Environmental Engineering Reference
In-Depth Information
respectively (Albert et al., 1995; Wu et al., 1997), acetate
will not play a role in the complexation of Hg(II) in sur-
face waters or sediment pore waters because of the strong
binding of Hg(II) by chloride, inorganic sulfi de, polysul-
fi des, and thiols associated with dissolved organic matter
(DOM)
(Benoit et al., 1999, 2001b; Dyrssen and Wedborg,
1991; Haitzer et al., 2003; Han and Gill, 2005; Lamborg
et al., 2004). Consequently, it is unlikely that acetate plays
a dominant role in MMHg production in marine waters.
Mediterranean are mixed on this point, with some report-
ing relatively high (100-300 fM) DMHg concentrations at
depth (Cossa and Coquery, 2005; Cossa et al., 1994, 1997)
but others reporting lower (
100 fM) DMHg concentrations
that are less than MMHg concentrations (Horvat et al.,
2003; Kotnick et al., 2007; Monperrus et al., 2007). Despite
the apparently confl icting data from the Mediterranean,
the general trends suggest that “methylated Hg” is MMHg
in surface waters but primarily DMHg in intermediate and
deep waters. Consequently, the link between particulate
organic carbon remineralization in intermediate waters
and methylated Hg in the North Pacifi c (Sunderland et al.,
2009) most likely attests to DMHg, rather than MMHg,
production in the water column at those depths.
Despite uncertainties as to the source of DMHg in the
ocean, it is well established that DMHg is generally absent
above the thermocline. Processes that could act to keep
DMHg levels low in surface waters include evasion of DMHg
to the atmosphere, photodegradation, thermal decomposi-
tion, and/or biotic degradation. Photodegradation has widely
been perceived to be the most important of these processes
based upon incubation experiments during which DMHg
losses were observed, as well as estimated degradation rates
required to offset calculated diffusive fl uxes between different
oceanic compartments (Mason and Fitzgerald, 1993; Mason
and Sullivan, 1999; Mason et al., 1995).
However, a study investigating the potential photo-
degradation of DMHg in seawater detected no loss of
DMHg in dark controls or light-exposed seawater samples
exposed to ambient sunlight for 1 day (Black et al., 2009a).
Photodegradation of DMHg still might have occurred, but
at a suffi ciently slow rate that it was within the analyti-
cal error of that study. Still, the upper limit of DMHg pho-
todegradation in that study would be substantially lower
than rates of MMHg photodegradation measured in sea-
water (Monperrus et al., 2007). Therefore, even if DMHg is
photodegraded in seawater and MMHg is the dominant prod-
uct, the steady-state concentration of MMHg from this reac-
tion will be relatively low because MMHg is itself rapidly pho-
todegraded. Other processes, such as evasion of DMHg to the
atmosphere, are therefore likely responsible for the low levels
of DMHg measured in the surface mixed layer.
Even if DMHg is not rapidly photodegraded, calculations
indicate that degradation of DMHg does occur in the ocean,
and this may represent a source of MMHg to the marine envi-
ronment (Mason and Fitzgerald, 1993; Mason and Sullivan,
1999; Mason et al., 1995). Indeed, it has been suggested that the
degradation of DMHg alone might account for all of the MMHg
in the ocean (Mason and Fitzgerald, 1993). The fact that DMHg
levels are highest in intermediate and deep waters suggests
either that rates of DMHg production are highest there and/or
that DMHg is more stable under conditions characterizing the
subsurface than those at the surface. Upwelling of deep waters
containing DMHg could represent an important mechanism
for increasing MMHg levels in surface waters if DMHg was
then degraded to MMHg (Niki et al., 1983a, 1983b).
DEGRADATION OF DIMETHYLMERCURY AS A SOURCE
OF MONOMETHYLMERCURY
DMHg is prevalent in the intermediate and deep waters
throughout the oceans (Figure 10.3), and it is often the
dominant methylated form of mercury at depth in the
open ocean (Cossa et al., 1994, 1997; Horvat et al., 2003;
Mason and Fitzgerald, 1993; Mason and Sullivan, 1999;
Mason et al., 1995). It is not clear exactly where or how
this DMHg is formed, but it has long been hypothesized
that DMHg production occurs in the water column and
is associated with heterotrophic activity and carbon rem-
ineralization at intermediate depths (Cossa and Coquery,
2005; Cossa et al., 1994, 1997; Mason and Fitzgerald, 1990,
1993; Mason and Sullivan, 1999; Mason et al., 1995). An
exception to this may be in the Arctic Ocean, where DMHg
has been detected in surface waters under sea ice (St. Louis
et al., 2007). The link between organic carbon remineral-
ization and DMHg production has been suggested based
upon: (1) mass balance calculations, (2) elevated DMHg lev-
els observed in low-oxygen intermediate waters or recently
formed deep waters below regions with high primary pro-
ductivity, and (3) correlations between DMHg (or “methyl-
ated Hg”) concentrations and apparent oxygen utilization,
carbon remineralization rates, and nutrient concentrations.
Two of the most recent studies to have investigated
relationships between methylated Hg distributions and
measurements of organic carbon utilization, nutrient con-
centrations, and hydrographic parameters were carried
out by Cossa et al. (2009) in the Mediterranean Sea and
Sunderland et al. (2009) in the North Pacifi c Ocean. DMHg
and MMHg concentrations were not measured separately
in either study; instead, samples were acidifi ed prior to
MMHg analysis, converting DMHg to MMHg (Black et al.,
2009a). Thus, these studies could report only concentra-
tions of “methylated Hg,” the sum of DMHg and MMHg.
While both studies showed correlations between methyl-
ated Hg and apparent oxygen utilization and different mea-
sures of organic carbon utilization and remineralization in
intermediate waters, it is unclear whether this relationship
applies to DMHg, MMHg, or both species. Other studies
measuring both DMHg and MMHg separately have shown
that DMHg is often in excess of MMHg in intermediate and
deep waters of the ocean (Cossa and Coquery, 2005; Cossa
et al., 1994, 1997; Mason and Fitzgerald, 1990, 1993; Mason
and Sullivan, 1999; Mason et al., 1995). Studies from the
