Differential trends in mercury concentrations in double-crested cormorant populations of the Canadian Prairies.

Mercury and selenium concentrations were measured in double-crested cormorants (Phalacrocorax auritus), piscivorous fish, and common prey items in five lakes in two ecoregions in Saskatchewan, Canada. Hg and Se concentrations in cormorants were within the natural ranges of birds living in un-impacted sites. Site explained a significant proportion of the variation in total Hg (THg) and methylmercury (MeHg) concentrations in both cormorant breast muscle and livers. Birds nesting on more northern lakes in the Boreal Plain ecoregion (THg range 0.11-1.06 and 0.26-9.27 μg g(-1) wet weight, for breast and liver respectively) had lower THg concentrations compared to those from lakes in the Prairie ecoregion (THg range 0.60-4.26 μg g(-1) ww and 1.59-25.11 μg g(-1), for breast and liver respectively). Concentrations of MeHg in livers was also lower in birds from northern sites (0.06-1.15 μg g(-1) ww) compared to those from prairie sites (0.22-4.06 μg g(-1) ww). We documented a wide range of %MeHg in livers (4.5-52 %), indicative of detoxifying MeHg via demethylation to inorganic Hg. Our data suggest that the threshold value where demethylation rates increase substantially appears to be ~10 μg g(-1) ww MeHg, similar to thresholds in other wildlife. Molar ratios of Hg:Se suggests that some birds from highly saline Reed Lake in the prairie region had insufficient Se available to bind to Hg, thereby removing Se binding as a mitigative strategy for high Hg levels for these birds.