Central Nebraska Basins Study Unit

Mercury in Stream Ecosystems

Methylmercury, the most toxic form of mercury, accumulates in low-level members of aquatic food webs, and its concentration in tissue progressively increases as higher-level predators consume lower-level organisms. At the top of the food web, fish, wildlife, and humans that consume fish can be exposed to harmful levels of methylmercury. As part of NAWQA, a study of mercury contamination in streams across the U.S. was designed to describe the occurrence of total mercury and methylmercury in water and sediment, and total mercury in fish. Sampling sites were selected to represent the large national range of mercury deposition, and across the ranges of the transport and methylation of mercury in differing aquatic ecosystems. The study of mercury occurrence complements additional NAWQA work to examine in detail the cycling of mercury in the environment and food webs at a small subset of the sites.

Of approximately 112 sites sampled nationally in 2002, 8 are in Nebraska ( map ). Samples of fish fillets, water, and bed sediments were collected during low-flow conditions in July 2002. Widely available predacious fish species, of approximately 3 to 4 years of age, were targeted for sampling. Channel catfish were sampled at five Nebraska sites, and black crappie, creek chub, and largemouth bass were sampled at one site each. Laboratories determined mercury concentrations in the water, sediment, and tissue, and will verify fish-specimen age.

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