Mercury Isotope Study of Sources and Exposure Pathways of Methylmercury in Estuarine Food Webs in the Northeastern US

We measured mercury (Hg) isotope ratios in sediments and various estuarine organisms (green crab, blue mussel, killifish, eider) to investigate methylmercury (MMHg) sources and exposure pathways in five Northeast coast (U.S.) estuaries. The mass independent Hg isotopic compositions (MIF; Delta Hg-199) of the sediments were linearly correlated with the sediment 1/Hg concentrations (Delta Hg-199: r(2) = 0.77, p < 0.05), but the mass dependent isotope compositions (MDF; delta Hg-202) were not (r(2) = 0.26, p = 0.16), reflecting inputs of anthropogenic Hg sources with varying delta Hg-202. The estuarine organisms all display positive Delta Hg-199 values (0.21 to 0.98 parts per thousand) indicating that MMHg is photodegraded to varying degrees (5-12%) prior to entry into the food web. The delta Hg-202 and Delta Hg-199 values of most organisms can be explained by a mixture of MMHg and inorganic Hg from sediments. At one contaminated site mussels have anomalously high delta Hg-202, indicating exposure to a second pool of MMHg, compared to sediment, crabs and fish. Eiders have similar Delta Hg-199 as killifish but much higher delta Hg-202, suggesting that there is an internal fractionation of delta Hg-202 in birds. Our study shows that Hg isotopes can be used to identify multiple anthropogenic inorganic Hg and MMHg sources and determine the degree of photodegradation of MMHg in estuarine food webs.