Mercury in the Marine Environment - Mercury in the Environment - page 176

halogen, and nitrogen oxide radicals (Lin et al., 2006;
Lindberg et al., 2002, 2007; Pal and Ariya, 2004; Seigneur
et al., 2006). This ionic Hg(II), often referred to as reactive
gaseous mercury (RGM), is presumed to exist as HgCl 2 ,
HgBr 2 , and HgOBr and is very particle-reactive and com-
monly becomes associated with aerosols or other particles
(Lin et al., 2006; Holmes et al., 2009). As a result, the
average residence time of RGM in the atmosphere is only
hours to weeks (Holmes et al., 2009; Lin and Pehkonen,
1999; Lindberg et al., 2007). The preindustrial spatial dis-
tribution of wet and dry deposition of Hg(0) and Hg(II)
to surface waters of the ocean, as estimated by Selin
et al. (2008), is shown in Figure 10.5, and their modeled
anthropogenic increase in that deposition is shown in
Figure 10.6.
Although earlier studies largely suggested that •OH and
O 3 were the most important oxidizers of Hg(0) in the atmo-
sphere (Bergan and Rodhe, 2001; Pal and Ariya, 2004),
this has been questioned more recently, as have the rate
constants typically used to model the reactions involved
(Carlvert and Lindberg, 2005 ; Seigneur et al., 2006). Based
on modeling results for the marine boundary layer (MBL),
the oxidation of Hg(0) by Br and O 3 , along with entrain-
ment of RGM-rich air from the free troposphere, appear to
account for the majority of RGM present in the MBL. RGM
concentrations in the MBL are highly diurnal, with a life-
time on the order of hours, with scavenging onto sea-salt
aerosols and subsequent deposition to surface waters as the
dominant source of Hg(II) to the surface ocean (Holmes
et al., 2009). However, scavenged RGM is also deposited
to the ocean via wet deposition, a process most important
in the subtropics, where global scale atmospheric down-
welling occurs alongside abundant precipitation (Selin
et al., 2008). The reduction of Hg(II) and the reoxidation
of Hg(0) in surface waters and the MBL is exceedingly
rapid, and mercury deposited to the oceans can be quickly
reemitted and recycled (Mason and Sheu, 2002; Mason
et al., 2001; Pal and Ariya, 2004; Selin et al., 2007; Strode
et al., 2007; Whalin et al., 2007).
The distribution of mercury in the ocean and profi les of
mercury concentrations in different oceanic basins (Figures
10.1 and 10.2) vary as the result of differences in the rela-
tive size of mercury sources and sinks, as well as variations
in ocean circulation. Open ocean concentrations of unfi l-
tered total mercury are lower than in estuary and coastal
regions, and generally fall within the range 0.4-4.0 pM.
However, concentrations greater than 7 pM have been
reported for multiple locations and depths in the ocean
(Mason et al., 1998, 2001; Laurier et al., 2004).
The dominant source of mercury to the open ocean
is atmospheric deposition; therefore, concentrations of
total mercury are often highest in the mixed layer and
decrease with depth because particle scavenging (Mason
and Fitzgerald, 1993; Mason et al., 1995, 2001; Mason and
Sullivan, 1999; Horvat et al., 2003; Laurier et al., 2004;
Cossa and Coquery, 2005; Kotnik et al., 2007; Sunderland

et al., 2009). Concentrations of mercury in some areas
of the ocean increase slightly in intermediate or bottom
waters, presumably because of remineralization of sinking
particles or sediment resuspension. The obvious exception
to this otherwise “scavenged type” profi le for total mercury
is found in the Arctic Ocean, where large areas are covered
with ice for much of the year and inputs of mercury from
rivers are believed to be more important than atmospheric
deposition (Outridge et al., 2008).

Importance of Monomethylmercury in the
Marine Environment

Methylated forms of mercury are the most toxic and read-
ily biomagnifi ed in aquatic food chains, with MMHg being
the most important for both ecologic and human health
implications. MMHg is bioaccumulated and biomagni-
fi ed much more effi ciently than inorganic Hg(II) (Mason
et al., 1996; Pickhardt and Fisher, 2007; Wang and Wong,
2003; Watras and Bloom, 1992). Phytoplankton bioac-
cumulate MMHg to concentrations ~10,000 times greater
than the natural waters in which they live (Pickhardt and
Fisher, 2007; Watras et al. 1998). This transfer from natural
waters to phytoplankton represents the single greatest bio-
concentration of MMHg that occurs at any trophic level
in aquatic food chains. This process is important because
most of the MMHg in fi sh at higher trophic levels comes
from dietary sources (Pickhardt et al., 2006; Wang and
Wong, 2003). As a result, two of the more important steps
in the biogeochemical cycling of mercury responsible for
its toxicity involve: (1) the methylation of inorganic mer-
cury to form the more toxic form MMHg, and (2) the sub-
sequent biomagnifi cation of MMHg up the food chain to
potentially dangerous levels in large predatory fi sh con-
sumed by humans and wildlife. This section will focus
on the sources and sinks of MMHg in ocean ecosystems,
while subsequent sections will describe MMHg bioaccu-
mulation in marine organisms and its biomagnifi cation in
marine food webs.

Sources and Sinks of Monomethylmercury in the
Marine Environment

Concentrations of MMHg in the compartments comprising
the marine environment (i.e., surface waters, deep waters, sus-
pended particles, sediments, sediment pore waters, etc.) are
the net result of its biotic and abiotic production and decom-
position and its partitioning or movement between different
reservoirs. Because the in situ production and decomposition
of MMHg occur simultaneously in aquatic and sedimentary
environments (Heyes et al., 2006; Monperrus et al., 2007;
Rodríguez Martín-Doimeadios et al., 2004), concentrations
of MMHg refl ect the net result of these competing processes
in conjunction with the transport of MMHg into or out of
the world's oceans. Concentrations of MMHg and DMHg in
marine waters are summarized in Table 10.1.