Mercury Isotope Ratios used to Determine Sources of Mercury to Fish in Northeast U.S. Streams

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Mercury isotope analyses were used to distinguish different sources of mercury to fish in 23 streams along a forested-rural to urban-industrial land-use gradient in the Northeastern United States – the first time biological tissues have been used to map mercury sources across a geographic region. The use of mercury isotope measurements in fish tissue allow for distinguishing different sources of mercury that are bioaccumulating into the food web. Mercury isotope signatures in fish in forested-rural areas indicated atmospheric sources of mercury. In contrast the isotope signatures in fish in urban-industrial areas are indicative of past (legacy) and present local sources.

Eating fish is a source of mercury exposure for many people in the United States

Fish are a source of food for many people in the United States.

(Credit: Dennis Wentz, USGS ex-employee. Public domain.)

Mercury in fish tissues is an important issue due to perceived and established health concerns. In streams within the Northeastern United States, mercury levels in fish tissue frequently exceed concentrations associated with toxicity to humans and wildlife. These streams receive mercury in varying proportions from distant atmospheric deposition as well as from local ongoing and legacy point sources, making it difficult to understand the sources of mercury in fish. Without this information it is difficult to link shifts in fish mercury concentrations directly back to changes in mercury source management and mitigation efforts.

U.S. Geological Survey (USGS) scientists provided a tool to differentiate between mercury sources in both sediments and fish using mercury stable isotope analysis. They analyzed mercury isotopes in fish tissue and streambed sediments collected from 23 streams across a forested-rural to urban-industrial land-use gradient in the Northeastern United States.

In this first regional scale effort, mercury isotope signatures in fish were associated to land-use variables. For example, mercury isotopes in prey and game fish in forested-rural areas were depleted and indicated atmospheric sources of mercury deposited onto the land surface that entered streams through runoff. In comparison, the prey and game fish from urban-industrial areas had enriched signals that indicated legacy sources. These unique isotope signatures were not related to differences in species, diet, habitat, growth, or potential photochemical effects. Isotopic signatures in bottom sediments corresponded to land use but the compositions were different than those in fish collected at the same site indicating that factors controlling mercury accumulation in fish and sediment differ.

Understanding sources of mercury in fish is important because fish are the dominant mercury exposure pathway of methylmercury to humans and to fish-consuming wildlife. This study revealed that mercury isotope measurements in fish tissue provide a tool for distinguishing different sources of mercury that are bioaccumulating into the food web.

The scientists on the USGS Ecologically-Driven Exposure Pathways Science Team are continuing to develop these advanced tools and analytical methods that provide an understanding of health risks associated with mercury exposure. These tools are critical to understanding the effectiveness of mercury-reduction efforts in reducing health risks to humans and other organisms.

This work was supported as part of the Northeast Stream Quality Assessment portion of the National Water-Quality Assessment (NAWQA), the USGS Toxic Substances Hydrology and Contaminant Biology Programs combined, and New York State Energy Research and Development Authority (NYSERDA) contract # 37346.