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Oil sands development in the lower Athabasca River watershed has raised considerable public and scientific concerns regarding perceived effects on environmental health. To address this issue for tributaries and the mainstem of the Athabasca River in the Athabasca Oil Sands Region, the Water Component of the Joint Oil Sands Monitoring (JOSM) plan produced monitoring assessments for seven integrated themes: atmospheric deposition, tributary water quality, river mainstem water quality, groundwater quality and quantity, water quality and quantity modelling, benthic invertebrate condition, and fish health. Our review integrates and synthesizes the large and diverse datasets assembled in the seven JOSM theme assessments to (i) evaluate possible environmental effects based on known sources and candidate proximal causes and (it) determine the importance of cause-and-effect pathways related to contaminant, sediment, and nutrient inputs. Although JOSM research identified ecological effects that appear to be associated with contaminant exposure, the source of this exposure is confounded by co-location of, and inability to differentiate between, oil sands operations (principally released by atmospheric emission) and inputs from the natural bitumen outcrops (e.g., erosional material transported by surface and groundwater flows). Nutrient enrichment from treated municipal sewage effluent was the dominant ecological effect observed for the mainstem Athabasca River, associated with increased fish size and changes in invertebrate assemblages, likely because this pollution source is discharged directly into the river. If the direct release of treated oil sands process water occurs in the future, then the potential ecological impact of these direct industry releases will need to be evaluated carefully. The ecological causal assessment method proved to be a useful tool for better understanding how stressor sources relate to ecological effects through candidate proximate causes. Factors that confound our ability to assess the ecological effects of oil sands development focus on our inability to adequately differentiate between contaminants supplied from natural and anthropogenic contaminant sources. Our causal synthesis identifies options for changes in future monitoring to better anticipate and detect degradation in the ecosystem health of the lower Athabasca River and its tributaries. Key words: oil sands, biomonitoring, cumulative effects, sediments, contaminants, nutrient enrichment. L'exploitation des sables bitumineux dans le cours inferieur du bassin versant de la riviere Athabasca a suscite une grande inquietude chez le public et les scientifiques quant aux effets percus sur la sante environnementale. Afin d'aborder cette question pour les affluents et le cours principal de la riviere Athabasca dans la region des sables bitumineux de l'Athabasca, la composante eau du programme de surveillance conjointe des sables bitumineux (SCSB) a produit des evaluations des surveillances pour sept themes integres : le depot atmospherique, la qualite de l'eau des affluents, la qualite de l'eau du cours principal de la riviere, la qualite et la quantite des eaux souterraines, la modelisation de la qualite et de la quantite de l'eau, la condition des invertebres benthiques et la sante des poissons. La synthese realisee par les auteurs integre et resume les vastes et divers ensembles de donnees rassembles dans les sept evaluations thematiques du programme de SCSB pour (i) evaluer les effets environnementaux possibles sur la base des sources connues et des causes proximales candidates et (it) determiner l'importance des voies de causalite liees aux apports de contaminants, de sediments et de nutriments. Bien que les recherches du programme de SCSB aient identifie des effets ecologiques qui semblent etre associes a l'exposition aux contaminants, la source de cette exposition est confondue par la co-localisation et l'incapacite a differencier entre les operations d'exploitation des sables bitumineux (principalement rejetes par emission atmospherique) et les apports provenant des affleurements naturels de bitume (par exemple, les materiaux formes par 1'erosion transported par le flux des eaux de surface et souterraines). L'enrichissement en nutriments des effluents d'eaux usees municipales traitees a ete l'effet ecologique dominant observe pour le cours principal de la riviere Athabasca, associe a l'augmentation de la taille des poissons et aux changements dans les assemblages d'invertebres, probablement parce que cette source de pollution est rejetee directement dans la riviere. Si le rejet direct des eaux de traitement des sables bitumineux se produit a l'avenir, l'impact ecologique potentiel de ces rejets directs de l'industrie devra alors etre soigneusement evalue. La methode d'evaluation des causes ecologiques s'est averee constituer un outil utile pour mieux comprendre comment les sources de stress sont liees aux effets ecologiques par des causes immediates candidates. Les facteurs qui confondent notre capacite a evaluer les effets ecologiques de l'exploitation des sables bitumineux se concentrent sur notre incapacite a differencier de maniere adequate les contaminants provenant de sources naturelles et anthropiques. Cette synthese sur les voies de causalite identifie des options de changements dans la surveillance future afm de mieux anticiper et detecter la degradation de la sante de l'ecosysteme du cours inferieur de la riviere Athabasca et de ses affluents. [Traduit par la Redaction] Mots-cles: sables bitumineux, biosurveillance, effets cumulatifs, sediments, contaminants, enrichissement en nutriments., 1. Introduction The Canadian oil sands in northeastern Alberta contain approximately 10% of the proven global oil reserves (Natural Resources Canada 2019), with development expanding substantially since 1980 and oil [...]

