Assessment of multi-trophic changes in a shallow boreal lake simultaneously exposed to climate change and aerial deposition of contaminants from the Athabasca Oil Sands Region, Canada

Abstract

The Athabasca Oil Sands Region (AOSR) has been intensely developed for industrial bitumen extraction and upgrading since the 1980s. A paucity of environmental monitoring prior to development raises questions about baseline conditions in freshwater systems in the region and ecological responses to industrial activities. Further, climatic changes prompt questions about the relative roles of climate and industry in shaping aquatic ecosystems through time. We use aquatic bioindicators from multiple trophic levels, concentrations of petrogenic contaminants (dibenzothiophenes), and spectrally-inferred chlorophyll-a preserved in well-dated sediments of a closed-basin, shallow lake ~50km away from the main area of industry, in conjunction with climate observations, to assess how the biotic assemblages of a typical AOSR lake have changed during the past ~75years. We examine the contributions of the area's stressors in structuring aquatic communities. Increases in sedimentary measures of petrogenic contaminants provide clear evidence of aerial contaminant deposition from local industry since its establishment, while climate records demonstrate consistent warming and a recent period of reduced precipitation. Quantitative comparisons of biological assemblages from before and after the establishment of regional industry find significant (p<0.05) differences; however, the magnitude and overall timing of the changes are not consistent with a threshold-type shift in response to the onset of regional industry. Rather, biotic assemblages from multiple trophic levels suggest transitions to an increasingly complex benthic environment and relatively warmer waters, which, like the increasing trends in inferred primary production, are consistent with a changing climate. These findings highlight the important role of climate conditions in regulating primary production and structuring aquatic communities in these shallow systems.

Keywords: Athabasca Oil Sands Region; Chironomid; Cladoceran; Diatom; Paleolimnology; Shallow lake.