Mercury in the Marine Environment - Mercury in the Environment - page 187

deep-sea sediments in the Mediterranean. In addition, the
highest methylation potentials, MMHg concentrations in
both the solid phase and pore water, and ratios of MMHg
to total mercury all occurred near the sediment-water
interface, potentially allowing for greater fl uxes of MMHg
to overlying waters than if the peak in these parameters
occurred at depth in the sediments. As a result, diffusive
benthic fl uxes of MMHg out of these deep-sea sediments
(range, 0.7-7.0 ng m -2 d 1 ) were similar to those calculated
for estuary and nearshore sediments (Table 10.3). Although
this suggests that deep-sea sediments may be an impor-
tant source of MMHg to the marine environment, both the
higher MMHg concentrations and higher %MMHg in the
water column of the Mediterranean as compared with other
open ocean waters (Cossa and Coquery, 2005; Horvat et al.,
2003; Kotnik et al., 2007) suggest that the MMHg benthic
fl uxes measured in the Mediterranean are likely higher
than those from other deep-sea sediments.
Using a benthic fl ux on the low end of those calculated
for deep-sea sediments of the Mediterranean Sea (5 pmol m -2
d -1 ) by Ogrinc et al. (2007) and scaling to deep-sea sediments
globally, which cover an estimated area of 3.26

does not discount the microbial methylation of mercury in
suboxic regions of the water column.
MMHg produced in the marine water column may be
more readily incorporated into marine food webs than
MMHg produced in marine sediments if it more easily dif-
fuses or is advected to adjacent oxygenated waters where it
can then be taken up by organisms. This could occur where
oceanic water masses of substantial size experience hypoxia
on a seasonal or persistent basis. These include stratifi ed and
stagnant basins and fjords along the coast of Scandinavia,
California, British Columbia, Japan, and Venezuela, as well
as the Black Sea, Caspian Sea, and the “dead zones” that
form annually at the mouths of the Mississippi, Yangtze,
and other rivers (Diaz, 2001). The microbial methylation of
mercury in these nearshore and shelf water masses has yet to
be the focus of substantial scientifi c exploration, but it war-
rants greater attention given the high biologic productivity
and human utilization of these regions, some of which are
also areas of local mercury pollution.
In addition to the biotic production of MMHg in sub-
oxic regions of the water column, the biotic methylation
of mercury has been measured in incubations of oxic sur-
faces waters of the Mediterranean, where mercury meth-
ylation rates as high as 6.3% d 1 were reported (Monperrus
et al., 2007). Concentrations of methylated Hg are some-
times greatest in the open ocean at intermediate depths,
where apparent oxygen utilization is high (Cossa et al.,
2009; Sunderland et al., 2009), a phenomenon related to
the remineralization of sinking organic carbon by hetero-
trophic bacteria. However, much of this methylated Hg, at
least in the Pacifi c, is likely to be DMHg. The importance of
Hg methylation across the entire oxic water column, and
the need to identify the predominant organo-form created
(DMHg or MMHg), clearly deserves further study.

10 14 m 2
(Ryther, 1969), we estimate that the fl ux of MMHg from deep-
sea sediments is 0.6 Mmol yr 1 . Again, it is quite likely that the
MMHg fl ux from deep-sea sediments in the Mediterranean
is higher than elsewhere in the open ocean, and depth pro-
fi les of MMHg do not show a consistent increase in MMHg
concentration near the sediment-water interface in the deep
ocean of the magnitude that would be expected if deep-sea
sediments were the predominant source of MMHg. However,
it is possible that large fl uxes of MMHg do occur out of deep-
sea sediments, but are accompanied by relatively high rates
of MMHg demethylation in bottom waters.

SUBOXIC AND OXIC WATER MASSES

PARTICLES

Elevated concentrations of MMHg have been reported
in the pycnocline of the Pettaquamscutt River estuary
(Mason et al., 1993) and in the water column of the Black
Sea at the oxic-suboxic boundary (Lamborg et al., 2008).
Such distributions suggest that MMHg is being produced
in the water column by sulfate-reducing or other bacteria
at this interface, and below this depth sulfate-reduction
rates are high but Hg(II) bioavailability is low because of
high sulfi de concentrations and the dominance of less bio-
available, negatively charged complexes HgS 2 2- and HgHS 2 -
(Benoit et al., 1999). Elevated MMHg levels have also been
reported for the low-oxygen bottom waters of the Chesa-
peake Bay (Mason et al., 1999), which is similar to the high
MMHg concentrations found in the anoxic hypolimnions
of freshwater lakes (Mauro et al., 2002; Regnell et al., 1997).
The high MMHg levels in such suboxic bottom waters of
lakes are likely due, at least to some extent, to diffusion and
advection of MMHg from sediments to overlying waters,
and those benthic fl uxes may be enhanced when the over-
lying water is hypoxic (Covelli et al., 2008). However, that

MMHg is particle-reactive, and scavenging by particles that
then sink to depth likely plays a part in maintaining the
low MMHg levels measured in oceanic surface waters. Con-
versely, these sinking particles may also be sites of MMHg
production, and thus a source of MMHg to intermediate
and deep waters. Particle partitioning coeffi cients (K d ) for
inorganic Hg(II) in open ocean waters are generally in the
range 10 5 10 6 (Mason and Fitzgerald, 1993; Mason et al.,

1998), and vertical movement of total mercury in the open
ocean is thought to occur primarily via particle transport
(Mason and Fitzgerald, 1993). Thus, persistent or sinking
particles and fecal pellets that are rich in organic carbon
and substrates for heterotrophic bacterial respiration are also
e in r i c h e d i in i in o r g a in i c H g ( I I ) t h at c o u l d b e m e t hy l at e d b y t h e
consortium of microbes colonizing these particles. Marine
snow, aggregates, and other particles are rich in microbes
and are hot spots of bacterial metabolism and enzymatic
activity (Simon et al., 2002). It is not certain whether
these heterotrophic microbes are capable of methylating