Are persistent organic
pollutants a threat to Arctic marine mammals?
Persistent Organic
Pollutants in the Arctic
Stiff and lean after six months curled in a den, a female
polar bear squeezes herself out of her winter home. Two
small cubs emerge tentatively at her heel for their first
view of the world beyond a snow cave. Entirely dependent
on their mother, the cubs follow obediently. Having used
up most of her fat stores, the female scans the sea ice
below and ponders a meal of seal blubber. But her cubs are
not yet ready to travel, and her milk will have to sustain
them for some time to come. The milk is rich and nourishing
but today it also harbors a threat. Chemicals from lands
far beyond her sea-ice domain taint the seals that the mother
has feasted on in the past, and will need again soon. The
chemicals that bind to the fat of the seals have accumulated
in her own fat stores. Unwittingly, the mother passes the
toxins to her young in her fat-rich milk, with effects that
are still unclear.
Persistent Organic Pollutants (POPs): A Background
In 1945, a booming chemical industry launched a new, effective
tool for dealing with insect pests: DDT. It held great promise,
including the hope of saving crops and eradicating disease-carrying
insects. Twenty years later, DDT and other similar chemicals
had indeed benefited agriculture and relieved some of the
problems associated with insects in many areas of the world.
However, these gains came at a price, as DDT is toxic to
many more organisms than those it was intended to kill.
In particular, birds of prey had trouble reproducing, and
their populations declined in many polluted parts of the
world.
As early as 1970, when it was detected in the blubber of
ringed seals, it was evident that DDT was present in the
Arctic. By the mid-1970s, DDT and other pesticides had been
detected in beluga whale, polar bear, and fish. Moreover,
birds of prey declined in northern areas that were thought
to be uncontaminated.
In addition to pesticides, most tests of animals also found
traces of industrial oil made of chemicals known as PCBs.
By 1980, there was evidence that these chemicals had reached
the Arctic via long-range transport. In the late 1980s,
there was evidence that human mother's milk at a location
in the Northwest Territories of Canada contained enough
PCBs to cause concern about effects on human health. The
most likely source was the food the woman had eaten.
Long-range transport via the atmosphere is the most likely
source of these persistent organic pollutants in the Arctic.
However, efforts to quantify the amount of POPs transported
in this way and to determine source regions, are quite limited.
The Biological Effects of POPs
Organic contaminants in the Arctic environment share many
characteristics that make them especially insidious for
people and wildlife.
POPs are stored in fat and are persistent
A common characteristic of most synthetic chemicals found
in Arctic animals is that they break down very slowly. This
persistence in the environment allows them to accumulate
in animals, and to pass up the food web. Most of the organic
pollutants are fat-soluble and accumulate in the fatty tissues
of animals. Arctic animals store energy as fat for survival
in the cold, and therefore fat is an important part of the
diet for both animals and people. Along with the fat in
their diet, animals and people take in the organic contaminants.
As predators take on the energy (fat) from their prey, the
contaminants too work their way up the food chain (from
fish, to seals, to polar bears, to people), often becoming
more concentrated in the top predators (bears, people).
A broad attack on reproduction
Most of the visible effects of POPs on animals are related
to the ability to conceive and raise young. For example,
thinning of their eggshells, which made it impossible for
the birds to hatch their chicks successfully, caused the
early declines in birds of prey. The effects of POPs on
mammals are well documented in polluted areas such as the
Baltic Sea. Malformations in reproductive organs, fewer
young or even complete failures to reproduce are some of
the detrimental signs of high contaminant levels.
One of the underlying causes of failure to reproduce is
that some of the chemicals interfere with sex hormones.
Such hormone disrupters can mimic or block hormones because
they are similar enough in structure to fit into the body's
biochemical receptors. Contaminants that block the estrogen
receptor can inhibit the growth of the reproductive tract
and the mammary glands. In fish the same receptor stimulates
the production of a precursor to egg yolk.
Sex hormones are important to the normal sexual development
of young animals. In polluted temperate environments, high
levels of hormone disrupters have been connected to malformations
in the reproductive organs, change of sex in some species,
and abnormal mating behavior.
The immune system is very sensitive
One of the most sensitive targets for organic contaminants
may be the immune system, the body's primary defense against
disease. The thymus, which normally produces antibodies
to fight infectious agents, can waste away and cease to
function. There are signs that animals with a high load
of organic contaminants are more susceptible to infections.
POPs also limit cell-mediated immunity, the branch of the
immune system that fights cancer cells and parasites.
Liver enzymes are telltale signs of
intoxication
In the body, many toxic chemicals are converted into less
toxic substances that can be excreted. The liver is the
site of most of this detoxification, and many organic contaminants
stimulate the production of specific liver detoxification
enzymes. Measurements of these enzymes are now used as biological
indicators of the load of contaminants in an animal. Unfortunately,
these same enzymes are also capable of breaking down hormones.
This side effect of detoxifying high levels of contaminants
can increase the breakdown of hormones and impair critical
hormone-dependent functions, such as reproduction.
Increased risk of tumors
Several POPs are suspected of being responsible for increased
rates of tumors in wildlife in polluted areas. There are
two ways by which a contaminant can increase the risk of
cancer. The fist is a mutation of hereditary material in
the cells, the DNA, which makes the cell lose control of
it growth. The second allows a cell damaged in this way
to turn into a tumor. Contaminants implicated in the latter
process are called promoters, and this group includes most
POPs. They do not cause cancer by themselves, but can act
together with DNA-damaging chemicals.
Porphyria
Some POPs disturb production of the pigment in red blood
cells, which in severe cases leads to the disease porphyria.
Symptoms include skin damage after exposure to sunlight
as well as damage to the nervous system. The biochemical
changes associated with porphyria, which are measurable
long before symptoms appear, are used as sensitive biological
markers of POPs in the environment.
Effect assessments include many uncertainties
Most of our knowledge of the toxicology of organic pollutants
comes from studies of laboratory animals, studies with a
few species of wild animals or studies of the association
between contaminant levels and effects in wild animals.
There are many uncertainties associated with these studies.
Thus, when POP levels in Arctic biota reach biological effects
thresholds determined from studies such as those mentioned,
it should be interpreted as a warning signal rather than
as evidence that such effects actually do occur in the Arctic.
Priorities of the Arctic Monitoring
and Assessment Program
The most recent AMAP recommendations for POPs give priority
to:
- Monitoring spatial distribution, contaminant levels
and biological effects in Arctic species having body burdens
of POPs at or above levels of concern;
- Improving our understanding of the adverse effects of
POPs on human populations, especially on child development;
and
- Filling data gaps, particularly in the U.S. and Russia.
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