Mercury in the Marine Environment - Mercury in the Environment - page 205

waters to be substantially greater (Lamborg et al., 2002;
Sunderland and Mason, 2007). In contrast, the two
mesopelagic feeders Bulwer's petrel ( Bulweria bulwerii )
and the band-rumped storm petrel ( Oceanodroma castro ),
also from the North Atlantic, showed Hg increases of
260% and 394%, respectively, which translates to a
change of 2.9- 4.1% per year. This increase is greater than
the change suggested by any of the models for subther-
mocline-mesopelagic waters for past or current situa-
tions (0.1-0.7% yr 1 ; Table 10.6).
There has been a spate of reports on temporal variations
of mercury concentrations in Arctic biota (Dietz et al.,
2006a, 2006b; Lockhart et al., 2005; Outridge et al., 1997,
2005; Riget and Dietz, 2000; Riget et al., 2005, 2007a).
Results from investigations of recent temporal variations
have been mixed (contrasting and/or not statistically sig-
nifi cant) for numerous reasons, as noted in those reports
and summarized in an article by Bignert et al. (2004).
They assessed the statistical power of existing data sets of
mercury in Arctic biota and determined that most data
sets were insuffi cient to detect temporal trends over the
past few centuries, for several reasons. These included
small sample size and interannual variations in season
of collection, specimen size, sex, and maturity. Even if
a statistically signifi cant change in mercury content of
the organisms occurs, that change may refl ect natural
variations (e.g., change in diet), rather than a response
to anthropogenic mercury fl uxes to the Arctic (Riget and
Dietz, 2000).
Results of the longer time-series analyses of mercury con-
centrations in Arctic biota are more straightforward—albeit
still subject to questions about the accuracy of mercury
measurements in historic samples. These long-term studies
consistently show measurable increases of mercury concen-
trations in organisms at or near the top of Arctic marine
food webs over the past century. For example, mercury lev-
els in the hair of contemporary polar bears were found to
be an order of magnitude higher that those of preindustrial
polar bears in Greenland—suggesting that ~90% of the
mercury at the top of that Arctic marine food web was due
to anthropogenic contamination (Dietz et al., 2006b).
There are a limited number of studies that have
attempted to determine temporal changes of mercury
concentrations in marine biota in the Antarctic. Efforts
to do so by comparing samples collected over the past few
decades have had mixed results (e.g., Honda et al., 2006;
Scheifl er et al., 2005). One of the more recent of those
studies (Sun et al., 2006) shows a systematic increase of
mercury in seal hair over the past century, relative to sam-
ples from the preceding two millennia, which correlates
with temporal increases in mercury observed in a peat bog
in southern Chile over the same period. In addition, earlier
increases of mercury concentrations in the seal hair and
peat bog appear to correlate with anthropogenic emissions
of mercury.

The development of long-term, high-quality monitor-
ing programs to detect changes in marine ecosystems
is an essential part of detecting ecologic impacts of mer-
cury emissions from human activities, and such programs
have been suggested and outlined (Evers et al., 2008).
Unfortunately, few of these monitoring programs currently
exist, and the changes that they must detect over the course
of a few decades are likely small—on the order of a few per-
cent. The Regional Monitoring Program for Water Quality
in the San Francisco Estuary is one such program (Flegal
et al., 2005). Despite a measured decrease of mercury in
parts of the estuary, sportfi sh in San Francisco Estuary
have shown no apparent trend in mercury concentration
over 1970-2000 (Greenfi eld et al., 2005). However, there
are interannual variations in fi sh mercury concentrations,
which are tentatively linked to changes in migration pat-
terns, diets, populations sampled, or to variation in fresh-
water discharge to the estuary.

Conclusion

Globally, mercury inputs to the atmosphere have decreased
in the European region, but continue to increase in areas
such as Asia (Pacyna et al., 2006). Models of mercury
cycling suggest that oceanic reservoirs of total mercury
have not yet reached steady state with current inputs, and
will not do so for decades to centuries (Selin et al., 2008;
Sunderland and Mason, 2007). As a result, mercury con-
centrations can be expected to rise in some marine reser-
voirs. Conversely, in regions with extensive point source or
legacy contamination, total mercury concentrations can be
expected to decrease over many decades as mercury is lost
by burial and natural attenuation (Macleod et al., 2005).
Ecosystems, however, are in a dynamic state, with multi-
ple infl uences on interannual, decadal, and multidecadal
time scales. The detection of small yearly changes (a few
percent) predicted by mercury cycling models might not be
measured as easily as changes in mercury accumulation by
organisms. The unambiguous identifi cation of such small
changes is made all the more problematic by the complex
nature of MMHg cycling, bioaccumulation, and biomagni-
fi cation in marine ecosystems.
In summary, there are many areas that need to be investi-
gated to increase our knowledge of mercury in marine ecosys-
tems and improve our understanding of how humans have
infl uenced its biogeochemical cycling in the marine environ-
ment. These include: (1) further research on the mechanisms,
rates, and locations of MMHg production and degradation in
the ocean, (2) the establishment of monitoring programs to
capture small changes in mercury and MMHg concentrations
over time scales of interest, and (3) an improvement in our
understanding of the structure and function of marine eco-
systems and how they affect the bioaccumulation and bio-
magnifi cation of mercury. Such efforts will require substan-
tial effort by scientists from many different disciplines.