Enclosure (1) TO COMDTINST 16478.12
Methyl mercury was measured in approximately 20% of the sediment samples to
asses the form of mercury present and provide insight to the potential
biological effects of mercury in the sediment. Methyl mercury is generally
considered to be more toxic than other forms of mercury and more easily
accumulated in tissue of aquatic organisms (USEPA 1985). However methyl
mercury is rarely measured in marine samples limiting the quantifability of
toxicity or bio availability of the methyl form in sediment. Consequently,
the actual levels of methyl mercury found could not be compared to any
screening levels or other bench marcks. Instead, the relative proportion of
methyl mercury present in sediments, compared to similar background stations
or values reported in the literature was used to assess results.
Similarly, methylation rates are difficult to measure and generally only
attempted under highly controlled conditions. It was not feasible to measure
methylation rates as part of this investigation. In the absence of direct
measurements, the percentage of total mercury present in the methyl form was
used as a relative measure of methylation rate. This is based on the
assumption that all other variables being similar, the higher the percentage
of methyl mercury, the higher the rate of methylation or the presence of a
source of methyl mercury. If higher rates, or a source of methyl mercury was
indicated, there could be cause for concern. Methyl mercury occuring as
approximately one percent of total mercury, as reported in the literature
(Baudo et al 1990), was used in this investigation as the expected range. A
percentage above this level was considered an indication of either higher
than expected methyation rates or a direct source methyl mercury. This is
only an assumption but no data was available to more accurately assess rates
of methylation or concern over levels of methyl mercury in sediments.
Organisms at the highest trophic levels bioconcentrate mercury predominantly
through the food chain; the higher the trophic level of the fish or animal,
the greater the likelihood of biomagnification. This is especially true for
longer-lived organisms such as predatory fish, fish-eating mammals, and
predatory birds. Since methyl mercury is more easily assimilated and it can
readily bioaccumulate, it is an important factor to be considered in sediment
studies involving ecological risk.
The assessed level of methyl mercury in all locations sampled during the
prototype investigations was less than one percent of total mercury. The
following graphs illustrate both the low percentage of methyl mercury in
sediment samples, and the fact the methyl mercury levels are uncorrelated to
the total mercury levels in sediments.
In the Chesapeake by, the methyl mercury values reported for AtoN stations
were minimal, less than 1 percent at all locations, and except for the very
silty
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