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Heavy crude oil transportation over land is increasing, yet the ecological impacts of spills, particularly of diluted bitumen, in freshwater environments remain poorly understood. We simulated spills of diluted bitumen in 1400 L land-based mesocosms containing water and sediments from a boreal, oligotrophic lake and monitored the response of natural planktonic communities over 11 days. Most species of phytoplankton (chrysophytes and dinoflagellates) and zooplankton (copepods and cladocerans) were sensitive to oil, exhibiting >70% overall abundance reductions in response to the spills. Declines in nano- and microphytoplankton were short-lived and began to recover after the oil sank, whereas picophytoplankton and zooplankton populations remained depressed at the end of the experiment. In contrast, oil spills stimulated bacteria known to degrade hydrocarbons, especially Alphaproteobacteria, whereas Gammaprotobacteria(a common marine oil spill bacterial class) increased less. This is the first experiment to examine the effects of diluted bitumen in a multitrophic freshwater community. Si le transport terrestre de petrole brut lourd est a la hausse, la comprehension des impacts ecologiques de deversements, particulierement de bitume dilue, dans les milieux d'eau douce demeure limitee. Nous avons simule des deversements de bitume dilue dans des mesocosmes terrestres de 1400 L contenant de l'eau et des sediments d'un lac oligotrophe boreal et surveille les reactions de communautes de plancton naturelles sur une periode de 11 jours. La plupart des especes de phytoplancton (chrysophytes et dinoflagelles) et de zooplancton (copepodes et cladoceres) etaient sensibles a la presence de petrole, presentant des baisses d'abondance globale de >70 % en reaction aux deversements. Les baisses etaient de courte duree en ce qui concerne le nanophytoplancton et le microphytoplancton, et ils commencaient a se remettre une fois le petrole coule au fond, alors que les populations de picophytoplancton et de zooplancton demeuraient reduites a la fin de l'experience. En comparaison, les deversements de petrole stimulaient les bacteries connues pour degrader les hydrocarbures, particulierement les alphaproteobacteries, alors que l'abondance de gammaprotobacteries (une classe repandue de bacteries associees aux deversements de petrole en mer) n'augmentait pas autant. Il s'agit de la premiere experience a examiner les effets du bitume dilue dans une communaute d'eau douce multitrophique. [Traduit par la Redaction], Introduction Heavy crude oils are complex mixtures of organic components and carry additional contaminants, including sulfur, and metals (vanadium, nickel, iron) (Martinez-Palou et al. 2011; ECCC 2013; Lee et al. [...]

Physical impacts of diluted bitumen (dilbit) and the application of surface washing agents (SWAs) in freshwater have not been characterized for aquatic invertebrates. These compounds are known to reduce surface tension in feather and fur microstructures of birds and mammals, and are thus likely to affect the buoyancy of surface‐dwelling aquatic insects. We evaluated impacts of fresh dilbit and a SWA on water striders (Metrobates sp.), which are surface‐dwelling organisms that rely on fine‐hair microstructures to remain buoyant. We report nominal sheen thickness values that cause 50% immobility in 48 h as determined from exposure studies in outdoor tanks. A comparison of our data with those from historic oil spill volumes in Canada and the United States in the past 12 yr indicates that our reported nominal sheen thicknesses could have been reached or exceeded in 99% of historic spills when scaled to a small reference lake. The addition of Corexit EC9580A, a SWA approved for marine use in Canada, led to 100% immobility in striders within minutes, both in combination with oil and alone. Our study reveals an acute sensitivity to Corexit EC9580A and dilbit by surface‐dwelling insects and may be driven by disruption of mechanisms of buoyancy. We highlight a need to evaluate physical impacts, typically excluded from standard toxicity testing, within the context of spill impact mitigation assessments. Environ Toxicol Chem 2021;40:1298–1307. © 2020 SETAC [ABSTRACT FROM AUTHOR]

• Morphological and metabarcoding identification are comparable up to genus-level. • Shoreline cleaner application had larger impact on zooplankton community than eMNR. • Metabarcoding could be more sensitive than morphology to detect community changing. Regulators require adequate information to select best practices with less ecosystem impacts for remediation of freshwater ecosystems after oil spills. Zooplankton are valuable indicators of aquatic ecosystem health as they play pivotal roles in biochemical cycles while stabilizing food webs. Compared with morphological identification, metabarcoding holds promise for cost-effective, high-throughput, and benchmarkable biomonitoring of zooplankton communities. The objective of this study was to apply DNA and RNA metabarcoding of zooplankton for ecotoxicological assessment and compare it with traditional morphological identification in experimental shoreline enclosures in a boreal lake. These identification methods were also applied in context of assessing response of the zooplankton community exposed to simulated spills of diluted bitumen (dilbit), with experimental remediation practices (enhanced monitored natural recovery and shoreline cleaner application). Metabarcoding detected boreal zooplankton taxa up to the genus level, with a total of 24 shared genera, and while metabarcoding-based relative abundance served as an acceptable proxy for biomass inferred by morphological identification (ρ ≥ 0.52). Morphological identification determined zooplankton community composition changes due to treatments at 11 days post-spill (PERMANOVA, p = 0.0143) while metabarcoding methods indicated changes in zooplankton richness and communities at 38 days post-spill (T-test, p < 0.05; PERMANOVA, p ≤ 0.0429). Shoreline cleaner application overall seemed to have the largest impact on zooplankton communities relative to enhanced monitored natural recovery, regardless of zooplankton identification method. Both metabarcoding and morphological identification were able to discern the differences between the two experimental remediation practices. Metabarcoding of zooplankton could provide informative results for ecotoxicological assessment of the remediation practices of dilbit, advancing our knowledge of best practices for remediating oil-impacted aquatic ecosystems while serving to accelerate the assessment of at-risk freshwater ecosystems. [ABSTRACT FROM AUTHOR]