Environmental Engineering Reference
In-Depth Information
Although there are numerous sources of MMHg and
pathways for its production in the marine environment,
the relative importance of these has yet to be completely
elucidated. MMHg production by sulfate-reducing, and
potentially iron reducing, bacteria in sediments is consid-
ered to be the predominant source of MMHg to both sedi-
ments and overlying surface waters in freshwater, estuary,
and coastal environments (Benoit et al., 2003; Compeau
and Bartha, 1985; Fleming et al., 2006; Gilmour et al.,
1992; Kerin et al., 2006; King et al., 2001). However, in the
open ocean, the dominant source of MMHg has yet to be
conclusively demonstrated, so the source of the MMHg
found in marine fi sh is unclear. As a result, the effects of
past or future changes in anthropogenic mercury emission
on mercury levels in seafood are uncertain.
Although there have been a number of mass balances and
models developed to quantify sources, sinks, and the biogeo-
chemical cycling of total mercury in both the marine envi-
ronment as a whole as well as in regional seas (Balcom et al.,
2004, 2008; Cossa and Coquery, 2005; Horvat et al., 1999;
Lamborg et al., 2002; Macleod et al., 2005; Mason and Sheu,
2002; Mason et al., 1994; Outridge et al., 2008; Rajar et al.,
2007; Selin et al., 2008; Strode et al., 2007; Sunderland and
Mason, 2007; Žagar et al., 2007), there has been a decided
lack of quantitative models of the biogeochemical cycling of
MMHg in the world's oceans (Mason and Gill, 2005). Efforts
to develop mass balances for MMHg in small coastal areas
(Balcom et al., 2004, 2008; Bloom et al., 2004a; Macleod
et al., 2005; Mason et al., 1999) represent the fi rst step to this
end. Below we discuss the importance of different sources
and sinks of MMHg in marine ecosystems and present a pre-
liminary mass balance for MMHg in the oceans.
et al., 2005), mangrove sediment (Quevauviller et al.,
1992), coastal upwelling (Conaway et al., 2009), or arctic
ice leads and polynyas (St. Louis et al., 2005, 2007).
Although MMHg generally represents only a few percent
of the total Hg in precipitation, it is possible that this MMHg
is more labile than MMHg from other sources, and may more
easily be assimilated into aquatic food webs. The wet depo-
sition of MMHg may also be more ecologically important
over the time scale of storm events, when it might represent
a pulse of bioavailable mercury to ecosystems. Nevertheless,
precipitation does not represent a predominant source of
MMHg to the marine environment. Assuming that MMHg
makes up 0.2% of the mercury in marine precipitation
(Mason et al., 1992; Lamborg et al., 1999), and using esti-
mates of wet deposition of total mercury (Lamborg et al.,
2002; Mason and Sheu, 2002), the wet depositional fl ux of
MMHg to the ocean is estimated to be only 0.02 Mmol yr
1
.
There is limited evidence suggesting that marine aero-
sols are a source of MMHg in the subarctic areas (Constant
et al., 2007). Evaporation of cloud droplets containing
MMHg and reports of MMHg in the atmosphere and its
volatilization from surface waters (Iverfeldt and Lindqvist,
1982; Lee et al., 2002; Mester and Sturgeon, 2002) suggest
that dry deposition of MMHg is possible. However, dry
deposition is not believed to be an importance source of
MMHg to the oceans.
SURFACE AND SUBSURFACE MONOMETHYLMERCURY
INPUTS FROM TERRESTRIAL SOURCES
RIVERINE AND ESTUARINE INPUTS
Riverine and estuarine inputs of total mercury to coastal
waters can be substantial, both in areas affected by local
anthropogenic sources of mercury (Balcom et al., 2004,
2008; Conaway et al., 2003; Mason et al., 1999) and in
remote regions where human-related inputs of mercury
are limited to atmospheric deposition (Leitch et al., 2007).
Although riverine inputs of total mercury can be impor-
tant locally, and are similar in size to the net burial of mer-
cury in marine sediments, they are small relative to atmo-
spheric inputs of total mercury to the oceans. Conversely,
the fl ux of MMHg to coastal environments via surface
waters of terrestrial origin is poorly quantifi ed for all but
the most extensively studied areas (e.g., Long Island Sound,
Chesapeake Bay, San Francisco Bay, and the Gulf of Trieste).
Rivers and estuaries could play a more important role as a
source of MMHg to the ocean than they do for total mercury
because the largest sources of total mercury to the ocean are
wet and dry atmospheric deposition (Figure 10.4), whereas
atmospheric deposition of MMHg is relatively inconsequen-
tial. The importance of riverine MMHg inputs to coastal
waters is illustrated by estimates that fl uvial inputs account for
55% of the MMHg fl uxes to the New York/New Jersey Harbor
Estuary (Balcom et al., 2008) and 18% of the MMHg fl uxes to
the water column of Long Island Sound (Balcom et al., 2004).
External Sources of Monomethylmercury to the
Marine Environment
ATMOSPHERIC DEPOSITION
The presence of MMHg in precipitation has been reported,
with MMHg concentrations in precipitation measured
in open ocean areas (
0.05 pM) (Lamborg et al., 1999;
Mason et al., 1992) being lower than those along the
coast or inland (0.1-3.0 pM) (Balcom et al., 2004; Bloom
et al., 2004; St. Louis et al., 2005). The sources of MMHg
in rainwater are unknown, but may include the abiotic
gas or aqueous-phase methylation of inorganic mercury in
the atmosphere (Hammerschmidt et al., 2007) by acetate
(Gårdfeldt et al., 2003a) or other organic compounds (Hall
et al., 1995), or the biotic methylation of Hg(II) by microbes
present in atmospheric aerosols or cloud droplets (Jones
and Harrison, 2004). Other potential sources of MMHg
in precipitation include the volatilization of MMHg com-
plexes from surface waters (Iverfeldt and Lindqvist, 1982;
Mester and Sturgeon, 2002) or landfi ll gas (Lindberg et al.,
2005) or the atmospheric degradation of DMHg (Niki et al.,
1983a, 1983b) originating from landfill gas (Lindberg
