Mercury levels were measured in colonial waterbird eggs collected from two sites in northern Alberta and one site in southern Alberta, Canada. Northern sites in the Peace-Athabasca Delta and Lake Athabasca were located in receiving waters of the Athabasca River which drains the oil sands industrial region north of Fort McMurray, Alberta. Temporal trends in egg mercury (Hg) levels were assessed as were egg stable nitrogen isotope values as an indicator of dietary change. In northern Alberta, California and Ring-billed Gulls exhibited statistically significant increases in egg Hg concentrations in 2012 compared to data from the earliest year of sampling. Hg levels in Caspian and Common Tern eggs showed a nonstatistically significant increase. In southern Alberta, Hg concentrations in California Gull eggs declined significantly through time. Bird dietary change was not responsible for any of these trends. Neither were egg Hg trends related to recent forest fires. Differences in egg Hg temporal trends between northern and southern Alberta combined with greater Hg levels in eggs from northern Alberta identified the likely importance of local Hg sources in regulating regional Hg trends. Hg concentrations in gull and Common Tern eggs were generally below generic thresholds associated with toxic effects in birds. However, in 2012, Hg levels in the majority of Caspian Tern eggs exceeded the lower toxicity threshold. Increasing Hg levels in eggs of multiple species nesting downstream of the oil sands region of northern Alberta warrant continued monitoring and research to further evaluate Hg trends and to conclusively identify sources.

Elders and Indigenous land users in the Peace-Athabasca Delta (PAD) have observed a dramatic decline in the relative abundance of muskrat in recent decades (~1935–2014). The main explanation for the decline has been reduction in suitable habitat as a result of decades with reduced frequency of ice-jam flooding on the Peace River. Under favourable conditions, ice jams can cause flooding of perched basins within the PAD that would otherwise receive no recharge from floodwaters. To examine whether abundance of muskrat in the PAD is driven by flooding, we tested the predictions that the density of muskrat (estimated by winter counts of houses) (1) was inversely related to the number of years since major ice jam floods and (2) increased with water depth. An ongoing collaborative monitoring program initiated in 2011, combined with analysis of data from past surveys (1973–2015), allowed Indigenous land users and scientists to document a 10 to 100-fold increase in the density of muskrat houses in 24 basins, over the two years following ice-jam flood events in the PAD. During 1973–2015, in the periods between major floods, density of houses dropped by approximately 79% for every year after a significant flood. In 27 basins surveyed from 2011 to 2015, density of muskrat houses increased by two orders of magnitude in the two years following a flood in the spring of 2014. Density of muskrat houses had a non-linear relationship with estimated depth of water at the time of fall freeze-up; the highest densities of muskrat houses were in basins with about 60 – 250 cm of water at the time of freeze-up. The depth of snow at the time of surveys did not have a strong relationship with the density of muskrat houses. However, few houses were counted in basins with more than 20 cm of snow, likely because deeper snow made it more difficult to conduct surveys and spot houses. Factors other than an increase in the depth of water at fall freeze-up may provide the mechanisms by which flooding affects muskrat. Density of muskrat houses is clearly tied to ice-jam flooding in the PAD. However, the local mechanisms by which floods affect muskrat are best understood by Indigenous land users and remain poorly understood by Western science. Indigenous peoples continue to regard muskrat as an indicator of ecological and cultural health of the PAD. This study highlights the value of consistent ecological monitoring that includes Indigenous knowledge.