Impacts of mercury on the environment
mercury in food webs
important factor in the impacts of mercury to the environment is its
ability to build up in the organisms and up along the food chain.
Although all forms of mercury
can accumulate to some degree, methylmercury is absorbed and accumulates
to a greater extent than other forms. Inorganic mercury can also be
absorbed, but is generally taken up at a slower rate and with lower
efficiency than is methylmercury (US EPA, 1997). The
biomagnification of methylmercury has a most significant influence on the
impact on animals and humans. Fish appear to bind methylmercury strongly,
nearly 100 percent of mercury that bioaccumulates in predator fish is
methylmercury. Most of the methylmercury in fish tissue is covalently
bound to protein sulfhydryl groups. This
binding results in a long half-life for elimination (about two years;
Wiener and Spry, 1996). As a
consequence, there is a selective enrichment of methylmercury (relative to
inorganic mercury) as one moves from one trophic level to the next higher
refers to the net accumulation over time of metals within an
organism from both biotic (other organisms) and abiotic (soil, air,
and water) sources.
The term biomagnification
refers to the progressive build up of some heavy metals (and some
other persistent substances) by successive trophic levels –
meaning that it relates to the concentration ratio in a tissue of a
predator organism as compared to that in its prey (AMAP, 1998).
contrast to other mercury compounds the elimination of methylmercury from
fish is very slow (US EPA, 1997). Given
steady environmental concentrations, mercury concentrations in individuals
of a given fish species tend to increase with age as a result of the slow
elimination of methylmercury and increased intake due to changes in
trophic position that often occur as fish grow to larger sizes (i.e., the
increased fish-eating and the consumption of larger prey items).
Therefore, older fish typically have higher mercury concentrations in the
tissues than younger fish of the same species.
mercury concentrations are lowest in the smaller, non-predatory fish and
can increase many-fold on the way up the food chain (AMAP, 1998).
Apart from the concentration in food, other factors affect the
bioaccumulation of mercury. Of most importance are the rates of
methylation and demethylation (see section 2.3) by mercury methylating
bacteria (e.g., sulphate reducers). When all of these factors are
combined, the net methylation rate can strongly influence the amount of
methylmercury that is produced and available for accumulation and
retention by aquatic organisms. As described in section 2.3, several
parameters in the aquatic environment influence the methylation of mercury
and thereby its biomagnification. While much is generally known about
mercury bioaccumulation and biomagnification, the process is extremely
complex and involves complicated biogeochemical cycling and ecological
interactions. As a result, although accumulation/magnification can be
observed, the extent of mercury biomagnification in fish is not
easily predicted across different sites.
At the top
levels of the aquatic food web are fish-eating species, such as humans,
seabirds, seals and otters. The larger wildlife species (such as eagles,
seals) prey on fish that are also predators, such as trout and salmon,
whereas smaller fish-eating wildlife (such as kingfishers) tend to feed on
the smaller forage fish. In a
study of fur-bearing animals in Wisconsin, the species with the highest
tissue levels of mercury were otter and mink, which are top mammalian
predators in the aquatic food chain. Top avian predators of aquatic food
chains include raptors such as the osprey and bald eagle (US EPA, 1997).
Thus, mercury is transferred and accumulated through several food web
levels (US EPA, 1997). Aquatic food webs tend to have more levels than
terrestrial webs, where wildlife predators rarely feed on each other, and
therefore the aquatic biomagnification typically reaches higher values.
compounds toxic to wildlife
is a central nervous system toxin, and the kidneys are the organs most
vulnerable to damage from inorganic mercury. Severe
neurological effects were already seen in animals in the notorious case
from Minamata, Japan, prior to the recognition of the human poisonings,
where birds experienced severe difficulty in flying, and exhibited other
grossly abnormal behaviour. Significant effects on reproduction are
also attributed to mercury, and methylmercury poses a particular risk to
the developing fetus since it readily crosses the placental barrier and
can damage the developing nervous system.
