Mercury in wintering American black ducks (Anas rubripes) downstream from a point-source on the lower Penobscot River, Maine, USA

Highlights: • Mercury (Hg) exceeds safe consumption limits in waterfowl during the fall hunting season along the lower Penobscot River, Maine • Primary feather Hg concentrations document low mercury exposure on breeding grounds • Mercury concentrations increase with residence time during fall and winter • Mercury concentrations in duck blood and breast muscle are strongly correlated • By spring, Hg concentrations in ducks exceed levels associated with reproductive toxicity and may carry-over to summer breeding grounds Waterfowl wintering along the lower Penobscot River, Maine continue to be exposed to elevated Hg concentrations from the HoltraChem chlor-alkali plant that operated along the river between 1967 and 2000. In American black ducks (Anas rubripes) total Hg in duck breast muscle increased with residence time on contaminated marshes, reaching means of 0.82 ± 0.21 μg/g ww (wet weight) by the end of the fall hunting season, and prompting Maine to issue a human consumption advisory on duck breast muscle. Methyl Hg comprised over 99% of the total Hg in breast muscle. The ratio of Hg concentrations in blood and muscle were strongly correlated and approached 1:1 after extended residence times. Primary feather (P1) total Hg concentrations averaged 2.2 ± 1.3 μg/g fw (fresh weight), verifying low Hg exposure during feather growth on distant breeding grounds the preceding summer. Mercury concentrations in black ducks, following winter residence along the lower Penobscot exceeded levels associated with reproductive toxicity. Carry-over of Hg to summer breeding grounds may limit the subsequent reproductive success of black ducks.