse set of authors. In the literature review,Thorstensen et al. describe in detail how different methods of tagging wild fish in natural 14 environments can be used to track movements and estimate population size. They also 15 describe the best practices for tagging without causing stress to the fish. They then explore 16 how physiological, isotopic, and contaminant measurements from small samples like small 17 tissue biopsies and blood can be integrated with movement data -providing a holistic 18 approach to studying and monitoring wild fish health and behavior.We also gathered two method papers that showcase safe and non-lethal methods for