birds, adverse effects of mercury on reproduction can occur at egg
concentrations as low as 0.05 to 2.0 mg/kg (wet weight). Eggs of certain
Canadian species are already in this range, and concentrations in the eggs
of several other Canadian species continue to increase and are approaching
levels of mercury in Arctic ringed seals and beluga whales have increased
by 2 to 4 times over the last 25 years in some areas of the Canadian
Arctic and Greenland (Muir et al., 2001; Wagemann et al.,
1996). In warmer waters as well, predatory marine mammals may also be at
risk. In a study of Hong Kong’s population of hump-backed dolphins,
mercury was identified as a particular health hazard, even more than other
Recent evidence suggests that mercury is
responsible for a reduction of micro-biological activity vital to the
terrestrial food chain in soils over large parts of Europe – and
potentially in many other places in the world with similar soil
characteristics. Preliminary critical limits to prevent ecological effects
due to mercury in organic soils have been set at 0.07-0.3 mg/kg for the
total mercury content in soil. (Pirrone
et al., 2001)
global scale, the Arctic region has been in focus recently because of the
long-range transport of mercury. However, impacts from mercury are by no
means restricted to the Arctic region of the world. The same food web
characteristics - and a similar dependence on a mercury contaminated food
source - are found in specific ecosystems and human communities in many
countries of the world, particularly in places where a fish diet is
water levels associated with global climate change may also have
implications for the methylation of mercury and its accumulation in fish.
For example, there are indications of increased formation of methylmercury
in small, warm lakes and in many newly flooded areas.
chapter is not intended to provide a comprehensive synthesis of the
literature on mercury exposure, effects and risks to ecological receptors.
Rather it represents a summary of selected reviews of the topic, as well
as data and comments submitted during the drafting process.
parts of the descriptive text in this chapter were based on Pirrone et
al. (2001), US EPA (1997), the Canadian government submission of
information to UNEP (sub42gov) and the submission from the Nordic Council
of Ministers (sub84gov).
the years numerous scientific papers, reports and reviews have been
published on mercury and methylmercury
toxicity and ecotoxicity. The reader is referred to the comprehensive
coverage in the WHO IPCS Monographs on Mercury (WHO/IPCS, 1991),
Methylmercury (WHO/IPCS, 1990) and Mercury - Environmental Aspects (WHO/IPCS,
1989) for detailed information. In this text a broader perspective is
adopted in combination with some of the data from the recent decade as
compiled in reviews (US EPA, 1997; Pirrone et
al., 2001; the Canadian submission to UNEP (sub42govatt1); and
section will primarily focus on the mercury concentrations and doses
resulting in effects in individual organisms. The data are mostly
laboratory results or from epidemiological studies. Despite a number of
field investigations of the potential effects of mercury on free-living
aquatic and terrestrial wildlife, the effects of mercury at higher levels
of biological organization (e.g., ecosystem, community, population) are
not well understood, as indicated in the review by US EPA (1997).
exposure may result in severe neurological effects, and this was seen in
Minamata, Japan, from about 1950-1952 (prior to the recognition of human
poisonings), where birds experienced severe difficulties in flying, and
exhibited other grossly abnormal behaviour (US EPA, 1997). Signs of
neurological disease including convulsions, fits, highly erratic movements
(mad running, sudden jumping, bumping into objects) were observed among
domestic animals, especially cats whose diets were high in seafood.
bulk of data on mammals have been generated through laboratory experiments
on mice, rats and other typical laboratory animals, for the evaluation of
risk for humans. These findings are not evaluated in this text, where
wildlife species are the main focus.
studies under controlled conditions have been used to assess the effects
of methylmercury (from a fish diet) on mink and otter (and several avian
species). According to the US EPA (1997), effects can occur at a dose of
0.18 mg/kg body weight per day or 1.1 mg/kg methylmercury in diet (LOAEL
established by US EPA for mink from Wobeser et al., 1976). Death may occur in species
at 0.1-0.5 mg/kg body weight per day or 1.0-5.0 mg/kg in the diet. Smaller
animals (for example, minks, monkeys) are generally more susceptible to
mercury poisoning than are larger animals (for example, mule deer or harp
EPA has developed methylmercury wildlife criteria for two mammal species
in the USA (mink and otter) relying on an aquatic diet (US EPA, 1997). The
wildlife criteria are based on a methylmercury level in water (from which
the animals get their food) that is thought not to harm the species. The
criteria were calculated from effect concentrations (LOEL and NOEL) and
Mammalian Wildlife Criteria for methylmercury were 57 picograms per litre
(pg/l) for mink and 42 pg/l for river otter. The
US EPA noted that the criteria reflect effect levels that are just over
two orders of magnitude higher than those forming the basis for their
human reference dose, and that the wildlife criteria do not cover more
subtle effects like those observed in humans recently (US EPA, 1997).
should be mentioned that methylmercury is rarely measured in water, and
that concentrations in the Wildlife Criteria are extraordinarily difficult
to measure. Recent total mercury concentrations in unpolluted (only
diffuse load) surface water are reported in the range of 0.1 to 5 ng/l.
