Environmental Origins of Methylmercury Accumulated in Subarctic Estuarine Fish Indicated by Mercury Stable Isotopes

Methylmercury (MeHg) exposure can cause adverse reproductive and neurodevelopmental health effects. Estuarine fish may be exposed to MeHg produced in rivers and their watersheds, benthic sediment, and the marine water column, but the relative importance of each source is poorly understood. We measured stable isotopes of mercury (delta Hg-202, delta Hg-199, and delta Hg-201), carbon (delta C-13), and nitrogen (delta N-15) in fish with contrasting habitats from a large subarctic coastal ecosystem to better understand MeHg exposure sources. We identify two distinct food chains exposed to predominantly freshwater and marine MeHg sources but do not find evidence for a benthic marine MeHg signature. This is consistent with our previous research showing benthic sediment is a net sink for MeHg in the estuary. Marine fish display lower and less variable delta Hg-199 values (0.78 parts per thousand to 1.77 parts per thousand) than freshwater fish (0.72 parts per thousand to 3.14 parts per thousand) and higher delta Hg-202 values (marine: 0.1 parts per thousand to 0.57 parts per thousand; freshwater: -0.76 parts per thousand to 0.15 parts per thousand). We observe a shift in the Hg isotopic composition of juvenile and adult rainbow smelt (Osmerus mordax) when they transition between the freshwater and marine environment as their dominant foraging territory. The Hg isotopic composition of Atlantic salmon (Salmo salar) indicates they receive most of their MeHg from the marine environment despite a similar or longer duration spent in freshwater regions. We conclude that stable Hg isotopes effectively track fish MeHg exposure sources across different ontogenic stages.