Environmental Engineering Reference
In-Depth Information
and the benefi ts from marine ecosystems are greater than
those from all other ecosystems combined (Costanza et al.,
1997). Those benefi ts include the consumption of seafood,
which has many health benefi ts. However, the consump-
tion of marine fi sh and shellfi sh accounts for more than
90% of the population-wide mercury intake in the United
States (Sunderland, 2007), as is true in much of the world.
Accordingly, the study of mercury biogeochemistry and
environmental toxicology in marine systems is of great
importance for both human and environmental health.
While mercury has long been recognized as a potent toxin,
full appreciation of the threat of mercury contamination
from environmental exposures has occurred only within the
latter half of the past century (Clarkson and Magos, 2006).
The environmental mercury problem was fi rst documented
in the 1950s in Minamata Bay, Japan (Harada, 1995), where
many individuals suffered severe mercury poisoning from
their consumption of seafood with elevated levels of meth-
ylmercury (5-35 µg g
1
). That contamination was traced to
wastewater discharges of monomethylmercury to the bay
from a chemical plant producing acetaldehyde and vinyl
chloride. Since then, other cases of mercury poisoning have
been chronicled, including in native populations of Arctic
and sub-Arctic regions who consume relatively high amounts
of fi sh, marine mammals, and marine birds (Burger et al.,
2007; Van Oostdam et al., 2005). More recently, the focus
has shifted to the potential sublethal or long-term toxicity of
mercury to individuals consuming relatively large amounts
of marine fi sh, and in a few cases marine mammals (National
Research Council, 2000).
The greatest concern for human health related to environ-
mental mercury pollution is the consumption of fi sh by the
most susceptible populations, specifi cally pregnant women
and children (Clarkson and Magos, 2006; Mergler et al., 2007).
The National Research Council (2000) estimated that ~60,000
children born in the United States each year are at risk of neu-
rodevelopmental problems associated with their in utero
exposure to mercury from their mother's consumption of fi sh,
while the Centers for Disease Control and Prevention (2001)
reported that as a result of fi sh consumption nearly 1 in 10
women in the United States could have blood mercury levels
that are hazardous for a developing fetus. As a result, fi sh con-
sumption advisories have been issued by government agencies
for numerous species of fi sh in areas of Canada, Europe, and
all 50 states in the United States (e.g., US Environmental
Agency [US EPA], 2007). Thus, while fi sh represent an impor-
tant protein source for large segments of the population, and
the consumption of fi sh has many health benefi ts, it is also the
pathway responsible for most human exposure to mercury.
The growing awareness of mercury's sublethal toxicity in
humans has increased concerns about mercury's adverse effects
on other organisms. Ecosystems can be threatened by elevated
environmental mercury levels, with higher trophic-level
organisms considered most at risk. These include top preda-
tor fi sh, as well as piscivorous birds and mammals, which
can have high body burdens of mercury (often described
as “potentially toxic”) because of the biomagnifi cation
of mercury in aquatic food chains (Brookens et al., 2008;
Scheuhammer et al., 2007; Sonne et al., 2007). Some of
the adverse effects associated with these elevated mercury
concentrations are reduced reproductive success in fi sh
and birds, altered behavioral traits or neurologic effects in
marine mammals, and immunotoxic effects in birds and
marine mammals (Scheuhammer et al., 2007).
Mercury in Saline Waters
Mercury is a naturally occurring element, and is released to
the environment by a variety of natural and anthropogenic
processes (this topic, chapter 2). It is then distributed and
redistributed by various processes, including atmospheric
exchange with both terrestrial and oceanic compartments,
export from terrestrial and freshwater systems to the ocean,
and deposition and burial in sediments (Sunderland and
Mason, 2007). Mercury concentrations in the ocean are
typically in the picomolar to subpicomolar range, making
quantifi cation of this element an analytical challenge.
There are four principal forms of mercury found in
the marine environment: elemental mercury, inorganic
Hg(II), monomethylmercury (MMHg) and dimethylmer-
cury (DMHg). Analytically, the sum of all of these forms is
referred to as “total mercury.” Dissolved gaseous mercury
(DGM) is a combination of volatile forms (elemental and
dimethylmercury) but is often measured only as dissolved
elemental mercury because of the relatively low concen-
tration of DMHg. Reactive mercury is an operationally
defi ned fraction of easily reducible mercury under acidic
conditions that includes some fraction of inorganic Hg(II)
and perhaps some organomercury. Depth profi les for vari-
ous mercury species in the North Pacifi c and the equatorial
and South Atlantic are shown in Figures 10.1-10.3.
Elemental mercury, Hg(0), is volatile and in seawater
usually exists as a dissolved gas at subpicomolar concentra-
tions. In rare cases, Hg(0) may be present as a liquid, which
can occur in association with marine geothermal metal-
bearing fl uids (Dekov, 2007; Stoffers et al., 1999) or anthro-
pogenic discharges to the ocean, such as the amphoras of
Sharm El Sheikh in the Red Sea or the wreck of the German
U-boat U-864 off the coast of Fedje, Norway.
Inorganic Hg(II) exists as a variety of dissolved complexes
in saline aquatic environments. Ligands important in bind-
ing mercury include chloride, inorganic reduced sulfur spe-
cies, and dissolved organic matter (including thiol functional
groups), with binding to colloidal and particulate matter
also being important (Dyrssen and Wedborg, 1991; Han and
Gill, 2005; Lamborg et al., 2004). The complexation of Hg(II)
in aquatic environments (both oxic and anoxic) has been
reviewed previously, and a more thorough treatment of the
subject can found elsewhere (Dyrssen and Wedborg, 1991;
Mason et al., 1996; Fitzgerald et al., 2007; Skyllberg, 2008).
Organic mercury species include MMHg and DMHg. MMHg
is only detectable in open ocean waters at very low concentra-
tions, and is likely present in saline water as a dissolved complex
bound to chloride, organic matter, or reduced sulfur species