A number of studies have shown that methylmercury typically amounts to
1-10 percent of total mercury in water. Assuming a mercury concentration
of 1 ng/l in the water,
methylmercury will range from 10-100 pg/l
and it will thus not be uncommon to exceed the Wildlife
or harmful effects in marine and terrestrial mammals are reported in AMAP
(1998) when mercury concentrations exceed 25 to 60 mg/kg wet weight in
kidneys and liver. Methylmercury is a central nervous system toxin and the
kidneys are the organs most vulnerable to damage from inorganic mercury.
Significant effects on reproduction are attributed to mercury, but in
particular methylmercury poses a risk to the developing fetus since it
readily crosses the placental barrier (AMAP, 1998).
thinning in birds was observed in the 1950's and 1960's as some of the
first environmental consequences of the spreading of mercury (and other
environmental toxins); in this case methylmercury was used as seed
dressing, and severe poisoning of wildlife was observed in Scandinavia and
North America. The populations of pheasants and other seed-eating birds,
as well as birds of prey (e.g. hawks and eagles), were drastically reduced
and in some areas nearly disappeared (Ramel, 1974). Therefore birds,
feathers and eggs have been used since then for monitoring the effects of
mercury, and a number of effect values are available.
poisoned birds usually have whole body residues of mercury in excess of 20
mg/kg wet weight (US EPA, 1997).
and Gochfeld (1997) quote a number of studies relating concentrations of
mercury in eggs to a variety of effects in birds, particularly reduced
hatchability, chick survival and other reproductive failures. The effect
concentrations range from 0.05-5.5 mg/kg wet weight in eggs with the
majority around 0.5-1.0 mg/kg wet weight, see table 5.1. It should be
noted that effect levels vary among species, depending on their feeding
preferences, for example, and that extrapolation to other species should
be done with caution.
Summary of acute and other adverse mercury effect levels in birds.
Acute effects level
mg/kg wet weight
Other adverse effect
mg/kg wet weight
mg/kg wet weight
and Gochfeld, 1997
mg/kg dry weight
and Gochfeld, 1997
mg/kg wet weight (in fish)
et al., 1998 in Pirrone et al., 2001.
- 0.4 mg/kg wet weight
sources quoted in Canadian submission, sub42gov (see text below).
0.5 mg/kg wet weight
quoted in Canadian submission, sub42gov (see text below).
particular, the ability of birds to demethylate methylmercury (which may
be related to their dietary preference – fish diet versus vegetable
diet) has important implications for avian risk assessment since most
tests have been conducted on non-fish-eating species.
In addition, the confounding effects of co-exposure to selenium on
methylmercury toxicity should be mentioned, as selenium has been shown in
laboratory studies to elicit protective and in some cases antagonistic
effects on mallards depending on the life stage (US EPA, 1997).
to mercury toxicity is species specific, making it difficult to predict
toxic thresholds for mercury in eggs of seabirds. Nevertheless, laboratory
studies on other bird species indicate that adverse effects of mercury on
reproduction can occur at egg concentrations as low as 0.5 to 2.0 mg/kg
wet wt. (Burgess and Braune, 2001). The eggs of Leach’s Storm-Petrel are
already in this range of mercury concentrations, and concentrations in the
eggs of several other Canadian species continue to increase and are
approaching these levels.
of mercury in feathers associated with adverse effects are reported in the
range 5-65 mg/kg dry weight (Burger and Gochfeld, 1997), see table 5.1.
controlled feeding studies concentrations of mercury down to 0.5 mg/kg wet
weight in the diet have been shown to produce reproductive and behavioural
effects. Field studies on free-living common loons indicate negative
impacts when mercury in prey fish reaches 0.2 - 0.4 mg/kg wet weight
(Barr, 1986; Nocera and Taylor, 1998; Scheuhammer, 1995).
been suggested (though not proven) that methylmercury may cause immuno-toxicological
effects and increased prevalence of chronic diseases in great white herons
(Spalding et al., 1994). This is consistent with immunotoxic findings of methylmercury
in laboratory mammals, and may be a particularly important consequence of
methylmercury exposure to wildlife populations, which frequently encounter
infectious diseases (USA, comm-24-gov). For reviews on immunotoxicological
and histopathological effects of methylmercury on wild birds, see Wolfe et
al. (1998) and Spalding et al.
criteria for birds were established by the US EPA for kingfisher, loon,
osprey and bald eagle, and range from 33 to 100 pg methylmercury/l water,
see table 5.2. The US EPA noted that the criteria reflect effect levels
that are just over two orders of magnitude higher than those forming the
basis for the human reference dose, and that the wildlife criteria do not
cover more subtle effects like those recently observed for humans (US EPA,
Wildlife Criteria for
methylmercury in water (US EPA, 1997).
