Environmental Engineering Reference
In-Depth Information
in increased Hg concentrations in each of four seston size
classes (0.2-2, 2-20, 20-200, and >153 µm), suggesting
that the supply of Hg to lake ecosystems is important to Hg
partitioning at the base of the food web. Mercury concen-
trations in sestons varied among the lakes and among the
three size fractions of the microbial planktonic food web.
Picoseston (0.2-2 µm) made up the majority of the particu-
late Hg pool in each lake and had the highest average Hg
concentrations (mean 361
enhanced deposition due to the scavenging of GEM and
Hg
2+
by the canopy. The shallow surfi cial deposits facili-
tate the transport of Hg
2+
from uplands to downstream
wetlands and lakes, particularly through the mobilization
of DOC during high-fl ow events. Wetlands are a promi-
nent feature of the Adirondack landscape and important
zones of MeHg production. Adirondack lakes are nutrient-
poor and unproductive. This condition limits plankton
and fi sh biomass and likely enhances fi sh Hg concentra-
tion. Finally, there is an important linkage between Hg
contamination and elevated acidic deposition in the
Adirondacks. Sulfate input from acidic deposition is a crit-
ical substrate for sulfate-reducing bacteria, which produce
MeHg. Also there is the widespread relationship between
increases in fi sh Hg with decreases in pH. The mechanism
driving this relationship is unclear, but trends in fi sh Hg
suggest that acidifi cation may limit fi sh growth, result-
ing in associated high concentrations of Hg in fi sh tissue;
improvements in fi sh growth may cause decreases in fi sh
Hg concentrations.
319 ng g
1
dry weight [dw]).
Mean Hg concentrations for the nanoseston (2-20 µm) and
microseston (
20-200 µm) size classes were lower than the
picoseston size class and ranged from 114
79 ng g
1
to
135
61 ng g
1
dw, respectively. Seston density across all
size fractions was negatively correlated with seston Hg con-
centrations and signifi cantly predicted Hg concentrations
in the 0.2-2 µm size class. Note that seston Hg was not
related to chlorophyll concentration, suggesting that bacte-
rial production is an important carbon source at the base
of the food chain in these unproductive lakes. Mercury in
bulk zooplankton (
153 µm) ranged from 105 to 613 ng g
1
with an average 38
18% of THg as MeHg (Adams et al.,
2009).
Over the past two decades, fi sh with elevated concen-
trations of Hg have been observed in remote lake dis-
tricts, including the Adirondacks (Driscoll et al., 1995).
Studies across eastern North America have shown that
fi sh Hg concentrations increase with decreases in lake pH.
Controls in emissions of sulfur
dioxide have resulted in
some improvement in the acid-base status of Adirondack
lakes. In addition, decreases in atmospheric Hg deposi-
tion have also occurred. In 1992-1993 and again in 2005-
2006, 25 Adirondack lakes were surveyed to analyze pat-
terns of Hg in the water column and yellow perch (
Perca
fl avescens
) and changes in these patterns (Dittman and
Driscoll, 2009). During the 1992-1993 survey 64% of the
yellow perch surveyed (n
Reservoirs
The impoundment of reservoirs for hydroelectric genera-
tion, fl ood control, recreation, waste management, and
other purposes continues to increase worldwide, modi-
fying global carbon cycling and exacerbating local Hg
contamination (Kelly et al., 1997; St. Louis et al., 2000).
Mercury sources in reservoirs are largely identical to
those of natural water bodies: atmospheric deposition
of Hg from local and distant sources due to combustion
of fossil fuels, waste incineration, and other combustion
processes; and, wastewater and industrial point source
discharges (Arnason and Fletcher, 2003; Abbott and
Kotchenruler, 2006; Driscoll et al., 2007; Park et al., 2008).
Despite this commonality, Hg concentrations in biota are
universally elevated in reservoirs relative to other fresh-
water aquatic environments (Evers et al., 2007), a fi nding
that has prompted increasing research and management
attention at the international (e.g., World Commission
on Dams, 2000) and regional levels (e.g., Mason and
Sveinsdottir, 2003). The United States Federal Energy
Regulatory Commission now routinely addresses Hg con-
tamination in their relicensing documents for U.S. hydro-
electric facilities. Mercury contamination in reservoirs is
a particular concern from the perspectives of human
health and ecologic integrity, as many of these systems
are vast in scale and support large fi sheries and complex
ecologic communities. In a general sense, elevated biotic
Hg concentrations in reservoirs stem from two major
effects: the creation of the reservoirs; and the ongoing
management of those reservoirs (Evers et al., 2007). In
this section, we describe factors within these two gen-
eral aspects that result in elevated Hg in biota due to
enhanced MeHg production.
725) exceeded the United States
Environmental Protection Agency action limit for fi sh Hg
of 0.3 µg g
1
(ww). The percentage of yellow perch exceed-
ing the 0.3 µg g
1
(ww) action limit decreased to 49% in
the 2005-2006 survey (n
1154). Twelve lakes exhibited
a decrease in perch Hg, six lakes showed an increase, and
seven lakes had no change. Four key variables infl uenced
the change in perch Hg concentrations in the Adirondacks:
watershed area, elevation, change in pH, and change in fi sh
condition. Dittman and Driscoll (2009) speculated that as
the acidity in lakes is attenuated, the lakes may become
more productive and water-quality conditions less stress-
ful to fi sh, leading to improved fi sh conditions. As fi sh
body conditions and growth rates improve, fi sh can exhibit
“growth dilution” of tissue contaminants, leading to lower
fi sh Hg levels, as seen in other studies (Karimi et al., 2007;
Ward et al., 2009).
In summary, Hg contamination is widespread in the
Adirondacks, despite its remote location and moderate
inputs of wet Hg deposition. These characteristics point to
the Hg sensitivity of the region. The forest cover facilitates
