Environmental Engineering Reference
In-Depth Information
CHAPTER 9
Mercury Hotspots in Freshwater Ecosystems
Drivers, Processes, and Patterns
CELIA Y. CHEN, CHARLES T. DRISCOLL, and NEIL C. KAMMAN
MECHANISMS THAT CONTROL MERCURY SENSITIVITY
Supply or Inputs and Transport
Bio-Availability and Transformation
Bio-Accumulation and Trophic Transfer
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s
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MERCURY HOT SPOT CASE STUDIES
Everglades
The Adirondacks
Reservoirs
Large Lake Ecosystems: Lake Michigan
The Nyanza Superfund Site on the Sudbury River, Massachusetts
5b
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5a
4a
4b
4c
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2
3a
3b
Ia
Ib
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CONCLUSIONS
The transformations and fate of the global, regional, and
local contaminant mercury (Hg) have been studied exten-
sively in freshwater ecosystems for the past 15 years (Watras
et al., 1998; Driscoll et al., 2007; Evers et al., 2007). Much
is known about the processes involved in methylmercury
(MeHg) production and fl ux and MeHg bio-accumulation
and trophic transfer in lakes, ponds, and reservoirs. More-
over, studies conducted in a wide range of freshwater
ecosystems have revealed specifi c suites of attributes that
predispose certain systems to being hot spots of MeHg bio-
accumulation. Evers et al. (2007) defi nes a biologic Hg hot-
spot as “a location on the landscape that, compared to the
surrounding landscape, is characterized by elevated con-
centrations of Hg in biota (e.g., fi sh, birds, mammals) that
exceed established human or wildlife health criteria.” Here,
the defi nition is based on bio-accumulation in a biotic
end point rather than levels of emission or deposition at a
particular location. Based on this defi nition and a dataset
of over 7000 observations of Hg concentrations in yellow
perch and common loon blood, fi ve confi rmed and nine
suspected hot spots were identifi ed in the northeast United
States and southeastern Canada (Figure 9.1 and Table 9.1)
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Biologic Mercury Hotspots
Suspected Biologic Mercury Hotspots
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FIGURE 9.1
Mercury hot spots in the northeastern United States and
southeastern Canada. (
Source:
Adapted from Evers et al., 2007.)
(Evers et al., 2007). These 14 areas encompass great variation
in the types of ecosystems and mechanisms that create Hg
hot spots.
Although elevated inputs of ionic Hg (Hg
2
) to ecosystems
are generally thought to result in high concentrations in fi sh,
there are many factors that mediate the ultimate fate and
trophic transfer of Hg in the environment. Elevated MeHg
bio-accumulation in fi sh and piscivorous birds and mam-
mals results from a complex sequence of biotic and abiotic
mechanisms that control the transport and availability of
Hg
2
, MeHg production, bio-accumulation, or biomagnifi ca-
tion. These mechanisms occur at critical points in the trans-
fer of Hg in the environment (Figure 9.2) which include:
(1) Hg supply and inputs to the ecosystem (e.g., deposition
to the landscape); (2) transport to aquatic ecosystems (e.g.,