Criterion (pg/l) *
* 1 pg (picogram) is 10-12
While toxic levels in adult fish are believed to occur at levels
well above those typically encountered in the environment (except in
grossly polluted systems), recent evidence suggests that mercury exposure
to early life stages in some fish can affect growth, development and
hormonal status at levels within a factor of 10 of levels encountered in
“pristine” lakes (i.e., lakes where there are no known mercury point
sources; US EPA, 1997(Volume VI);
Friedman et al., 1996; Wiener and Spry, 1996). Furthermore, Wiener
and Spry (1996) concluded that while direct waterborne exposure to
methylmercury is generally not a serious concern to adult fish, effects
from indirect exposure via dietary uptake and maternal transfer of
methylmercury to eggs and developing embryos occur at 1 percent of levels
affecting adult fish, and may be a concern (i.e., embryo mortality in lake
trout eggs at
0.07 - 0.10 µg/g w.w. versus toxicity in adults at 10-30 µg/g).
Although not conclusive, they further suggest that the reproductive
success of some walleye populations may be impaired by existing levels of
mercury exposure (USA, comm-24-gov).
Mercury concentrations and biomagnification in fish have been
assessed extensively due to the risks of mercury to humans through fish in
the diet. In general, acute toxicity (96 hour LC50) ranges from
33-400 µg/l for freshwater fish, with seawater fish being less sensitive
Mercury is toxic to micro-organisms and has long been used to
inhibit the growth of bacteria in laboratory experiments (WHO/IPCS, 1990).
Effects of inorganic mercury have been reported at concentrations of 5
µg/l in cultures of micro-organisms, and of organic mercury compounds at
concentrations at least 10 times lower (WHO/IPCS, 1991). As mentioned,
organic mercury compounds have been used as fungicidal seed dressings.
Investigations in temperate forest soils have shown that adverse
effects on microbial processes can be expected at concentrations
corresponding to the present level increased by a factor of about 3. (Rundgren
et al., 1992 ; Tyler, 1992,
in Pirrone et al., 2001).
Recent research indicates, however, that
impacts may already be evident in soils over large parts of Europe
(Johansson et al., 2001; Johansson, 2001) – and potentially in
many other places in the world with similar soil characteristics.
Recently, preliminary critical limits to prevent ecological effects
from mercury in organic soils have been set to 0.07–0.3 mg/kg for the
total mercury content in soil. The limits were developed by an
international expert group on effect-based critical limits for heavy
metals, working within the framework of the UN ECE Convention on
Long-Range Transboundary Air Pollution (Curlic et
al., 2000; quote from Pirrone et
al., 2001). The bioavailability of mercury
in soil has a strong influence on its toxicity. This means that mainly the
water-dissolved fraction of the mercury present is the determining factor
for its toxicity in soil environments.
plants are affected by mercury in the water at concentrations approaching
1 mg/l for inorganic mercury, but at much lower concentrations of organic
mercury (WHO/IPCS, 1991). High concentrations of inorganic mercury affect
macroalgae by reducing the germination (AMAP, 1998).
Aquatic invertebrates vary greatly in their susceptibility to
mercury. Generally, larval stages are more sensitive than adults. In
48-hour exposures, 50 percent mortality in larvae often occur at
concentrations around 10 µg/l, which typically is 100 times lower than in
adults. Oyster larvae are even more sensitive to mercury (WHO/IPCS, 1989).
Toxicity is also affected by temperature, salinity, dissolved oxygen, and
water hardness (Boening, 2000).
other classes of animals (e.g. reptiles, amphibians), little data exist
from which to draw conclusions regarding risk levels. Several species
(e.g. alligator, snapping turtle) are expected to experience significant
methylmercury exposure due to their piscivorous feeding habits.
Some data on residues in alligators are available, but
corresponding effect levels are lacking (USA, comm-24-gov).
is very limited information on toxicity in the terrestrial environment,
apart from the mammals, birds and the recent micro-organism data.
Terrestrial plants are fairly insensitive to the toxic effects of mercury
compounds. Mercury is, however, accumulated in higher plants, especially
in perennials (Boening, 2000). The primary effect observed in plants is
associated with root tips (Boening, 2000).
