Regional differences in mercury levels in aquatic ecosystems: A discussion of possible causal factors with implications for the Tennessee river system and the Northern Hemisphere |
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Authors: | J Devereux Joslin |
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Institution: | (1) Atmospheric Science Department Tennessee Valley Authority, Oak Ridge National Laboratory, P. O. Box 2008, 37831-6034 Oak Ridge, Tennessee, USA |
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Abstract: | Concern about mercury pollution from atmospheric deposition has risen markedly in the last decade because of high levels of
mercury in freshwater fish from relatively pristine waters. Whereas high concentrations have been found principally in Canada,
the northern United States, and Scandinavia, they have also recently been observed throughout much of Florida. Recent surveys
of the Tennessee River system, however, have found no locations where fish levels exceed EPA guidelines for fish consumption.
This paper evaluates a number of factors that may cause certain regions in the northern hemisphere to experience unacceptable
fish mercury levels while other regions do not. Relevant regional differences include: (1) Waters of the Tennessee River system
are generally nonacidic (pH>6) and well buffered, whereas 16%, 22%, and 40% of the lakes in upper Midwest, Northeast, and
Florida, respectively, have acid-neutralizing capacities below 50 μeq/liter. Acidity correlates highly with fish mercury levels
in a number of lake surveys, and experimental manipulations of acidity have significantly raised fish mercury levels. (2)
The ratio of land area to water surface area in the Tennessee Valley averages about 30, whereas it is 15 in the upper Midwest
and 6 in Florida. Low ratios allow mercury in precipitation to be directly deposited to aquatic bodies, without an opportunity
for the mercury to be sequestered by terrestrial ecosystems. (3) Stream organic matter concentrations in Florida, the upper
Midwest, and Sweden are 2–10 times those in the Tennessee Valley. Mercury binds strongly to organic matter, and organic matter
transport in runoff is a major pathway by which mercury enters aquatic ecosystems. |
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Keywords: | Mercury Tennessee River Fish toxicity Organic matter Acidification Atmospheric Deposition |
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