Researchers looked at levels of mercury in tern and gull eggs collected from two sites in northern Alberta, at Egg Island and Mamawi Lake near Fort Chipewyan, and one site in the southern part of the province in 2012.
The study, published earlier this month in the Environmental Science & Technology journal, found that California and Ring-billed gulls at Mamawi Lake exhibited “statistically significant increases” in mercury concentrations in their eggs in 2012, up 139 per cent from levels measured in the earliest year of sampling in 2009.
Caspian and Common tern eggs at Egg Island also showed an increase, though it was much smaller.
Mercury levels in eggs collected at the Langdon Reservoir in southern Alberta, however, declined by 57 per cent from 2008 to 2012.
While concentrations in gull and Common tern eggs were generally below generic toxic thresholds, mercury levels in the majority of Caspian tern eggs in 2012 exceeded the lower toxicity threshold for effects in birds, though the study concluded “it is unlikely that current mercury levels pose a threat to the birds studied there.”
The study noted that neither mercury trends were related to the birds’ diet or from nearby forest fires, and were instead the result of “local sources” possibly related to the oilsands, though more research would be required to conclusively identify the causes.
“Increasing (mercury) levels in eggs of multiple species nesting downstream of the oilsands region of northern Alberta warrant continued monitoring and research to further evaluate (mercury) trends and to conclusively identify sources,” the authors noted.
The birds studied are considered top predators in the region, selected to give researchers insight into mercury contamination throughout the food chain. Gulls and terns predominantly eat small fish, which respond “rapidly to changes in levels of mercury in their environment,” according to the study.