Ecosystems at risk and vulnerable species
This section describes the increased risks to ecosystems and to
various species due to the specific properties of mercury and the
environment. On the global scale, the Arctic region has been in focus
recently because of mercury’s particular tendency to long-range
transport. It is important to acknowledge, however, that impacts of
mercury are by no means restricted to the Arctic region. The same food web
characteristics and similar dependence on a mercury contaminated food
source are found in specific ecosystems and human communities in many
countries around the world, particularly where a fish diet is predominant.
Consequently, fish-eating birds and mammals are more highly exposed to
mercury than any other known denizens of the aquatic ecosystem (Pirrone et
absence of a specific local mercury source, the pattern of mercury
deposition over a country or continent strongly influences which
eco-regions and eco-systems are more highly exposed.
example, in Canada and the Northern USA
the mercury levels in loons decreases from east to west (Canadian
submission, sub42gov), see figure 5.1.
Burgess, 1998; Evers et al., 1998 in the Canadian submission
Mean mercury levels in loon blood in Canada and the Northern USA
from East to West
(Canadian submission, sub42gov)
Aquatic food webs
top marine predators are especially vulnerable to mercury exposure for
reasons previously discussed. The levels of mercury in Arctic ringed seals
and beluga whales have increased 2- to 4-fold over the last 25 years in
some areas of the Canadian Arctic and Greenland (Muir et al., 2001;
Wagemann et al., 1996). However, it is not yet fully understood how
much of the mercury found in the biological environment is derived from
natural sources versus human activity.
warmer waters as well, predatory marine mammals may be exposed to mercury
levels that are health threatening. In a study of Hong Kong’s population
of hump-backed dolphins, mercury was identified as a particular health
hazard, even more than other heavy metals (Parsons, 1998).
recent knowledge points to the sub-surface parts of the oceans,
which are low in oxygen, as a source of conversion of mercury to
methylmercury, fueling the latter’s bioconcentration in fish and food
web. Concentrations of methylmercury in fish species increased 4-fold from
a depth of less than 200 m to more than 300 m, with no further increases,
however, even down to about 1200 m (Monteiro et al.,
their recent report, the US EPA (1997) presented a number of characteristics
of the freshwater ecosystems that are most at risk from airborne releases
are located in areas where atmospheric deposition of mercury is high;
include surface waters already affected by acid deposition;
possess characteristics other than low pH that result in high levels
of bioaccumulation; and/or
include sensitive species.
It could be
added, for other parts of the world, that freshwater bodies subject to
local direct releases of mercury are also at risk.
Canadian environmental authorities likewise recognise that “fish-eating
species in regions with higher mercury deposition, and in areas that
favour methylation such as partially acidified watersheds, watersheds with
large wetlands high in dissolved organic carbon, and reservoirs, are
expected to be most at risk from increased dietary mercury exposure”
(Canadian submission, sub42govatt1).
have shown that approximately 30 percent of Ontario lakes sampled
contained small fish (<250 g) with mercury concentrations averaging
more than 0.3 ppm, the level suggested as the dietary threshold for severe
reproductive impairment in fish-eating birds (loons) (Scheuhammer and
Blancher, 1994, in Canada submission, sub42gov).
map in figure 5.2 (subject to updates, with additional data for Atlantic
Canada forthcoming) indicates, by range, the mercury concentrations in
freshwater fish from 3,200 different locations in Canada.
Draft Status and Trends Report, Environment Canada, 2001.
Mercury levels in freshwater fish in Canada (Canadian submission,
factors remaining constant, mercury contamination of fish tends to be
higher in small lakes than in large lakes.
This may be explained by small lakes being warmer, increasing the
methylation of mercury. This relationship may have further important implications for
the methylation of mercury and its accumulation in fish in the context of
long-term climate change (Canadian Dept. of Fisheries and Oceans, 1998).
rising water levels and newly flooded areas, which might occur as a result
of climate change, could possibly influence the rate at which mercury is
released and methylated, as such events have been shown to be a source of
increased mercury release and methylmercury formation (Canadian
submission, sub32gov, and Canadian comments, comm-20-gov).
The terrestrial food web
Historically, the use of organic mercury compounds for agricultural
seed dressing has resulted in mercury exposures of seed-eaters,
particularly birds and rodents (Fimreite, 1970; Johnels et al., 1979, in Pirrone et
al., 2001). Where the use of mercury-coated
seeds continues, some impact on the terrestrial environment is expected.
recently, inorganic mercury was not considered a major source of effects
in the soil compartment because it is bound to the soil particles and is
not very bioavailable to plants or organisms. In fact, the uptake of
gaseous elemental mercury through leaves is much more efficient than the
uptake of soil mercury (Hg(II)) in roots, and the main exposure of plants
may therefore be through the air.
