Persistent Organic Pollutants and Heavy Metals (Including Mercury) - The Changing Arctic Environment: The Arctic Messenger - page 149

(isotopes), only one is usually the most common in nature, but very small quantities of the
other isotopes will be found. For example, most carbon is 12 C, but approximately 1% is
13 C. Similarly, for nitrogen, 14 N is much more common than 15 N. The word stable means
we are thinking only about isotopes that do not transform into another element. Although
stable isotopes share identical chemical behaviour, their mass varies according to the num-
ber of neutrons. It turns out that biological and geochemical processes sometimes have a
slight favour of one isotope over another, resulting in a change in the ratio of occurrence of
the two isotopes before and after the biological or geochemical process. The stable isotope
content is described as delta values ( δ ), meaning parts per 1,000 (‰) differences from an
international standard. You can use a mass spectrometer to calculate the ratio of the two
isotopes in a sample.
The metabolism of animals tends to enrich the 15 N content of the body relative to the
14 N content. When animals in the lowest trophic level are eaten by animals in the next
level, they pass along their δ 15 N enrichment. The story proceeds from one level to the next
until youfinally arrive at, forexample, the polar bear orourselves. Inmarine systems, there
is roughly a 3.2‰ enrichment of 15 N from one trophic level to the next. The ratios of 13 C
and 14 C vary between different species of phytoplankton and some plants. This signature
is passed on to herbivores that eat them and thus differences in δ 13 C between different an-
imals suggest dietary differences. The main value of this approach is that it provides a way
of quantifying the trophic underpinning of biomagnification. It has also often solved fas-
cinating mysteries in data sets where, for example, a population of walrus was found to
have organochlorine concentrations much higher than usual. Walrus feed on clams and are
therefore not too high in the food chain and generally do not carry worrisome body burdens
of POPs. This group was carrying burdens more akin to polar bears. Stable isotopes solved
the puzzle. They had added two more trophic levels to their food web by enjoying the taste
of fish-eating seals.
Returning to the POPs state of knowledge report, about half the document was de-
voted to showing that international action to ban or to severely restrict the use of POPs was
achievable. It occupied so much space because these substances had so successfully insinu-