Freshwater Contaminants in Southwest Alaska
Mercury is a toxic element with no known essential biological function. It occurs naturally as a solid in various minerals and as a gas in volcanic eruptions. It is also released into the environment as a gas by human activities, particularly mining and coal burning. Gaseous elemental mercury can be transported atmospherically to areas far from its original emission sources. Then it can be oxidized and deposited on the earth’s surface in the form of rain or snow. Under anaerobic conditions, typically in wetland sediments, bacteria can convert this deposited inorganic mercury to organic methylmercury, which is five to ten times more toxic. Methylmercury tends to accumulate to greater concentrations in aquatic (rather than terrestrial) organisms. In fish, methylmercury readily crosses biological membranes, excretes slowly relative to its rate of uptake, and accumulates to concentrations vastly exceeding those in surface waters.
FindingsThe National Park Service (NPS) partnered with the U.S. Geological Survey (USGS) in a multi-year study of the factors controlling fish mercury levels at thirteen lakes in Katmai and Lake Clark national parks and preserves. The first two years of the study involved sampling water, sediment, plankton, and fish—both lake trout (resident) and sockeye salmon (migratory). Preliminary results suggest that lake trout exhibit a wide range of mercury concentrations among parks and among lakes (Figure 1). Also, median mercury concentrations of lake trout in six of the thirteen lakes exceed the State’s guidance on unlimited consumption of Alaska-caught fish by at-risk groups (200 ng/g wet weight or 16 meals/month; Hamade 2014). By comparison, median mercury concentrations of sockeye salmon are consistently low and exhibit little variability. Differences in mercury levels between these two species are likely due to differences in age. Sockeye salmon rarely live past six years. Lake trout age in this study ranged from seven to twenty-six years.
MethodsWe collected samples from thirteen lakes selected to represent a range of local factors capable of influencing the distribution of distantly derived mercury (Nagorski et al. 2014; Figure 2). Samples were then analyzed for total mercury, methylmercury, and other analytes including stable isotopes of mercury, carbon, and nitrogen. We are currently developing quantitative models to relate mercury in lake trout with potential local drivers. We are also drafting a protocol for long-term monitoring of mercury in lake trout in Southwest Alaska lakes.
ReferencesHamade, A.K. 2014. Fish consumption advice for Alaskans: A risk management strategy to optimize the public’s health. State of Alaska, Section of Epidemiology. Anchorage, Alaska.
Nagorski, S. A., and others. 2014. Spatial distribution of mercury in southeastern Alaskan streams influenced by glaciers, wetlands, and salmon. Environmental Pollution 184:62-62.