Environmental Engineering Reference
In-Depth Information
Bio-Accumulation and Trophic Transfer
control the exposure of humans and wildlife to this toxic
element. Mercury inputs to ecosystems largely occur as
Hg
2
, which enters aquatic ecosystems via atmospheric
deposition or from point sources such as wastewater and
storm drainage discharges or contaminated industrial sites
(Driscoll et al., 2007). Inputs of Hg can be retained in soil
or sediments, reduced to gaseous elemental mercury (GEM)
and emitted to the atmosphere, or transported via drainage
water to sites of net methylation (Grigal, 2003). This trans-
port is often facilitated by an Hg carrier, generally either dis-
solved organic matter or suspended particulate matter. Note
that hydrologic fl ow paths are a critical factor controlling
Hg transport. Concentrations of total Hg in streams typi-
cally increase with discharge. These increases typically occur
through both the dissolved and particulate fractions of total
Hg. Increases in particulate Hg with discharge are undoubt-
edly due to increases in sediment transport with fl ow.
Increases in the dissolved fraction of total Hg are generally
associated with increases in DOC, particularly the hydropho-
bic fraction of DOC (Dittman et al., 2009). This change likely
refl ects increased transport of Hg
2
due to shifts toward more
shallow hydrologic fl ow paths during high discharge. A criti-
cal factor concerning the response of ecosystems to changes
in Hg inputs is ecosystem storage. Upland soils and lake sedi-
ments are large and important net sinks of Hg inputs. This
Hg could be permanently buried and sequestered or could be
remobilized and become available for methylation and tro-
phic transfer in downstream aquatic ecosystems.
The fi nal process in the sequence is bio-accumulation
(uptake from food and water) and trophic transfer of Hg
2
and MeHg. MeHg and Hg
2
are bioconcentrated (from
water) and assimilated by phytoplankton. MeHg in phy-
toplankton is assimilated more effi ciently by zooplankton
than Hg
2
, resulting in an enrichment in MeHg content
(Mason et al., 1996). Phytoplankton in the water column
exhibit the largest bioconcentration step in aquatic food
webs resulting in log bio-accumulation factors of 5-6
(Driscoll et al., 2007). A bio-accumulation factor is the ratio
of the concentration of a contaminant (e.g., MeHg) in the
organism to the value in water. A number of mechanisms
infl uence the bio-accumulation of MeHg in phytoplank-
ton, including bio-availability of MeHg to algal uptake and
phytoplankton growth. Phytoplankton bio-accumulate
both Hg
2
and MeHg from water but partition the MeHg to
the cytoplasm, leaving Hg
2
bound to the cell wall (Mason
et al., 1996). Zooplankton appear to assimilate the MeHg
inside algal cells and egest the Hg
2
with the cell walls
(Mason et al., 1995). Fish also obtain MeHg from ingestion
of food and take up far less directly from water (Hall et al.,
1997). Enrichment of MeHg continues up the food chain
to top consumers, who are found to have virtually all their
Hg occurring as MeHg (Driscoll et al., 2007). Several fac-
tors infl uence this trophic transfer of MeHg in aquatic eco-
systems. Elevated inputs of growth-limiting nutrients (e.g.,
P, N) increase biomass, resulting in a decrease in the Hg
concentration of individual organisms; this phenomenon
is called biodilution (Pickhardt et al., 2002; Chen and Folt,
2005). Higher growth rates in individuals also result in
lower mass-specifi c concentrations in their tissues because
of growth dilution (Karimi et al., 2007; Ward et al., 2009).
Growth dilution has been hypothesized as the mechanism
explaining lower MeHg concentrations in fi sh and shell-
fi sh from the more productive lakes (Beckvar et al., 2000;
Essington and Houser, 2003).
Differences in food-web structure and type affect the fate
of MeHg as well. Several studies have found chain length to
be positively related to MeHg concentrations in top-trophic-
level fi sh, suggesting that the increased number of trophic
levels results in greater biomagnifi cation of MeHg (Cabana
et al., 1994; Stemberger and Chen, 1998). Finally the Hg con-
centration of fi sh is greater for those species feeding from a
pelagic food chain rather than a benthic food chain (Becker
and Bigham, 1995; Power et al., 2002; Gorski et al., 2003;
Kamman et al., 2005). Some studies have shown that bio-
accumulation of MeHg also occurs along the terrestrial food
chain (Rimmer et al., 2005).
Bio-Availability and Transformation
An important factor controlling concentrations of Hg to fi sh
and other biota is net methylation. Methylation is the pro-
cess by which microbes convert Hg
2
to MeHg. This process
is important because MeHg is the form that readily bio-
accumulates in the terrestrial and aquatic food chain result-
ing in potential exposure to wildlife and humans. Although
methylation can be mediated by a variety of microbes, the
most important of these is sulfate-reducing bacteria (Benoit
et al., 2003). Sulfate-reducing bacteria fl ourish in reducing
environments, such as wetlands and lake and river sedi-
ments. As a result, zones of high methylation activity occur
only at restricted locations on the landscape. Conditions that
promote the methylation of Hg
2
are low oxygen, high inputs
of labile organic carbon, and a supply of sulfate and Hg
2
.
Inputs of oxygen or nitrate inhibit sulfate reduction and
therefore methylation of Hg
2
(Todorova et al., 2009). In the
presence of elevated concentrations of sulfi de, methylation of
Hg
2
is also restricted because the sulfi de binds Hg
2
, making
it unavailable for methylation and because of the formation
of charged Hg-sulfi de complexes, which cannot be readily
assimilated by sulfate-reducing bacteria (Gilmour et al., 1992;
Benoit et al., 2003). MeHg can be demethylated by oxidative
or reductive processes, resulting in the formation of either
Hg
2
or GEM. Therefore, the net production of MeHg is the
result of two processes: methylation and demethylation.
Mercury Hot Spot Case Studies
The following fi ve case studies have been chosen in part
because they represent systems that have been extensively
studied for their Hg dynamics and also to illustrate the
