Environmental Engineering Reference
In-Depth Information
spiders, and amphipods, which is supported by their
13
C
Mercury in Coastal Wetland Ecosystems
and
15
N values. The percentage of mercury as MMHg in
amphipods (
Orchestia traskiana
) and beetles (
Cicindela
sp.)
was relatively high (83-94%) in this study, and the excep-
tionally high values of mercury in sparrows are likely a
result of this MMHg-rich diet. In a study of bird colonies
in coastal salt pans in Portuguese coastal wetlands, Tavares
et al. (2008) found that mercury concentrations in black-
winged stilt (
Himantopus himantopus
) chicks correlated
with mercury concentrations in some macroinvertebrate
groups (Corixidae, Chironomidae, and Hydrophilidae).
Surprisingly, they found that mercury levels in the chicks
did not correlate with trophic levels as predicted by
Distributed at the middle and high latitudes along shores
worldwide, salt marshes are among the most highly pro-
ductive ecosystems in the world—comparable to subsi-
dized agriculture (Mitsch and Gosselink, 2000). Ecosystem
function in salt marshes is dominated by belowground
production, anaerobic conditions, and detrital food webs.
Studies of MMHg in salt marshes attest to the importance
of its belowground production (Canário et al., 2007b). In
contrast to this, scientifi c study of these ecosystems often
focuses on birds and other organisms living predominantly
above ground. Those birds often have relatively high con-
centrations of total Hg and MMHg, and can have varied
diets covering a broad range of habitat types (Ackerman
et al., 2007; Eagles-Smith et al., 2009a). The export of
MMHg from tidal wetlands, either fl ushed out tidally, or via
bioadvection due to fi sh or bird migration, is a potentially
important source of MMHg to adjacent ecosystems (Hall
et al., 2008; Heim et al., 2007; Mitchell and Gilmour, 2008).
Most studies on mercury and primary producers in salt
marshes have been on macrophytes, although algal pro-
duction in these coastal wetlands can be as high or higher
(Zedler, 1980). Indeed, direct grazing by consumers on mac-
rophytes is thought to be minor
compared to grazing on
algae (Mitsch and Gosselink, 2000). Nonetheless, certain
plants or plant assemblages create or take up more mer-
cury than others, and the decay of these plant materials by
microbial consumers can play an important role in MMHg
production (Rajan et al., 2008).
Best et al. (2007) found that uptake of inorganic Hg(II)
and MMHg by plants differed substantially among vegeta-
tion types in a salt march, with cordgrass (
Spartina foliosa
)
accumulating more of both types of mercury than pick-
leweed (
Sarcocornia pacifi ca,
formerly
Salicornia virginica
).
Using stable isotope ratios (
15
N
values. This disparity was attributed to spatial and tempo-
ral differences in prey availability between colony sites.
Long-Term Changes of Mercury
in Marine Ecosystems
One of the key reasons for studying mercury in ecosystems
has been to determine whether, and how, humans have
infl uenced mercury concentrations in fi sh. Unfortunately,
these are not simple questions to answer. Although there
has clearly been an increase in mercury concentrations in
many environmental compartments, there are few reliable
long-term data sets on mercury concentrations in marine
organisms. Another diffi culty is that marine ecosystems are
not static, and in most cases they have been fundamentally
changed by human activity. In addition, the way humans
have used these ecosystems has changed markedly through
time, and will continue to do so in the future.
Measuring Human Impact on Mercury
Concentrations in Biota
The importance of attributing some fraction of the mer-
cury in fi sh to human activity lies in how we perceive and
manage mercury contamination. A great deal of money,
legislation, and research has been focused on linking the
atmospheric deposition of mercury from industrial dis-
charges to mercury concentrations in fi sh. The combustion
of fossil fuels, primarily coal, now accounts for the larg-
est anthropogenic emission of mercury to the atmosphere
globally (Pacyna et al., 2006). Regulation of coal combus-
tion is already an environmentally and politically charged
topic because of rising atmospheric CO
2
levels and the
uncertain supply of crude oil. It is inevitable that coal use
will rise, but regulation of mercury from this source glob-
ally is not assured. Linking the bulk of mercury in marine
biota to human contamination would provide a human
health incentive to enact such regulation. Evidence for
such a link in freshwater systems has been rapidly accumu-
lating in recent years, with multiple studies demonstrating
that the atmospheric deposition of mercury is linked to the
production and accumulation of MMHg in aquatic ecosys-
tems (Hammerschmidt and Fitzgerald, 2006c; Harris et al.,
2007; Orihel et al., 2006, 2007; Paterson et al., 2006).
15
N) and marsh veg-
etation structure, Best et al. (2007) found no relationship
between trophic level (based on
13
C and
15
N) and MMHg concen-
tration and only a weak relationship between MMHg bio-
accumulation and the defi ned habitat types. But they did
fi nd that plant detritus was enriched in mercury relative to
live plants—consistent with the plant litter experiments by
Windham et al. (2004) and Zawislanski et al. (2001)—and
suggested that detritivores would have a higher mercury
exposure than herbivores. This and other studies (Rajan
et al., 2008) have also showed that the rates and timing
of MMHg production during plant decay differed substan-
tially under aerobic and anaerobic conditions.
Some studies have highlighted the importance of preda-
tor macroinvertebrates with high percentages of MMHg as
a key source of MMHg in salt marsh food webs, consistent
with similar observations in freshwater and terrestrial sys-
tems (Cristol et al., 2008). Song sparrows (
Melospiza melodia
samuelis
) had the highest mercury concentrations in the
Best et al. (2007) survey, at 1.7 µg g
1
, with 80% as MMHg.
These birds are thought to feed primarily on insects,
