Biomagnification Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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Biomagnif icat ion refers to the tendency of some chemicals to concentrate in living organisms and to pass up the food chain as they are consumed. Especially pernicious are those chemicals that are easily absorbed, not easily metabolized or excreted, have long half-lives, and an affinity for fat. These include methyl mercury; polychlorinated biphenyls (PCBs); and the chlorinated hydrocarbon pesticides such as DDT, aldrin, diel drin, lindane, and heptachlor. Substances that are water-soluble and biodegradable are not generally subject to biomagnification.

The loads of these chemicals increase in small, ostensibly harmless increments, but are concentrated in higher trophic levels, as organisms eat and in turn are eaten. Over time, the repeated consumption of contaminated prey ultimately results in the accumulation of high and possibly toxic levels within the consumer, compared with ambient levels of the contaminant in air, soil, and water that may be quite low. Additionally, since predators may preferentially select prey with the highest levels of contaminants (that is, those experiencing sublethal symptoms of chemical poisoning), predators may be exposed not to average pollutant concentrations but to maximum levels.

Because of the slow and incremental nature of biomagnification, there is often no indication that poisoning has occurred until toxic levels are reached. Poisoning at high levels can be lethal; while at lower doses, neurological, developmental, reproductive, and behavioral disorders-among other problems-may arise.

Species at Risk

Biomagnification is a special concern for species that occupy the higher trophic levels and those that have long average life spans. Particularly at risk are terrestrial mammalian carnivores, marine mammals, birds of prey, and predator fish species. Studies have demonstrated that the degree of biomagnification, however, is not merely related to the trophic levels of the consumer, but also to the species’ bioenergetic conversion efficiency, which relates activity and energy expenditures to growth and bioaccumulation rates. It is also associated with the species’ physiological ability to detoxify the chemical.

Predatory birds are especially vulnerable to biomagnification because their detoxification capability is poor. In fish-eating birds such as the bald eagle and osprey the biomagnification of DDT created toxic levels of DDE, a metabolic product of DDT that interferes with calcium absorption. This led to the thinning of eggshells and caused eggs to crack under the weight of the roosting parent, leading to plummeting numbers of these species. In the decades following the prohibition of DDT use in the United States in the 1970s, some of these populations have recovered.

As a high trophic level feeder, humans are not excluded from the process of biomagnification. A tragic example of this occurred in Minamata, Japan, in the 1950s. Mercury was released into a nearby river by a factory and then methylated by bacteria. The methyl mercury was bioconcentrated by organisms and subject to biomagnification, eventually poisoning the humans who consumed contaminated fish. This resulted in a host of health disorders and deaths, creating health problems even for successive human generations.

Mobile and migratory organisms have the capacity to move chemical contaminants in their bodies from one ecosystem to another. Global air and ocean circulatory patterns, in addition to transport by rivers, can move contaminants long distances. Thus, biomagnification of toxins occurs in organisms living in ecosystems far removed from the original sources of contamination.

Salmon are high trophic level feeders, relatively large, and have high lipid content. As such, they may accumulate a high concentration of toxins over their life span. Because of their anadromous life history, they can transport toxins from marine to freshwater systems. In the Great Lakes, salmon transport contaminants from the lakes to small tributaries. After spawning in freshwater tributaries, salmon die. Many species, both aquatic and terrestrial, consume their carcasses and take on their burden of toxins, thereby passing them to terrestrial food webs.

In aquatic environments, biomagnification occurs at every trophic level and toxins are ingested not only through the consumption of food, but also through the intake of water and sediments containing contaminants. As a result of the global transport of pollutants, the amplified process of biomagnification in aquatic systems, and the impact of cold climates on processes of chemical transformation, elevated levels of persistent organic pollutants (POPs) have been found in the arctic far from their place of origin in arctic-dwelling species of fish, birds, and mammals, including polar bears and marine mammals. With diets high in these species, humans living in arctic communities, such as the Inuit, have been found to have high levels of POPs in their bodies. The breast milk of Inuit women has been found to have DDT and PCB concentrations many times higher than that of women in more temperate latitudes. In a final step of biomagnification, these toxins are passed via breast milk to infants. The implications of this on the health and development of these infants is not yet completely understood.

Bibliography:

Rachel Carson, Silent Spring (Houghton Mifflin Company, 1962);
Gary A. Polis, Mary Power, Gary R. Huxel, eds., Food Webs at the Landscape Level (University of Chicago Press, 2004);
Carl Sindermann, Coastal Pollution: Effects on Living Resources and Humans (Taylor and Francis, 2006);
H. Walker, S.P. Hopkin, R.M. Sibley, D.P. Peakall, Principles of Ecotoxicology, 3rd ed. (Taylor and Francis 2006).