Mercury in the Marine Environment - Mercury in the Environment - page 192

Mass Balance of Monomethylmercury
in the Oceans

with fi shing (0.04 Mmol yr 1 ) being of lesser importance.
The dark biotic and/or abiotic degradation of MMHg in the
water column and volatilization to the atmosphere are other
potential, but poorly quantifi ed, sinks for MMHg.
The sum of all identifi ed and estimated sources for
MMHg in the mass balance is ~1.2 Mmol yr 1 , while the
sum of all identifi ed and estimated sinks is ~1.0 Mmol yr 1 .
If correct, this disparity indicates that the ocean is not at
steady state with respect to MMHg concentrations and that
levels of MMHg are on the rise, as is true for total mercury.
However, this difference (0.2 Mmol yr 1 ) is well within the
error of this preliminary mass balance, and a number of
potentially important sources and sinks (e.g., production
in the water column) cannot yet be accurately estimated.
In addition, the magnitude of some of the sinks for MMHg
in the ocean may be substantially greater than estimated
here, while some of the sources of MMHg are quite possi-
bly signifi cantly smaller. Such discrepancies and uncertain-
ties further highlight the need for additional research in
these areas if we are to better understand the biogeochemi-
cal cycling and bioaccumulation of MMHg in marine
ecosystems.
Based on our estimated standing stock of MMHg in the
ocean (70 Mmol) and the annual sources and sinks for
MMHg (1-1.2 Mmol yr 1 ), the residence time of MMHg in
the oceans would be on the order of 60-70 years. As is true
of most of the MMHg fl uxes estimated above, this value is a
preliminary estimate based on limited data, which are them-
selves subject to large errors. Thus, the uncertainty associ-
ated with this residence time is likely at least a factor of 5.

Using estimates and calculations presented above for the
various MMHg fl uxes of interest, we present a preliminary
mass balance for MMHg in the oceans in Figure 10.7. We
have not attempted to quantify the errors associated with
each of these fl uxes because in many cases the MMHg data
are so limited in number or extent as to preclude our doing
so. That said, uncertainties associated with the estimated
fl uxes are unquestionably quite large, likely approaching
an order of magnitude in some cases. We, therefore, pres-
ent this mass balance not as an authoritative model of
MMHg biogeochemical cycling in the marine environ-
ment, but rather as a fi rst attempt at a comprehensive
estimate for the ocean that can be revised in the future, as
will certainly be necessary. We do not provide estimates for
fl uxes for which insuffi cient data prevent their calculation,
but include them in Figure 10.7 to illustrate areas requiring
more research in order to improve our understanding of
MMHg cycling in marine ecosystems.
As summarized in Figure 10.7, the in situ production
and fl ux of MMHg out of deep-sea sediments is estimated
to be the dominant source of MMHg to the marine envi-
ronment (0.6 Mmol yr 1 ). We acknowledge that this value
was estimated by extrapolation from near the lower limit
of MMHg fl uxes measured for deep-sea sediments in the
Mediterranean Sea (Ogrinc et al., 2007), and that fl uxes
from deep-sea sediments elsewhere are likely to be con-
siderably smaller. Even if this were the case, and MMHg
fl uxes from deep-sea sediments were only one third those
measured in the Mediterranean, the export of MMHg
from open ocean sediments would be ~0.2 Mmol yr 1 , and
still constitute one of the largest sources of MMHg to the
oceans. Recent research has suggested that the microbial
methylation of Hg in the water column may be one of the
largest sources of methylated Hg to the open ocean water
column, although this may well be DMHg, not MMHg.
Thus, the production of MMHg in deep-sea sediments and
intermediate waters of the open ocean are potentially the
largest sources of MMHg to the oceans, but these are also
two of the least understood and studied.
Other important sources of MMHg to the ocean include
geothermal activity (0.2 Mmol yr 1 , much of which may be
quickly demethylated), nearshore sediments (0.18 Mmol yr 1 ),
and riverine and estuarine inputs (0.21 yr 1 , much of which
is deposited in nearshore sediments). Minor sources include
atmospheric deposition (0.02 Mmol yr 1 ) and submarine
groundwater discharge (0.004 Mmol yr 1 ). Additional
potential sources include export from coastal wetlands, the
degradation of DMHg, and the abiotic methylation of mer-
cury in the water column.
The sinks for MMHg in the marine environment are even
less well understood than the sources. Photodemethylation
(0.7 Mmol yr 1 ) and sediment burial (0.21 Mmol yr 1 ) appear to
be the most important sinks that can currently be estimated,

Mercury in Marine Organisms

Mercury concentrations systematically increase with
increasing trophic level in freshwater, estuary, and marine
food chains (Figure 10.8 and Table 10.5). Mercury levels in
apex predators in the marine environment may be more
than 1 million-fold higher than in surrounding waters.
This enrichment underlies mercury's persistent and bio-
accumulative nature as a toxicant, representing a serious
environmental and human health concern. Table 10.5
summarizes data from various marine ecosystems, illus-
trating the general trend of increasing total mercury con-
centrations and increasing %MMHg with trophic level.
Central to mercury's biomagnifi cation is that only
MMHg, not Hg(II), is biomagnifi ed in marine food
webs. The preferential bioaccumulation of MMHg, due
to its relatively low elimination rate and relatively high
assimilation effi ciency, has been shown in kinetic models,
laboratory studies, and fi eld studies of mercury uptake
by aquatic organisms (Baeyens et al. 2003; Bargagli et al.,
1998; Francesconi and Lenanton, 1992; Hammerschmidt
and Fitzgerald, 2006a; Lawson and Mason, 1998; Mason
et al., 1996; Mathews and Fisher, 2008; Moye et al., 2002;
Pickhardt and Fisher, 2007; Tsui and Wang, 2004; Wang