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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 [...]

Our study aimed to determine drivers of longitudinal variation in stream metabolism and benthic macroinvertebrate (BMI) composition and assess concordance of these ecological measures for two Canadian rivers (Rat River and Tobacco Creek). Gross primary production was associated with longitudinal position in both rivers but also with the percentage of the watershed used for agriculture and hydrogeomorphic zone. However, within- and among-zone differences in stream metabolism indicated that longitudinal variation followed a staircase pattern rather than a clinal pattern. BMI composition was associated with network position in both rivers, but hydrogeomorphic zones were only important in Tobacco Creek. Among-zone differences in BMI communities in Tobacco Creek depended on season. Concordance between stream metabolism and BMI composition was not observed within either river despite metabolism and BMI composition being associated with longitudinal position. For these rivers, segment-scale hydrogeomorphic conditions appear to be important modifiers of longitudinal patterns observed at the whole river scale. The lack of concordance between stream metabolism and BMI composition suggests reach-scale processes are driving ecological differences within sampling sites. Notre etude visait a determiner les facteurs de variation longitudinale du metabolisme des eaux et de la composition des macroinvertebres benthiques (MIB) et a evaluer la concordance de ces parametres ecologiques pour deux rivieres canadiennes (la riviere Rat et le Tobacco Creek). La production primaire brute est associee a la position longitudinale dans les deux rivieres, mais egalement au pourcentage du bassin versant utilise pour l'agriculture et la zone hydrogeomorphologique. Les differences au sein des zones et entre elles sur le plan du metabolisme des eaux indiquent toutefois que la variation longitudinale suit un motif en escalier plutot qu'un motif clinal. La composition des MIB est associee a la position dans le reseau pour les deux rivieres, mais les zones hydrogeomorphologiques ne sont importantes que dans le Tobacco Creek. Les differences entre zones des communautes de MIB dans le Tobacco Creek dependent de la saison. Une concordance entre le metabolisme des eaux et la composition des MIB n'est observee dans aucune des deux rivieres malgre le fait que le metabolisme et la composition des MIB soient associes a la position longitudinale. Pour ces rivieres, les conditions hydrogeomorphologiques a l'echelle du tronqon semblent etre d'importants modificateurs des motifs longitudinaux observes a l'echelle de la riviere. L'absence de concordance entre le metabolisme des eaux et la composition des MIB porte a croire que des processus a l'echelle du tronqon expliqueraient les differences ecologiques dans les sites d'echantillonnage. [Traduit par la Redaction], Introduction Detecting patterns of ecological condition in lotic systems and then discerning the processes that regulate these patterns are critical steps towards effective management of aquatic resources. According to the [...]

Climate change poses a significant threat to Arctic freshwater biodiversity, but impacts depend upon the strength of organism response to climate‐related drivers. Currently, there is insufficient knowledge about Arctic freshwater biodiversity patterns to guide assessment, prediction, and management of biodiversity change.As part of the Circumpolar Biodiversity Monitoring Program's first freshwater assessment, we evaluated diversity of diatoms, benthic macroinvertebrates, and fish in North American Arctic rivers. Alpha diversity was assessed in relation to temperature, water chemistry, bedrock geology, and glaciation history to identify important environmental correlates. Biotic composition was compared among groups to evaluate response to environmental gradients.Macroinvertebrate α‐diversity declined strongly with increasing latitude from 48°N to 82°N, whereas diatom and fish diversity peaked around 70°N without a clear latitudinal decline. Macroinvertebrate diversity was significantly positively related to air temperature. Diatom diversity was related to bedrock geology and temperature, whereas fish diversity was related to glaciation history.Fish and macroinvertebrate assemblages differed between sites in western Canada, where invertebrate composition was more variable, and Alaska, where fish composition was more variable. In sites with both diatom and macroinvertebrate data, diatom composition was distinct in Alaska, where richness was highest in former glacial refugia. Macroinvertebrate composition was distinct in lowest‐latitude eastern and high‐latitude western Canadian sites where temperature was highest.Temperature, precipitation, geology, calcium, and substrate size were important environmental correlates for diatoms and macroinvertebrates, although the relative importance of each correlate differed. Diatom taxa were most strongly associated with water chemistry, whereas benthic invertebrate composition related most strongly to precipitation and temperature.This large‐scale study provides the most substantial integration and analysis of river diatom, macroinvertebrate, and fish data from the North American Arctic to date. Findings suggest that macroinvertebrates will show the strongest response to climate‐related shifts in temperature, whereas diatoms and fish are more likely to respond to climate‐induced shifts in nutrients and hydraulic connectivity. However, significant gaps in data coverage limited our ability to reliably evaluate spatial patterns and detect change. These gaps could be reduced by improving collaborative efforts between the U.S.A. and Canada to harmonise future monitoring. [ABSTRACT FROM AUTHOR]

Comprehensive assessments of contemporary diatom distributions across the Arctic remain scarce. Furthermore, studies tracking species compositional differences across space and time, as well as diatom responses to climate warming, are mainly limited to paleolimnological studies due to a lack of routine monitoring in lakes and streams across vast areas of the Arctic.The study aims to provide a spatial assessment of contemporary species distributions across the circum‐Arctic, establish contemporary biodiversity patterns of diatom assemblages to use as reference conditions for future biomonitoring assessments, and determine pre‐industrial baseline conditions to provide historical context for modern diatom distributions.Diatom assemblages were assessed using information from ongoing regulatory monitoring programmes, individual research projects, and from surface sediment layers obtained from lake cores. Pre‐industrial baseline conditions as well as the nature, direction and magnitude of changes in diatom assemblages over the past c.200 years were determined by comparing surface sediment samples (i.e. containing modern assemblages) with a sediment interval deposited prior to the onset of significant anthropogenic activities (i.e. containing pre‐1850 assemblages), together with an examination of diatoms preserved in contiguous samples from dated sediment cores.We identified several biotypes with distinct diatom assemblages using contemporary diatom data from both lakes and streams, including a biotype typical for High Arctic regions. Differences in diatom assemblage composition across circum‐Arctic regions were gradual rather than abrupt. Species richness was lowest in High Arctic regions compared to Low Arctic and sub‐Arctic regions, and higher in lakes than in streams. Dominant diatom taxa were not endemic to the Arctic. Species richness in both lakes and streams reached maximum values between 60°N and 75°N but was highly variable, probably reflecting differences in local and regional environmental factors and possibly sampling effort.We found clear taxon‐specific differences between contemporary and pre‐industrial samples that were often specific to both ecozone and lake depth. Regional patterns of species turnover (β‐diversity) in the past c.200 years revealed that regions of the Canadian High Arctic and the Hudson Bay Lowlands to the south showed most compositional change, whereas the easternmost regions of the Canadian Arctic changed least. As shown in previous Arctic diatom studies, global warming has already affected these remote high latitude ecosystems.Our results provide reference conditions for future environmental monitoring programmes in the Arctic. Furthermore, diatom taxa identification and harmonisation require improvement, starting with circum‐Arctic intercalibrations. Despite the challenges posed by the remoteness of the Arctic, our study shows the need for routine monitoring programmes that have a wide geographical coverage for both streams and lakes. [ABSTRACT FROM AUTHOR]