Persistent Organic Pollutants and Heavy Metals (Including Mercury) - The Changing Arctic Environment: The Arctic Messenger - page 154

doubled, while at lower depths, it has increased by 10-25%. Elemental inorganic mercury
(which makes up most of the emissions) is not very available to most biota, but under the
right conditions, microorganisms are able to convert inorganic mercury into methylmer-
cury, which then biomagnifies through the food web. It is thought that methylmercury re-
mains in upper ocean layers for about 11 years before removal by uptake into the marine
food web and by photochemical processes. The most significant biomagnification is found
in freshwater and marine trophic pathways, and once again, predators at the top of the
food chain accumulate the heaviest body burdens. For example, the concentration differ-
ence between water and a beluga whale in the Arctic can be 10 orders of magnitude. It is
the amount of methylmercury in animal or human tissue that is of interest to a toxicologist.
In the Arctic, methylation mainly takes place in freshwater lake sediments and wetlands
depleted of oxygen (anoxic) but which have a supply of organic matter and in sediments
of shelf regions of the Arctic Ocean. Publications by Elsie Sunderland provide a good in-
troduction to the literature. Figure 8.4 illustrates how the proportion of the three mercury
species changes with progression along the Arctic food web. Note that in top predators,
such as the beluga whale, almost all the mercury present is in the form of methylmercury
(MeHg).