New studies from both the field and laboratory have shown that a
mercury-related reduction of microbiological activity in soils is likely
taking place in southern Sweden (Bringmark and Bringmark 2001a; 2001b;
al., 2001; all in Pirrone et al., 2001). The findings in Sweden and
in other countries show that the microbiological activity in the topsoil
appears to be very sensitive to the mercury burden, and that significant
impacts may already be taking place in forest soils over large parts of
Europe – and potentially in many other places in the world with similar
soil characteristics (Johansson et al., 2001; Johansson, 2001; all
in the submission from the Nordic Council of Ministers (sub84gov).
microbiological activity in soil is vital to the processing of carbon and
nutrients in the soil, and the health of the microbiological community has
a great effect on the living conditions of trees and soil organisms, which
form the basis for the terrestrial food chain.
Arctic region is affected by long-range transported mercury. In the Arctic
sediments mercury shows increasing concentrations, and there is some
evidence that the concentration in some marine mammals has increased by a
factor of 2- to 4-fold over the last 25 years in some areas of the
Canadian Arctic and Greenland (Muir et al., 2001; Wagemann et al.,
1996; both in Canadian submission, sub42gov).
To what extent that is due to increased mercury levels, or to
increases in the fraction of the total mercury that is bio-available - a possible outcome of the current warming trend and increased
biotic activity in the Arctic - is a subject of current discussion
in AMAP (Canadian comments, comm-20-gov). The Arctic marine food web is often in the spotlight
regarding the risk to ecosystems and the impact on human populations from
mercury. In the Arctic, the aquatic food web is very long, with three
levels of predators (including humans) at the top, and therefore high
concentrations of biomagnified mercury occur there.
wealth of information is available on concentrations and trends for
mercury, particularly from AMAP, which published a comprehensive
assessment report in 1998, with another assessment report due in
2002/2003. However, it remains uncertain whether mercury poses a health
threat to the most highly exposed groups of Arctic marine mammals.
and exposure of top predators also occurs in subarctic and temperate
regions where the biomagnification is seen most clearly in aquatic
environments (US EPA, 1997). The animals considered at most risk of
adverse effects from mercury are again the species depending on a diet of
fish (e.g. otters, seals, eagles) or a diet of the fish-eating species
comparison, figure 5.3 shows mercury concentration levels found in
different tissue types from Arctic fish, birds and mammals. Note that
concentrations are presented on a logarithmic scale, meaning that large
differences in concentrations between trophic levels visually appear
small. The figure was developed in AMAP (1998).
Summary of ranges of mercury concentrations found in Arctic
marine organisms (means). Solid parts of the lines indicate ranges
for Greenland data from Dietz et al. (2000), where the analytical
data have been critically evaluated. The figure with concentration
levels was originally
produced in AMAP (1998), and is shown here courtesy of AMAP.
quantities of mercury are released to the waters of the Amazon and to the
air of vast gold mining areas where mercury is used for amalgamation of
the precious metal. This leads to impacts far beyond the local area, as
seen in the Pantanal floodplain wetland in western Brazil, and parts of
Bolivia and Paraguay (Leady and Gottgens, 2001). Post gold-rush mercury
deposition was more than 1.5 times higher than the deposition rate at the
Acurizal reference site, confirming a regional mercury effect due to gold
mining. Post gold-rush (1980) mercury accumulation in Acurizal was also
2.1 times the rate reported for a global reference during that time
period, suggesting an additional basin-wide effect over such reference
sites. The authors estimated that only 2-8 percent of the total mercury
released from gold mining was secured in sediments. The remainder of the
mercury was lost to the atmosphere, downstream areas or stored in biota.
sources of intermediate increases in mercury mobilisation in tropical rain
forests include slash-and-burn clearing of land for agriculture use or for
mining operations. These activities permit mercury already present in the
soil to be more exposed to mobilising mechanisms.
there is a general difference between tropical and temperate ecosystems
that may make tropical systems more vulnerable. In tropical ecosystems,
more species are sustained and the niche of each species becomes smaller.
In both ecosystems the top predators are the vulnerable species, but there
are relatively fewer of each species in the tropics and this will magnify
the effect of loss of individuals (Burger, 1997).
