Abstract
This study examined factors contributing to temporal variability (2009–2017) in total mercury (THg) concentrations in aquatic bird eggs collected in the Peace-Athabasca Delta and Lake Athabasca in northern Alberta. Factors examined included year of egg collection, site of collection, bird species, bird diets, annual surface-mineable oil sands production, forest fires, and flow of the Athabasca River. Surface mining activities associated with Alberta’s Athabasca oil sands are situated north of Fort McMurray, Alberta, adjacent to the northward-flowing Athabasca River. Previous studies have found that oil sands industrial operations release mercury into the local (within ~50 km) environment. An information-theoretic approach revealed that the best model for explaining egg THg levels included Athabasca River flow, bird food source, and bird species. Variability in egg THg levels was partly a reflection of differences in food sources, e.g. proportions of aquatic versus terrestrial food in bird diets. Annual fluctuations in maximal flow of the Athabasca River were also important with eggs collected following years of high maximal flow exhibiting higher THg concentrations. Furthermore, eggs collected in years of high versus low flow differed in their stable Hg isotope composition with less mass-independent fraction of 199Hg and 201Hg in years of high flow. Riverine processes associated with suspended sediment were likely critical in regulating Hg availability to nesting birds. This study highlights the importance of the Athabasca River as a conduit for Hg transport to ecologically-sensitive downstream ecosystems such as the Peace-Athabasca Delta and Wood Buffalo National Park (a UNESCO World Heritage Site). Human activities that increase atmospheric Hg deposition to the Athabasca River watershed, or that enhance Hg releases to the river through erosion of Hg-bearing soils, will likely increase the availability of Hg to organisms inhabiting downstream areas.
Highlights
The Athabasca River is a major river flowing northeast 1200 km from the Rocky Mountains to Lake Athabasca
Egg THg concentrations were not correlated with year of collection for Egg Island Caspian Terns (n = 80, r = -0.01, p = 0.97), Egg Island Common Terns (n = 70, r = 0.08, p = 0.53), or Mamawi Lake Ring-billed Gulls (n = 83, r = -0.13, p = 0.24) (Fig 3A and 3B)
An information-theoretic approach based upon ΔAICc indicated that egg THg data were best explained by a model including bird species, egg δ13C values, and Athabasca River flow (see S3 Table for a complete list of model ΔAICc and weights)
Summary
The Athabasca River is a major river flowing northeast 1200 km from the Rocky Mountains to Lake Athabasca. Along the way, it passes through Alberta’s oil sands, a region of large-scale, open-pit mines used to extract bitumen for synthetic oil production. Snow Hg levels declined rapidly with distance from such developments with most Hg deposited within approximately 50 km. These studies [2, 3] highlighted the possibility that spring snowmelt could result in the release of chemicals, such as Hg, into the aquatic environment.
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