The Athabasca River regulates methylmercury burdens of waterbirds breeding downstream

This study characterized the transport pathways and sources of mercury in downstream food webs of the Athabasca River Basin in northern Alberta, Canada. Flowing through a large boreal watershed in the Athabasca Oil Sands Region, the river drains into the ecologically-sensitive Peace-Athabasca Delta and western Lake Athabasca. Mercury stable isotope measurements on abiotic and biotic matrices were combined with spatial and temporal sampling to evaluate the importance of the Athabasca River as a conduit for mercury. Mixing model estimates derived from mercury isotopes indicated the Athabasca River was the source of 62–94% of the bioaccumulated mercury in otter, fish, and tern eggs collected from the delta or lake. A time series from 2009 to 2022 showed mercury loads from the Athabasca River enhanced bioaccumulation in western Lake Athabasca with the doubling of total mercury (THg) concentration in eggs of breeding terns following high flow years compared to low flow years. Mercury isotope data from potential abiotic sources (Athabasca River and Lake sediment, leaf litter, soil, air, rain) suggested a predominately terrestrial origin of mercury in fish of the Athabasca River. The similarity of terrestrial mercury isotope signatures with oil sands-related matrices (natural bitumen seeps, industry samples) precluded an estimation of contributions from oil sands operations to mercury bioaccumulation. Catchment processes or disturbances that increase mercury loads in the Athabasca River will increase mercury concentrations of aquatic biota in downstream receiving waters.