Reservoirs and wetlands
and wetlands are often mentioned as sources of methylmercury due to the
methylation of inorganic mercury in the sediment (Canadian submission,
to the Canadian submission (sub42gov), the “creation of reservoirs is an
important source of mercury contamination of fish in Canada”, because
the mercury present in newly flooded land becomes more available, and then
more toxic due to the increased rate of conversion to methylmercury.
Most fish caught in new reservoirs have mercury concentrations that
exceed the consumption limit of 0.2 mg/kg wet weight recommended by Health
Canada for people who frequently consume fish (Canadian submission,
investigation of mercury in feathers of birds from a number of tropical
locations, Burger (1997) reported that although fish-eating birds
generally had the highest mercury content, a similar content was found in
Cattle Egrets from the Aswan dam area, although this species is an
insect-eating bird. The author suggested that this may have been caused by
more methylmercury in the food web due to a recent flood in the area
initiating the methylation process.
experiment in a wetland and pond at the Experimental Lakes Area in
Northwestern Ontario demonstrated that natural wetlands are important
sites of mercury methylation, and that flooding of wetlands increases
methylation rates by a factor of more than 30 (Canadian submission,
sub42gov). Increased concentrations of methylmercury were found in water,
the food chain and eventually fish. Monitoring
of boreal reservoirs indicates that concentrations of methylmercury in
fish may return to normal 10 to 50 years after flooding.
Birds of prey and fish-eating birds
It is through fish consumption that mercury exposure in fish-eating
birds occurs. Fish-eating birds in regions with high mercury in fish may
be at risk of reproductive and behavioural affects (Scheuhammer, 1995, in
use of seabirds as biomonitors of marine environmental quality is widely
recognised. Environment Canada (2001) stated that because of their
widespread foraging habits and long lifespan, seabirds integrate mercury
exposure over large geographic areas, and may be an excellent bioindicator
of trends in long-range atmospheric transport of mercury. With birds the
use of non-invasive monitoring strategies, such as collection of feathers
and eggs, can be used.
levels of mercury in Canadian Arctic seabird eggs have increased 2- to
3-fold over the last 20 years (Braune et al., 1999), similar to the
increases reported in Arctic ringed seals and beluga whales over the same
period. In a detailed survey
of Canadian conditions, Burgess and Braune (2001) stated already at the
time of the investigation that the mercury content in eggs indicated a
mercury levels were highest in Leach’s Storm-Petrel and showed the
greatest increase over time. Levels and increases over time were similar
for Atlantic Puffins, Thick-billed Murres/Brünnich's Guillemots and
Northern Fulmars. All these species occupy Arctic or North Atlantic waters
year-round and forage offshore. In contrast, mercury levels in
Double-crested Cormorant and Black-legged Kittiwake eggs did not increase
over time. These species overwinter further south in the Atlantic Ocean.
The levels indicate a potential threat to reproduction in some seabird
species that will increase if trends continue”.
the concentrations in feathers have pointed to increasing levels of
mercury, geographical distributions, and differences in food preference.
and Furness (1997) have recently shown that feathers from fish-eating
birds, which catch fish from the deeper mesopelagic layer, accumulate
higher concentrations of mercury than birds feeding on fish from the upper
parts of the water column. Based on comparison with feathers from pre-1931
museum samples, they have shown that the accumulation has also increased
by 65-397 percent.
In a companion study, Monteiro et
al. (1999) reported a similar relationship
between bird populations in the Portuguese Atlantic islands and mainland
colonies. The egg mercury concentrations were typically 1-5 mg/kg dry
weight, depending on geographical location and species. These birds from
rather isolated locations had egg mercury concentrations well above the
lowest adverse effect level of 0.5 mg/kg dry weight proposed by Burger and
Gochfeld (1997). Mercury levels in feathers were also higher than the
adverse effect level of 5 mg/kg dry weight. Comparing to the adverse
effect levels, Burger and Gochfeld (1997) mentioned that the birds of prey
and fish-eaters most vulnerable include: hawks and eagles, gulls and skuas,
herons and egrets, penguins, albatrosses, ducks, shorebirds, terns,
puffins and alcids.
