Mercury in the Marine Environment - Mercury in the Environment - page 197

organism low in the food web, typically have lower total
mercury concentrations and lower percentages of MMHg.
Even when only comparing mercury levels in axial muscle
tissue, large differences in the percentage of total Hg exist-
ing as MMHg in marine fi sh exist and can be infl uenced by
numerous variables. These include factors that can be readily
measured (e.g., species, sex, size, age, trophic level, life histo-
ries, and physiologic condition), and others that cannot be
as easily documented (e.g., metabolic rate, elimination rate,
ecophysiologic activity, variations in trophic structure, prey
dynamics, feeding location, and time spent in proximity to
pollution). Consequently, addressing all of those variables to
determine the factors regulating mercury and MMHg con-
centrations in marine fi sh can be quite daunting.
One approach to resolve the relative importance of factors
has been to investigate mercury concentrations, along with
δ

average total mercury concentrations (µg g 1 dw) of 0.41

0.08 in walruses ( Odobenus rosmartus ), 0.84 0.17

in polar bears ( Ursus maritimus ), 1.07 0.11 in ringed

seals ( Phoca hispida ), 2.25 0.97 in beluga whales

( Delphinapterus leucas ), and 2.32 in narwhal ( Monodon
monoceros ). Of special concern are the high levels of mer-
cury in pilot whales ( Globicephalus meleanus ), because
they are an important part of the human diet in the
Faroe Islands (Andersen et al., 1987).
Mercury concentrations in most marine mammals are typ-
ically higher than those of their prey. One exception is polar
bears, whose muscle tissue is lower than that of their princi-
pal prey, ringed seals. This inconsistency has been explained
by polar bears' preferential consumption of ringed seal tis-
sues (skin and blubber) that have relatively low mercury con-
centrations (Atwell et al., 1998). In a human parallel, the skin
of narwhal and beluga whales, which have mercury concen-
trations ranging from 0.3 to 1.5

15 N, and other measures of trophic level in similar
species from the same location. Using this approach, it was
concluded that most of the differences in the bioaccumula-
tion of two sympatric snapper species ( Lutjanus campecha-
nus and L. griseus ) in the Gulf of Mexico could be accounted
for by “modest differences in their trophic position, and, to
a lesser degree, carbon sources, which had low variation and
high overlap among species” (Bank et al., 2007). This study
concluded that even small differences in trophic position
and food habits of similar species could result in relatively
large differences in their bioaccumulation of mercury.
Another diffi culty with assessing the role of fi sh in the
cycling of mercury within an ecosystem is that many fi sh
move from one ecosystem to another. Fish transport con-
taminants, including mercury, with them via bio-advection
(Blais et al., 2005, 2007; Krummel et al., 2003). This transport
is most evident for long-lived anadromous fi sh that accu-
mulate and concentrate MMHg in oceanic waters and then
return to spawn and die in freshwater systems (Zhang et al.,
2001). For example, mass balance calculations by Sarica
et al. (2004) indicated that: (1) salmon are an important
source of mercury in a Lake Ontario spawning stream, and
(2) invertebrates and vertebrates that feed on the salmon
detritus in that stream are important vectors for the trans-
port of that mercury to terrestrial ecosystems.
Biotransport is not limited to lateral movement. It has
been reported that fi sh spending more of their time feed-
ing in deep waters have higher Hg concentrations than epi-
pelagic fi sh feeding primarily in surface waters at the same
location (Monteiro et al., 1996). A consequence of those
deep-water fi sh migrating to surface waters is that mercury
concentrations in seabirds were found to depend upon
whether they were feeding on mesopelagic or epipelagic fi sh,
rather than on their trophic level (Monteiro and Furness,
1997; Monteiro et al., 1998; Thompson et al., 1998).

13 C,

δ

g g 1 , are considered a deli-
cacy by native Canadian and Greenland people (Wagemann
and Kozlowska, 2005; Wagemann et al., 1998).
In contrast to most measurements of mercury in fi sh,
which generally focus on muscle, most measurements of
mercury concentrations in marine mammals have been
conducted on several different tissues (Dehn et al., 2006;
Dietz et al., 2006b; Johansen et al., 2007; Lockhart et al.,
2005; Outridge et al., 2005; Riget and Dietz, 2000; Riget
et al., 2007a; Sonne et al., 2007; Wagemann et al., 1998;
Wagemann and Kozlowska, 2005). These include tissues
consumed by other marine organisms and humans (e.g.,
skin, muscle, fat, liver, and kidney) and tissues that may
be used to chronicle temporal changes of mercury concen-
trations in those organisms (e.g., hair, teeth, and baleen).
The rationale for these multiple analyses is that: (1) marine
mammals are too large for whole body measurements, (2)
the distribution of mercury in tissues of marine mammals
is a better measure of the potential toxicity of mercury
for them, (3) other marine mammals and people typically
consume only certain tissues of marine mammals, and (4)
some preserved marine mammal tissues (e.g., teeth and
baleen) may be used as biomonitors of historic mercury
concentrations in those organisms and their environment.
Riget et al. (2005) reported that spatial variations in
mercury content of the liver and kidney of ringed seals
were consistent with those observed in previous studies of
ringed seals, beluga whales, and polar bears (Wagemann
et al., 1998; Muir et al., 1999) that showed the highest con-
centrations in the western Canadian Arctic. The reason
for this geographic variation was tentatively attributed to
regional differences in geology, but this has yet to be resolved
(Riget et al., 2005), and regional differences in diet or natural
and anthropogenic fl uxes of mercury may also play roles.

μ

Mercury in Marine Mammals Mercury in Marine Birds

Mercury in the fl esh of marine mammals can be excep-
tionally high. For example, Atwell et al. (1998) reported

As with marine mammals, mercury concentrations in some
marine birds are high because of their position at or near