Mercury in the Marine Environment - Mercury in the Environment - page 186

Scaling a mean MMHg benthic fl ux of 50 pmol m -2 d 1
in coastal areas (see Table 10.3) and 9 pmol m -2 d 1 for the
continental shelf (Hammerschmidt and Fitzgerald, 2006b)
to the area of the ocean covered by these two respective
regions globally (Ryther, 1969), coastal and shelf sediments
together might be responsible for a MMHg fl ux to the
ocean of 0.18 Mmol yr 1 . This is substantially greater than
the riverine inputs of MMHg to the open ocean estimated
above (0.03 Mmol yr 1 ), but less than inputs from the deep-
sea sediments estimated below (0.6 Mmol yr 1 ).
Most MMHg in coastal sediments is found associated
with the solid phase. Partition coeffi cients (K d ) for MMHg
generally range from 10 2 to 10 3.5 in coastal sediments,
which is approximately 1 to 2 orders of magnitude lower
than typical K d values for both total mercury in sediments
and MMHg in surface waters (Benoit et al., 1998; Black
et al., 2009b; Bloom et al., 1999; Choe et al., 2004; Covelli
et al., 2008; Hammerschmidt and Fitzgerald, 2004, 2006b;
Hammerschmidt et al., 2004, 2008; Heyes et al., 2004;
Hines et al., 2006; Sunderland et al., 2004). The benthic
fl ux and partitioning of MMHg between the dissolved
and sediment solid phase are affected by the mineral and
organic matter composition and content of sediments,
sediment grain size, redox state, the abundance of iron
and manganese oxyhydroxides, and sulfi de concentra-
tions in the dissolved and solid phases (Benoit et al., 1998;
Covelli et al., 2008; Gill et al., 1999; Hammerschmidt and
Fitzgerald, 2004, 2006b; Hammerschmidt et al., 2008;
Heim et al., 2007; Lambertsson and Nilsson, 2006; Mason
and Lawrence, 1999; Stoichev et al., 2004). Research by Liu
et al. (2009) has highlighted the potential importance of
sediment disturbance on Hg biogeochemistry in nearshore
sediments. The redistribution of sediment and organic car-
bon associated with hurricanes in the Gulf of Mexico was
accompanied by changes in sediment Hg concentrations
and speciation, which in some cases included increases in
Hg methylation potentials and %MMHg.
Although the factors capable of controlling the microbially
mediated production and decomposition of MMHg in coastal
sediments are largely known (e.g., Benoit et al., 2003; Heyes
et al., 2006), the complex interplay between these factors in
determining MMHg net production, concentrations, and dis-
tribution remain poorly understood. For example, some stud-
ies of coastal sediments have concluded that both sulfi de and
organic matter inhibit MMHg production (Hammerschmidt
and Fitzgerald, 2004; Hammerschmidt et al., 2008), while
other studies have concluded that sulfi de and organic matter
increase MMHg production and accumulation (Lambertsson
and Nilsson, 2006; Ouddane et al., 2008; Sunderland et al.,
2006). These seemingly contradictory results highlight the
ability of sulfur chemistry and organic matter to infl uence
MMHg production in multiple ways, depending upon ambi-
ent concentrations and conditions—e.g., as substrates for
respiration, sulfate and organic matter can increase MMHg
production by increasing sulfate reduction, while as ligands
capable of binding Hg(II), both sulfi de and organic matter

can reduce MMHg production by making Hg(II) less bioavail-
able to microbes. But more importantly, such contrasting
results underscore the complexity of net MMHg production,
accumulation, and distribution in coastal sediments and the
need to consider the multitude of factors that simultaneously
play roles in the processes involved and how the importance
of these varies spatially and temporally.

DEEP SEA SEDIMENTS

While sulfate reduction in surfi cial deep-sea sediments is
not as important in the oxidation of organic matter as it is
in nearshore sediments (Canfi eld, 1989), sulfate reduction
associated with underlying microbial methane production
in buried ( 2 m below the seafl oor) deep-sea sediments

can be substantial (D'Hondt et al., 2002). Thus, it is not
unreasonable to expect mercury to be methylated in deep-
sea sediments via the same processes observed elsewhere.
Appreciable concentrations of mercury exist in surfi cial
layers of deep sea sediments (Cossa and Coquery, 2005;
Gobeil et al,. 1999; Ogrinc et al., 2007), which could act as
a substrate for methylation. It remains to be seen, however,
whether microbial activity results in substantial net produc-
tion of MMHg and appreciable benthic fl uxes of MMHg to
overlying deep waters of the open ocean.
Because concentrations of MMHg in deep waters of the
open ocean are not systematically much greater than those
in the rest of the water column, the importance of deep-
sea sediments as a source of MMHg to both the ocean and
marine fi sh has been questioned (Fitzgerald et al., 2007).
Given the much larger surface area of sediments in the open
ocean relative to nearshore and shelf environments, MMHg
production and fl uxes could be much lower in deep-sea sedi-
ments and still overwhelm nearshore sedimentary sources of
MMHg. Even if this were the case, in order for MMHg pro-
duced in deep-sea sediments to make its way into epipelagic
or mesopelagic food webs, it would fi rst have to be trans-
ported from the deep ocean toward the surface and coastal
zones. The time required for such transport may allow for
substantial demethylation or particle scavenging of MMHg,
and as mentioned earlier, depth profi les of MMHg in differ-
ent regions of the ocean are not completely consistent with
the deep ocean being a dominant source of MMHg.
We are aware of only one study that has measured
MMHg concentrations, MMHg methylation potential, and/
or MMHg effl ux in deep sea sediments (Ogrinc et al., 2007).
That study showed that potential rates of mercury methyla-
tion in deep-sea sediments of the Mediterranean Sea ranged
from 0.16 to 0.71% d 1 . Those rates are relatively low com-
pared to those in estuary or coastal sediments, where poten-
tial methylation rates typically range from 0.5 to 10% d 1
(Hammerschmidt and Fitzgerald, 2004; Lambertsson and
Nilsson, 2006; Monperrus et al., 2007). However, the pro-
portion of solid-phase mercury existing as MMHg (mean
of ~2%) was higher than in coastal or estuary sediments,
as were MMHg concentrations in the pore waters of these