Mercury and Stable Isotopes in Common Terns (Sterna Hirundo) from the St. Lawrence River: A Comparison Between Breeding and Winter Habitats
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The Common tern (Sterna hirundo) is considered a sentinel wildlife species for monitoring mercury (Hg) and other contaminants within the St. Lawrence River Cornwall/Massena Areas of Concern (AOC). Here, I investigate the relationship between Hg bioaccumulation and diet using stable isotopes of carbon (δ13C) and nitrogen (δ15N) in adult and chick Common terns from three colonies along a 160 km transect of the upper St. Lawrence River. The foraging range of the colony furthest downstream (EMC) includes both the Massena and Cornwall AOC’s while the two upstream colonies (213 and Ice) are more removed from known point sources of Hg. I also sampled winter- and summer-grown breast feathers to compare diet and Hg exposure on the terns’ breeding ground vs. the terns’ wintering grounds. Hg exposure in summer-grown feathers was significantly higher than Hg exposure in winter grown feathers. Stable carbon (δ13C) and nitrogen (δ15N) isotopes revealed a switch from a marine-based diet during the winter months to a freshwater-based diet on the breeding grounds. Among colonies in summer-grown and chick feathers, the only significant difference in total mercury (THg) exposure was found in chick feathers where Hg was significantly lower at 213 than Ice – both of which are upstream from the AOC’s. However, THg was negatively correlated with δ13C in both winter and summer feathers, and the most parsimonious multi-regression model for winter and summer feathers indicated that δ13C explains 24 and 25% of the variation in Hg exposure, respectively. This suggests terns foraging offshore bioaccumulate more Hg than individuals foraging inshore or in freshwater (winter feathers), and that during the breeding period, terns foraging in pelagic habitats bioaccumulate more Hg than terns foraging in littoral habitats (summer feathers). For the upper St. Lawrence River, these results provide strong evidence that foraging habitat is more important than colony location in determining Hg exposure in a top trophic consumer.