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August 20, 2019

Isotopes of mercury in fish can indicate the source of that mercury, reports a new study from the USGS Regional Stream Quality Assessment. 

The isotopic signature of mercury in fish in urban regions of the Northeast indicates that the mercury comes from current and legacy point sources, such as industrial activities. In more rural areas, the mercury signature in fish indicates that the mercury has undergone long-range atmospheric transport.

Mercury is a potent neurotoxin that accumulates in the tissue of fish and other organisms, and humans can be exposed to mercury by consuming fish that have bioaccumulated mercury. There are numerous statewide and water-body-specific fishing advisories across the United States for mercury. In areas with many potential mercury sources, such as the Northeast, one challenge is to determine which sources are the greatest contributors to mercury in fish. Identification of mercury sources can inform management actions in regions with mercury fishing advisories and improve our understanding of the bioavailability of mercury to fish and the rest of the food web.

Using 69 fish from 23 small streams in the Northeast and 3 isotopic tracers of mercury in the fish tissue, researchers determined that point sources, such as industrial emissions, were the predominant source of mercury in fish in urban areas with a relatively large amount of current or past industrial activity. In contrast, mercury in fish in rural streams was from atmospheric deposition of mercury onto the land surface that then washes into streams with storm runoff. These patterns were consistent across both game and prey fish. This is one of the first studies to use mercury isotopes in fish tissue to track mercury sources.

Sediments also had mercury isotopic compositions that corresponded to land use but the compositions were different than those in fish collected at the same site. This indicates that mercury in both fish and sediment is affected by factors such as land use and proximity to point sources, but that mercury in fish and sediment are not necessarily directly linked, as has been commonly assumed.

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