From: Roger Greenway, ENN
Published August 20, 2009 06:39 AM

Mercury Persists in Fish in Many Parts of US

The U.S. Geological Survey released a study that showed mercury contamination in every fish sampled in 291 streams across the country. The work was part of the National Water-Quality Assessment Program (NAWQA) which provides an understanding of water-quality conditions such as whether conditions are getting better or worse over time and how natural features and human activities affect those conditions. Regional and national assessments are possible because of a consistent study design and uniform methods of data collection and analysis.

About a quarter of the fish were found to contain mercury at levels exceeding the criterion for the protection of people who consume average amounts of fish, established by the U.S. Environmental Protection Agency. More than two-thirds of the fish exceeded the U.S. EPA level of concern for fish-eating mammals.

"This study shows just how widespread mercury pollution has become in our air, watersheds, and many of our fish in freshwater streams," said Secretary of the Interior Ken Salazar. "This science sends a clear message that our country must continue to confront pollution, restore our nation’s waterways, and protect the public from potential health dangers."

Some of the highest levels of mercury in fish were found in the tea-colored or "blackwater" streams in North and South Carolina, Georgia, Florida and Louisiana — areas associated with relatively undeveloped forested watersheds containing abundant wetlands compared to the rest of the country. High levels of mercury in fish also were found in relatively undeveloped watersheds in the Northeast and the Upper Midwest. Elevated levels are noted in areas of the Western United States affected by mining.

Mercury, a neurotoxin, is one of the most serious contaminants threatening our nation's waters. The main source of mercury to natural waters is mercury that is emitted to the atmosphere and deposited onto watersheds by precipitation. However, atmospheric mercury alone does not explain contamination in fish in our nation's streams. Naturally occurring watershed features, like wetlands and forests, can enhance the conversion of mercury to the toxic form, methylmercury. Methylmercury is readily taken up by aquatic organisms, resulting in contamination in fish.

An unexpected finding was that the production of methylmercury in channel sediment within the streams themselves appears to be relatively unimportant for controlling methylmercury in stream water.

Stream water provides methylmercury to the base of the food chain, and it is the amount of methylmercury in the water that is the primary driver of how much mercury that accumulates in top predator fish. In general, concentrations in fish increased with increasing concentrations of methylmercury in water.

Once in the food web, methylmercury biomagnifies at a fairly consistent rate from algae to invertebrates to fish—even among diverse stream ecosystems. In the ecosystems studied, foodweb characteristics have less impact on the amount of mercury in fish than do methylmercury levels in water.

The map shows the streams in the study with mercury levels noted by color.

For more information, including links to a list of advisories on what streams are safe to fish:

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