Bioconcentration

Bioconcentration refers to the processes that lead to elevated levels of contaminants in biota. It causes contaminant levels in some fish and wildlife species to become significantly higher than the background levels. This is a major environmental concern in the Arctic due to the increased environmental presence in cold climates of many contaminants from long-range sources, and the importance of fish and wildlife, including those high up in the food chain, in the diets of many northern peoples.

Bioconcentration is a broad term that encompasses bioaccumulation and biomagnification, the two processes that jointly lead to contaminants concentrating in biota. These processes are distinct, yet combine to produce a synergistic effect that results in the higher levels of contaminants in wildlife at higher levels of food chains.

Most contaminants are taken up physiologically through diet when the contaminants in a prey species are passed to the predator that eats it. Certain contaminants are persistent—they last for a long time in bodies and resist metabolic breakdown. Older animals have typically eaten more over their lives than have younger ones, resulting in progressively higher levels of contaminants in the same animal. Bioaccumulation refers to this building up of contaminants in an individual plant or animal over the course of time. Two animals of the same species, at the same trophic level (or same position on the food chain), can have widely different contaminant levels if their ages are different. As a general rule, younger individuals are likely to have lower contaminant levels than older individuals of the same species in the same area because they have had less chance to bioaccumulate contaminants.

Many contaminants tend to reach exponentially higher concentrations further up the food chain. That is, they build up in predator species at much higher concentrations than they do in their prey species. This is due to the process of biomagnification. Each trophic level rests on a wider food base. An individual herbivore (plant eater) consumes many primary producers, such as plants, and can receive the contaminants from each of these. The predator that eats many herbivores will receive all the contaminants from all the thousands of plants each of the herbivores ate, and so on. Within the same food web, an animal at a lower trophic level (or lower position on the food chain) will generally have lower contaminant levels than one at a higher trophic level.

It is the combination of these two processes that causes high body burdens in some wildlife species, due to a combination of life span (which may increase bioaccumulation) and diet (which may increase bio-magnification).

The beluga whale (Delphinapterus leucas) is an example of an Arctic marine species that bioconcen-trates contaminants. In the ocean, primary producers take up contaminants from the surrounding abiotic environment (water, sediment). Primary consumers such as small fish, squid, and crustaceans eat thousands of these tiny plants and animals during their lifetimes, and accumulate their contaminants. Belugas eat thousands of larger fish, squid, and crustaceans, building up all of the contaminants in all of the millions of tiny plants and animals those fish ate. The result is levels of contaminants that, due to the combination of bioaccumulation and biomagnification, are several thousand times higher in belugas than in the surrounding environment. Because beluga is a common part of the Inuit diet in the Arctic, food advisors suggest limits to the amount of muktuk (the fat layer beneath the skin) that can be safely consumed due to levels of mercury, cadmium, polychlorinated biphenyls (PCBs), DDT, and other contaminants. Polar bears, as top predators, are further up the Arctic food web, feeding on seals, and have much higher levels of contaminants than most other Arctic wildlife (Norstrom et al., 1988).

Caribou (Rangifer tarandus) is an example of a terrestrial species that bioconcentrates. Caribou eat lichen, a long-lived type of vegetation that is believed to bioaccumulate airborne contaminants. These are passed to caribou through their diets. This is believed to be the cause of elevated cadmium in caribou livers. Due to biomagnification, the wolves (Canis lupus) that feed on caribou have higher levels (Elkin, 1994).

Background levels of contaminants in the Arctic are higher than in temperate zones, due to naturally high levels of certain heavy metals in regional geology and the "cold condensation" effect where low Arctic temperatures cause volatile compounds that have traveled over long distances through the atmosphere to settle in the Arctic and Antarctic regions (Barrie et al., 1992). This results in the Arctic serving as a sink for several contaminants, with higher levels of environmental contaminants in soil and water than most other parts of the globe (CACAR, 1997). These contaminants are more available for bioconcentration in the Arctic than in most temperate zones. Other contaminants that may bioconcentrate in fish or wildlife can be naturally present at high levels, such as mercury released through the weathering of rocks in the North West Territories. This is typically variable from location to location.

The main types of contaminants that bioconcentrate in the Arctic are persistent organic pollutants (POPs) (organic chemical compounds largely from industrial compounds and chlorinated pesticides) and heavy metals (Pacyna, 1995). Cadmium and mercury are major heavy metal concerns in the Arctic (CACAR, 1997). Each contaminant concentrates in a certain body part. Most POPs are lipophilic (meaning that they build up in fat), while cadmium builds up primarily in the liver and kidneys and mercury muscle.

The amount of bioconcentration that occurs depends on both the nature of the contaminant and the wildlife species. Natural rates of uptake of contaminants can vary even between similar individuals in the same genus. For example, within the family Salmonidae, lake trout

(Salvelinus namycush) in the Arctic is likely to have higher levels of mercury than Arctic char (Salvelinus alpinus) the same size and age, feeding on a similar diet.

Contaminants are of special concern in the Arctic because the traditional diet consumed by aboriginal peoples in the North, such as the Inuit, is composed primarily of wildlife. The species eaten are primarily the long-lived wildlife species that can accumulate high levels of contaminants through the combination of bioaccumulation and biomagnification. Further, traditionally eaten parts of animals such as muktuk are the same body parts that accumulate POPs. Studies have observed unusually high levels of contaminants in Inuit as a result of diet. It is believed that this concern is partly responsible for northern people avoiding traditional foods in favor of store-bought foods. However, risk/benefit studies suggest that the many health benefits of traditional diets far outweigh the minor health risks posed by contaminants (e.g., Kuhnlein, 1995). Even with biomagnification, traditional diets appear to be healthy choices for northern aboriginal peoples.

Alan Ehrlich

See also Food Chains; Food Use of Wild Species; Heavy Metals; Persistent Organic Pollutants (POPs)

Further Reading

Barrie, L.A., D. Gregor, B. Hargrave, R. Lake, D. Muir, R. Shearer, B. Tracey & T.F. Bidleman, "Arctic contaminants: sources, occurrence and pathways." Science of the Total Environment, 1992: 1-74 Canadian Arctic Contaminants Assessment Report (CACAR), Jensen, J., K. Adare & R. Shearer, Ottawa: Dept. of Indian Affairs and Northern Development, 1997 Elkin, B., "Organochlorine, heavy metal and radionuclide transfer through the lichen-caribou-wolf food chain." In Synopsis of Research Conducted under the 1993/94 Northern Contaminants Program, Environmental Studies No. 72, edited by J.L. Murray & R.G. Shearer, Indian and Northern Affairs Canada, 1994, pp. 356-361 Kuhnlein, H.V., "Benefits and risks of traditional food for indigenous peoples: focus on dietary intakes of Arctic men." Canadian Journal of Physiology and Pharmacology, 73 (1995): 765-771 Lockhart, 1998 Norstrom, R.J., M. Simon, D.C.G. Muir & R. Schweinsburg, "Organochlorine contaminants in Arctic marine food chains: identification, geographical distribution and temporal trends in polar bears." Environment Science and Technology 22 (1998): 1063-1071 Pacyna, J.M., "The origin of Arctic air pollutants: lessons learned and future research." The Science of the Total Environment, 160/161 (1995): 39-53

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