Snowpacks in the Alberta Oil Sands Region (AOSR) o f Canada contain elevated loadings o f methylmercury (MeHg; a neurotoxin that biomagnifies through foodwebs) due to oil sands related activities. At sites ranging from 0 to 134 km from the major AOSR upgrading facilities, we examined sources of MeHg by quantifying potential rates of MeHg production in snowpacks and melted snow using mercury stable isotope tracer experiments, as well as quantifying concentrations of MeHg on particles in snowpacks (pMeHg). At four sites, methylation rate constants were low in snowpacks (k[sub m] = 0.001-0.004 d[sup -1] ) and nondetectable in melted snow, except at one site (k[sub m] = 0.0007 d [sup -l]). The ratio o f methylation to demethylation varied between 0.3 and 1.5, suggesting that the two processes are in balance and that in situ production is unlikely an important net source o f MeHg to AOSR snowpacks. pMeHg concentrations increased linearly with distance from the upgraders (R[sup 2] = 0.71, p < 0.0001); however, snowpack total particle and pMeHg loadings decreased exponentially over this same distance (R[sup 2] = 0.49, p = 0.0002; R[sup 2] = 0.56, p < 0.0001). Thus, at nearfield sites, total MeHg loadings in snowpacks were high due to high particle loadings, even though particles originating from industrial activities were not MeHg rich compared to those at remote sites. More research is required to identify the industrial sources o f snowpack particles in the AOSR. [ABSTRACT FROM AUTHOR]

A warming climate is rapidly changing the distribution and exchanges of carbon within high Arctic ecosystems. Few data exist, however, which quantify exchange of both carbon dioxide (CO2) and methane (CH4) between the atmosphere and freshwater systems, or estimate freshwater contributions to total catchment exchange of these gases, in the high Arctic. During the summers of 2005 and 2007-2012, we quantified CO2 and CH4 concentrations in, and atmospheric exchange with, common freshwater systems in the high Arctic watershed of Lake Hazen, Nunavut, Canada. We identified four types of biogeochemically-distinct freshwater systems in the watershed, however mean CO2 concentrations (21-28 μmol L-1) and atmospheric exchange (-0.013-0.046 g C-CO2 m-2 d-1) were similar between these systems. Seasonal flooding of ponds bordering Lake Hazen generated considerable CH4 emissions to the atmosphere (0.008 g C-CH4 m-2 d-1), while all other freshwater systems were minimal emitters of this gas (< 0.001 g C-CH4 m-2 d-1). Measurements made on terrestrial landscapes in the same watershed between 2008-2012 determined that the near-barren polar semidesert was a very weak consumer of atmospheric CO2 (-0.004 g C-CO2 m-2 d-1), but an important consumer of atmospheric CH4 (-0.001 g C-CH4 m-2 d-1). Alternatively, meadow wetlands were very productive consumers of atmospheric CO2 (-0.96 g C-CO2 m-2 d-1) but relatively weak emitters of CH4 to the atmosphere (0.001 g C-CH4m-2 d-1). When using ecosystem-cover classification mapping, we found that freshwaters were unimportant contributors to total watershed carbon exchange, in part because they covered less than 10% of total cover in the watershed. High Arctic watersheds are experiencing warmer and wetter climates than in the past, which may have implications for the net uptake of carbon greenhouse gases by currently underproductive polar semidesert and freshwater systems. [ABSTRACT FROM AUTHOR]