The information in two recent reviews of the Canadian environment
al., 1999; Braune et al., 1999) provided a very detailed
picture of the status and trends for mercury and other contaminants. The
following section is built on these references.
bears, ringed seals and beluga whales from western Arctic Canada had
elevated mercury levels, apparently due to differences in sedimentary
geology compared to the eastern Arctic. Belugas in contaminated
environments (St. Lawrence estuary) had higher kidney and liver mercury
content than belugas from five Arctic locations. Due to the lack of
dose/response data for Arctic animals, the data cannot be directly
interpreted with respect to impact, but rates of accumulation of mercury
are higher (1.5-2.5 times) in recent samples of ringed seals and belugas
than they were 10-20 years previously. This is in contrast to cadmium,
which in the same period remains unchanged.
of mercury in muscle of most species of Canadian Arctic freshwater fish
cross the US EPA (1997) threshold (between 0.077 and 0.30 ppm for trophic
level three fish) for protection of fish-eating birds and mammals. A
number of lakes in the Northwest Territories and Northern Quebec have fish
populations with levels exceeding the human consumption guidelines. The
higher mercury levels are typically associated with larger, older fish.
5.4 shows average mercury levels in fish from Lake St. Clair, Ontario in
southern Canada. Again,
higher mercury levels are associated with larger, older fish.
An example of observed fish mercury concentrations as compared to
hardly released from fish at all, and methylmercury accounts for
approximately 90 percent of the mercury in fish. In comparison with the
terrestrial environment, virtually all mercury in the kidneys of caribou
is of the less toxic inorganic form.
Ecological risk assessments
ecological risk assessments have been conducted in various places around
the world. Table 5.3 contains examples of risk assessment and criteria
Examples of risk assessment and criteria development efforts, as
aggregated by USA
EPA Mercury Study Report to Congress
ppm methylmercury is the estimated threshold in forage fish for
protection of piscivorous wildlife.
that it is probable that individuals of some highly exposed
wildlife subpopulations are experiencing adverse toxic effects due
to airborne mercury emissions
Popular Creek Risk Assessment
risks to mink (24% probability of at least a 15% mortality)
risks to kingfisher (50% probability of at least a 12-28% decline
et al., 1999
Everglades Risk Assessment
59% probability of exceeding methylmercury NOAEL for Wood Stork,
Great Egret, Great Blue Heron
et al., 2000
Canada Tissue Guidelines
0.033 ppm methylmercury in fish tissue recommended for wildlife
et al., 2000
studies that attempt to associate mercury exposure with effects measured
in natural field settings offer another important line of evidence.
While these studies are usually insufficient to conclusively
establish causal relationships between stressor and response, they
nonetheless add significantly to the evaluation of methylmercury impacts
on wildlife populations. Field
data contain important strengths such as reduced uncertainty associated
with extrapolating effects between the laboratory and the field.
This uncertainty is particularly important for methylmercury
because several ecological risk assessments tend to be sensitive to
relatively small amounts of uncertainty (i.e., a factor of 2 or 3 has
important implications for the findings). Selected reviews of field
epidemiological studies are found in US EPA (1997) for loons, bald eagles
and other species in addition to Wolfe et
variations in ecosystem sensitivity
is important to note the complex biogeochemistry of mercury with respect
to a given food chain and in specific environments. The sensitivity of
local ecosystems varies depending on natural conditions and anthropogenic
influence. This also implies that the “critical loading” – the input
of mercury that leads to enhanced mercury contamination and serious
concerns for human health and the environment – varies according to
local conditions. In some environments, fairly heavy mercury loads have
only a limited effect on living matter, as either mercury is not
efficiently bioaccumulated throughout the particular configuration of the
local food chain, or the mercury is not easily methylated (Canadian
comments, comm-20-gov). In
other cases, ecosystems may be particularly sensitive to mercury loading.
A good example is the Arctic region, where food chain characteristics seem
to mediate biomagnification to very high levels, resulting in a high
exposure of humans and other species at the highest trophic levels (see
section 4.4.3). Another example may be the high sensitivity of the
micro-flora in terrestrial environments of organic forest soils reported
in Sweden (as described in section 5.3.2 above).
Mercury concentrations in environmental media
amounts of data on mercury concentrations in various environmental media
(air, water, soil, sediments) and biota (plants, animals and other living
organisms) have been referenced in submissions to this assessment, as well
as in the literature. For further detail, the reader is invited to
consult, inter alia:
would be very important to investigate and review all such available data,
which would likely add to our understanding of the impact of mercury as a
global pollutant, and could provide a baseline for monitoring. However,
this has not been possible within the time and resource constraints
imposed on UNEP’s global mercury assessment process. Therefore, the
information submitted from different parts of the world on mercury
concentrations in fish (see section 4.5) serves as an indicator
illustrating the omnipresence of mercury in the global environment.