Chemistry Reference
In-Depth Information
Even if the Arctic comprises only 1-2% of the earth's
total surface, it is estimated that 6-10% of the total
anthropogenic emission of mercury to the atmosphere
is deposited there. In the months of spring and sum-
mer, the combination of ultraviolet light, ozone, bro-
mine, and low temperature results in a more effi cient
transformation of the elemental mercury of the atmos-
phere to reactive mercury species (that are deposited)
than at other locations on the earth (AMAP, 2002).
underlines that the rate by which inorganic mercury
in a specifi c ecosystem is methylated, participates in
determining which total mercury concentrations will
be attained by the top predators in the system.
The fact that the mercury concentration in fi sh typi-
cally increases with increasing size (and age) of the
fi sh is, of course, partly a result of the effi cient uptake
and retention of methylmercury, but the fact that the
food preferences of an individual fi sh (and thereby
its trophic level in the food chain) changes with size,
also play a role. As an example, small perches will
feed mainly on plant material (with a low mercury
content), whereas larger perches feed on zooplankton
with a somewhat higher mercury content. As adults,
the perch feed on other fi sh (with an even higher mer-
cury content), and this means that the mercury dose
that the perches ingest with the food increases with the
size and age of the fi sh (Meili, 1997).
It is known from investigations in lakes that the
main fraction of the total methylmercury burden in
such ecosystems is bound in the biota—especially at
the upper levels of the food chains. Generally, the fi sh
in lakes with a low pH and/or alkalinity show the
highest concentrations of methylmercury (Gilmour
and Henry, 1991; Spry and Wiener, 1991). Contrary to
that, the largest pool of inorganic mercury is bound in
the sediments of the lakes (Meili, 1997).
Specifi c problems are associated with metal con-
tamination of the tropical zones, where regulations
concerning prevention of environmental pollution
are not always followed. Unregulated gold mining
has resulted in extensive freshwater mercury pol-
lution, resulting in contamination of fi sh and birds.
Biomagnifi cation has been demonstrated from gas-
tropod molluscs (
Ampullaria
sp) to accipiters (
Ros-
trhamus sociabilis
) and from invertebrates to fi sh,
birds, and humans (Alho and Vieira, 1997; Pfeiffer
et al
., 1993).
8.10.4 Uptake of Mercury in Organisms and
Transport in Food Webs
As a general rule, organisms take up and retain
methylmercury far more effi ciently than inorganic
mercury, and the relatively high content of mercury at
the highest trophic levels of aquatic food chains is due
to incorporation of organic mercury into the organisms
at the lower trophic levels and subsequent transport
in the food chains (Kraepiel
et al
., 2003; Morel
et al
.,
1998).
The uptake of inorganic mercury in unicellular
algae typically takes place by Hg
++
absorbing to the
surface of the algal cell without penetrating the sur-
face. Methylmercury, on the other hand, penetrates the
surface of the algae and is thereby taken up into the
intracellular parts of the cell (Watras and Bloom, 1992;
Watras
et al
., 1998). Mercury concentrations in phyto-
plankton organisms are typically 10
5
-10
6
times higher
than the concentrations in the surrounding water, the
bioconcentration factor for methylmercury generally is
higher than that for inorganic mercury (Watras
et al
.,
1998).
Invertebrates generally take up and retain organic
mercury more effi ciently than inorganic mercury.
Shrimps,
Crangon crangon,
exposed to the two mercury
species in food for 3 weeks assimilated approximately
75% of the ingested organic mercury and only 4% of
the inorganic mercury, and uptake of organic mercury
from water was far more effi cient than uptake of inor-
ganic mercury (Riisgard and Famme, 1986).
Fish generally assimilate organic mercury effi ciently
from the food, and retention times for organic mercury
may be very long as illustrated in experimental studies
with rainbow trout in which half-lives of more than 200
days were found (Giblin and Massaro, 1973; Ruohtula
and Miettinen, 1975). In piscivorus fi sh in the top of
food chain, methylmercury typically constitutes more
than 90-95% of the total mercury content of the fi sh.
In aquatic food chains, the methylmercury concen-
tration that the top predators attain will depend on the
amount of methylmercury that enters the food chain
at the lower levels, because the food (also in predatory
fi sh) is the main source for uptake of mercury. This
8.10.5 Effects of Mercury in Wildlife
Fish may reach relatively high concentrations of
methylmercury in their tissues (easily ~1
g Hg/g wet
weight in low-alkalinity freshwater systems), so high
that adverse effects might be predicted in some cir-
cumstances. Sublethal adverse effects may be seen in
the most sensitive species at body burdens of methyl-
mercury exceeding 1.4
µ
g Hg/g wet weight, whereas
lethal effects are observed in some species at whole
body concentrations of 5-10
µ
µ
g Hg/g wet weight (Spry
and Wiener, 1991).
Canadian investigations have shown that the repro-
duction in fi sh eating birds—especially the effects on
the common loon
Gavia immer
—are affected negatively
