Biomagnification

Biomagnification, also known as bioamplification, or biological magnification is the increase in concentration of an element or compound that occurs in a food chain as a consequence of a) food chain energetics, b) lack of, or very slow, excretion or degradation of the substance.

It is an important concept in ecology, environmental science, and ecotoxicology: it says that the solution to certain types of pollution is not dilution, because food chains will concentrate the pollutant.

Although sometimes used interchangeably with 'bioaccumulation,' an important distinction is drawn between the two. Bioaccumulation occurs within a trophic level, and is the increase in concentration of a substance in an individual's tissues due to uptake from food and sediments In an aquatic milieu. Bioconcentration is defined as occurring when uptake from the water is greater than excretion. (Landrum and Fisher, 1999). Thus bioconcentration and bioaccumulation occur within an organism, and biomagnification occurs across trophic (food chain) levels.

Fat soluble (lipophilic) substances cannot be excreted in urine, a water-based medium, and so accumulate in fatty tissues of an organism if the organism lacks enzymes to degrade them. When eaten by another organism, fats are absorbed in the gut, carrying the substance, which then accumulates in the fats of the predator. Since at each level of the food chain there is a lot of energy loss, a predator must consume many prey, including all of their lipophilic substances.

For example, though mercury is only present in small amounts in seawater, it is absorbed by algae (generally as methylmercury). It is efficiently absorbed, but only very slowly excreted by organisms (Croteau et al, 2005). Bioaccumulation and biomagnification build it up in the fat tissues of successive trophic levels: consumer plankton, small fish, larger fish. Anything which eats these fish also consumes the higher level of mercury the fish have accumulated. This process explains why predatory fish such as swordfish and sharks or birds like osprey and eagles have higher concentrations of mercury in their tissue than could be accounted for by direct exposure alone. For example, herring contains mercury at approximately 0.01 ppm and shark contains mercury at greater than 1 ppm (EPA 1997).

History

According to the ISI Science Citation Index, the first use of the term in the title of a peer-reviewed article was in Johnson & Kennedy (1973). However, the concept traces back to Rachel Carson's book, Silent Spring, published in 1962. In Chapter 3 of Silent Spring, Carson describes the process but does not name it as biological magnification. Interestingly, she focusses on terrestrial systems, but most research has been done in aquatic systems.

==Carson drew attention to the issue, and other ecologists and toxicoligists examined its occurrence in many systems. As DDT, PCBs, mercury, and other substances were found through the 1970s to occur at strikingly high concentrations in the upper reaches of food chains, the concept of biomagnification of lipophilic substances became firmly established. It is presented in most introductory ecology and environmental science texts.

However, by the 1990s, some researchers began to question the roles of bioaccumulation versus biomagnification. For one thing, tissue concentrations of substances did not always increase uniformly with the trophic level (Landrum and Fisher, 1999). LeBlanc (1995) proposed that what is really bioaccumulation to different degrees is mistaken as biomagnification, because:

lipid contents of organisms increase with the trophic level
elimination efficiency of the substances decreases with trophic level (because the larger organisms have relatively less surface area to process and excrete substances, for their body size).

Thus the pattern of increased tissue concentration with higher trophic levels could be due to these differences in bioaccumulation. However, this proposal was based on rather limited data.

In 1990, Rasmussen et al., compared PCB levels in lake trout sampled from lakes with different numbers of trophic levels. Inputs to these lakes were small and relatively constant. The shorter the food chain in the lake, the lower the concentration of PCBs in the tissue of trout, which feed at the top of the chain (at least when they are large). This pattern is what is expected if biomagnification occurs. Additionally, they noted that the amount of PCB in tissues increased 3.5 times per trophic level, but the amount of lipids as a proportion of tissues increases much less, only 1.5 times per trophic level (Rasmussen et al., 1990).

Current status

In a review of a large number of studies, Suedel et al (1994) concluded that although biomagnification is probably more limited in occurrence than previously thought, there is good evidence that DDT, DDE, PCBs, toxaphene, and the organic forms of mercury and arsenic do biomagnify in nature. For other contaminants, bioconcentration and bioaccumulation account for their high concentrations in organism tissues. More recently, Gray (2002) reached a similar conclusion. However, even this study was criticized by Fisk et al., (2003) for ignoring many relevant studies. Such criticisms are spurring researchers to study carefully all pathways, and Croteau et al. (2005) recently added Cadmium to the list of biomagnifying metals.

The above studies refer to aquatic systems. In terrestrial systems, direct uptake by higher trophic levels must be much less, occurring via the lungs.

This critique of the biomagnification concept does not mean that we need not be concerned about synthetic organic contaminants and metal elements because they will become diluted. Bioaccumulation and bioconcentration result in these substances remaining in the organisms and not being diluted to non-threatening concentrations. The success of top predatory-bird recovery (bald eagles, peregrine falcons) in North America following the ban on DDT use in agriculture is testamnet to the importance of biomagnification.

Substances that biomagnify

There are two main groups of substances that biomagnify. Both are lipophilic and not easily degraded. Novel organic substances are not easily degraded because organisms lack previous exposure and have thus not evolved specific detoxification and excretion mechanisms, as there has been no selection pressure from them. These substances are consequently known as 'persistent organic pollutants' or POPs.

Metals are not degradable because they are elements. Organisms, particularly those subject to naturally high levels of exposure to metals, have mechanisms to sequester and excrete metals. Problems arise when organisms are exposed to higher concentrations than usual, which they cannot excrete rapidly enough to prevent damage. These metals are transferred in an organic form.

Novel organic substances

Inorganic substances

References

Carson, Rachel. 1962. Silent Spring. Houghton Mifflin.

Croteau, M., S. N. Luoma, and A. R Stewart. 2005. Trophic transfer of metals along freshwater food webs: Evidence of cadmium biomagnification in nature. Limnol. Oceanogr. 50 (5): 1511-1519.

EPA (U.S. Environmental Protection Agency). 1997. Mercury Study Report to Congress. Vol. IV: An Assessment of Exposure to Mercury in the United States . EPA-452/R-97-006. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards and Office of Research and Development.

Fisk AT, Hoekstra PF, Borga K,and DCG Muir, 2003. Biomagnification. Mar. Pollut. Bull. 46 (4): 522-524

Gray, J.S., 2002. Biomagnification in marine systems: the perspective of an ecologist. Mar. Pollut. Bull. 45: 46–52.

Johnson, BT, and Kennedy, JO (1973) Biomagnification of p,p'-DDT and methoxychlor by bacteria. Applied Microbiology 26, 66-71.

Landrum, PF and SW Fisher, 1999. Influence of lipids on the bioaccumulation and trophic transfer of organic contaminants in aquatic organisms. Chapter 9 in MT Arts and BC Wainman. Lipids in fresh water ecosystems. Springer Verlag, New York.

LeBlanc, GA 1995. Trophic level differences in the bioconcentration of chemicals: Implication in assessing environmental biomagnification. Env. Sci. Tech. 29:154-160.

Rasmussen, J.B., Rowan, D.J., Lean, D.R.S. and Carey, J.H., 1990. Food chain structure in Ontario lakes determines PCB levels in lake trout (Salvelinus namaycush) and other pelagic fish. Can. J. Fish. Aquat. Sci. 47, pp. 2030–2038

Suedel, B.C., Boraczek, J.A., Peddicord, R.K., Clifford, P.A. and Dillon, T.M., 1994. Trophic transfer and biomagnification potential of contaminants in aquatic ecosystems. Reviews of Environmental Contamination and Toxicology 136: 21–89.